JAVA Interview Q &A
Q 1What do we call an operator that operates on only one operand?
A - An operator that operates on only one operand is called a unary operator.
Q 2- What do we call an operator that operates on two operands?
A - An operator that operates on two operands is called a binary operator.
Q 3- Is the minus sign a unary or a binary operator, or both? Explain your answer.
A - Both. As a binary operator, the minus sign causes its right operand to be subtracted from its left operand. As a unary operator, the minus sign causes the algebraic sign of the right operand to be changed.
Q 4- Describe operator overloading.
A - For those languages that support it (such as C++) operator overloading means that the programmer can
redefine the behavior of an operator with respect to objects of a new type defined by that program.
Q5- Java programmers may overload operators: True or False?
A 6- False: Unfortunately, Java does not support operator overloading.
Q7 - Show the symbols used for the following operators in Java: assignment, not equal, addition,cast.
A - The above listed operators in order are: = != + (type)
Q8 - Is any operator automatically overloaded in Java? If so, identify it and describe its overloaded
behavior.
A - The plus sign (+) is automatically overloaded in Java. The plus sign can be used to perform arithmetic
addition. It can also be used to concatenate strings. However, the plus sign does more than concatenate strings. It also performs a conversion to String type. When the plus sign is used to concatenate strings, the operand on the right is automatically converted to a character string before being concatenated with the operand on the left. This assumes that the compiler knows enough about the operand on the right to be able to successfully perform the conversion. It has that knowledge for all of the primitive types and most or all of the built-in reference types.
Q 9- What is the purpose of the cast operator?
A - The cast operator is used to purposely convert from one type to another.
Q10 - The increment operator is a binary operator: True or False?
A - False: The increment operator is a unary operator.
Q11 - Show the symbol for the increment operator.
A - The symbol for the increment operator is two plus signs with nothing between them (++).
Q 12- Describe the appearance and the behavior of the increment operator with both prefix and
postfix notation. Show example, possibly incomplete, code fragments illustrating both notational
forms.
A - The increment operator may be used with both prefix and postfix notation. Basically, the increment operator causes the value of the variable to which it is applied to be increased by one. With prefix notation, the operand appears to the right of the operator ( ++X), while with postfix notation, the operand appears to the left of the operator (X++).
The difference in behavior has to do with the point in time that the increment actually occurs if the operator and its operand appear as part of a larger overall expression. With the prefix version, the variable is incremented before it is used to evaluate the larger overall expression. With the postfix version, the variable is used to evaluate the larger overall expression and then it is incremented.
Q13 - Show the output that would be produced by the following Java application.
class prg1 { //define the controlling class
public static void main(String[] args){ //define main method
int x = 5, X = 5, y = 5, Y = 5;
System.out.println("x = " + x );
System.out.println("X = " + X );
System.out.println("x + X++ = " + (x + X++) );
System.out.println("X = " + X );
System.out.println();
System.out.println("y = " + y );
System.out.println("Y = " + Y );
System.out.println("y + ++Y = " + (y + ++Y) );
System.out.println("Y = " + Y );
}//end main
}//End class. Note no semicolon required
//End Java application
A - The output from this Java application follows:
x = 5
X = 5
x + X++ = 10
X = 6
y = 5
Y = 5
y + ++Y = 11
Y = 6
Q 14 - Binary operators use outfix notation: True or False? If your answer is False, explain why.
A - False: Binary operators use infix notation, which means that the operator appears between its operands.
Q15 - In practice, what does it mean to say that an operator that has performed an action returns a
value (or evaluates to a value) of a given type?
A - As a result of performing the specified action, an operator can be said to return a value (or evaluate to a
value) of a given type. The type depends on the operator and the type of the operands. To evaluate to a value means that after the action is performed, the operator and its operands are effectively replaced in the expression by the value that is returned.
Q 16 - What are the four categories of operators described in Baldwin's Java tutorial on operators? Do
you agree with this categorization? If not, explain why not.
A - Some authors divide Java's operators into the following categories:
arithmetic
relational and conditional (typically called relational and logical in C++)
bitwise and logical
assignment
Q17 - Show and describe at least five of the binary arithmetic operators supported by Java
(Clarification: binary operators does not mean bitwise operators).
A - Java support various arithmetic operators on floating point and integer numbers. The following table lists five of the binary arithmetic operators supported by Java.
Operator Description
+ Adds its operands
- Subtracts the right operand from the left operand
* Multiplies the operands
/ Divides the left operand by the right operand
% Remainder of dividing the left operand by the right operand
Q18 - In addition to arithmetic addition, what is another use for the plus operator (+)? Show an
example code fragment to illustrate your answer. The code fragment need not be a complete
statement.
A - The plus operator (+) is also used to concatenate strings : "IMR " + " global Ltd."
Q19 - When the plus operator (+) is used as a concatenation operator, what is the nature of its
behavior if its right operand is not of type String? If the right operand is a variable that is not of type
String, what is the impact of this behavior on that variable.
A - In this case, the operator also coerces the value of the right operand to a string representation for use in the expression only. If the right operand is a variable, the value stored in the variable is not modified in any way.
Q 20- Show and describe four unary arithmetic operators supported by Java.
A - Java supports the following four unary arithmetic operators.
Operator Description
+ Indicates a positive value
- Negates, or changes algebraic sign
++ Adds one to the operand, both prefix and postfix
-- Subtracts one from operand, prefix and postfix
Q20 - What is the type returned by relational operators in Java?
A - Relational operators return the boolean type in Java.
Q 21- Show and describe six different relational operators supported by Java.
A - Java supports the following set of relational operators:
Operator Returns true if
> Left operand is greater than right operand
>= Left operand is greater than or equal to right operand
< Left operand is less than right operand
<= Left operand is less than or equal to right operand
== Left operand is equal to right operand
!= Left operand is not equal to right operand
Q 22- Show the output that would be produced by the following Java application:
class prg2 { //define the controlling class
public static void main(String[] args){ //define main method
System.out.println("The relational 6<5 is " + (6<5 ) );
System.out.println("The relational 6>5 is " + (6>5 ) );
}//end main
}//End class. Note no semicolon required
//End Java application
A - This program produces the following output:
The relational 6<5 is false
The relational 6>5 is true
Q23 - Show and describe three operators (frequently referred to as conditional operators in Java and
logical operators in C++) which are often combined with relational operators to construct more
complex expressions (often called conditional expressions). Hint: The && operator returns true if
the left and right operands are both true. What are the other two and how do they behave?
A - The following three logical or conditional operators are supported by Java.
Operator Typical Use Returns true if
&& Left && Right Left and Right are both true
|| Left || Right Either Left or Right is true
! ! Right Right is false
Q24 - Describe the special behavior of the || operator in the following expression for the case where
the value of the variable a is less than the value of the variable b.
(a < b) || (c < d)
A - An important characteristic of the behavior of the && and || operators in Java is that the expressions are
evaluated from left to right, and the evaluation of the expression is terminated as soon as the result of evaluating the expression can be determined. For example, in the above expression, if the variable a is less than the variable b , there is no need to evaluate the right operand of the || to determine the value of the entire expression. Thus, evaluation will terminate as soon as it is determined that a is less than b.
Q25 - Show the symbols used for the bitwise and operator and the or operator.
A - The & operator in Java is a bitwise and and the | operator is a bitwise or.
Q26 - The logical and operator is represented in Java by the && symbol. What is the representation
of the bitwise and operator in Java?
A - The bitwise and operator is represented by the & symbol in Java.
Q27 - Show and describe five operators in Java that perform actions on the operands one bit at a time
(bitwise operators).
A - The following table shows the seven bitwise operators supported by Java.
Operator Typical Use Operation
>> OpLeft >> Dist Shift bits of OpLeft right by Dist bits (signed)
<< OpLeft << Dist Shift bits of OpLeft left by Dist bits
>>> OpLeft >>> Dist Shift bits of OpLeft right by Dist bits (unsigned)
& OpLeft & OpRight Bitwise and of the two operands
| OpLeft | OpRight Bitwise inclusive or of the two operands
^ OpLeft ^ OpRight Bitwise exclusive or (xor) of the two operands
~ ~ OpRight Bitwise complement of the right operand (unary)
Q28 - In Java, the signed right shift operation populates the vacated bits with the zeros, while the left
shift and the unsigned right shift populate the vacated bits with the sign bit: True or False. If your
answer is False, explain why.
A - False: In Java, the signed right shift operation populates the vacated bits with the sign bit, while the left shift and the unsigned right shift populate the vacated bits with zeros.
Q29 - In a signed right-shift operation in Java, the bits shifted off the right end are lost: True or False.
If your answer is False, explain why.
A - True: For both Java and C++, bits shifted off the right end are lost.
Q30 - Using the symbols 1 and 0 construct a table showing the four possible combinations of 1 and 0.
Using a 1 or a 0, show the result of the bitwise and operation on these four combinations of 1 and 0.
A - The answer is:
1 and 1 produces 1
1 and 0 produces 0
0 and 1 produces 0
0 and 0 produces 0
Q 31- Using the symbols 1 and 0 construct a truth table showing the four possible combinations of 1
and 0. Using a 1 or a 0, show the result of the bitwise inclusive or operation on these four
combinations on these four combinations of 1 and 0.
A - The answer for the inclusive or is:
1 or 1 produces 1
1 or 0 produces 1
0 or 1 produces 1
0 or 0 produces 0
Q 32- Using the symbols 1 and 0 construct a truth table showing the four possible combinations of 1
and 0. Using a 1 or a 0, show the result of the bitwise exclusive or operation on these four
combinations on these four combinations of 1 and 0.
A - The answer for the exclusive or is:
1 xor 1 produces 0
1 xor 0 produces 1
0 xor 1 produces 1
0 xor 0 produces 0
Q 33- For the exclusive or, if the two bits are different, the result is a 1. If the two bits are the same,
the result is a 0. True or False? If your answer is False, explain why.
A - True.
Q 34- Is the assignment operator a unary operator or a binary operator. Select one or the other.
A - The assignment operator is a binary operator.
Q 35- In Java, when using the assignment operator, the value stored in memory and represented by the
right operand is copied into the memory represented by the left operand: True or False? If your
answer is False, explain why.
A - True.
Q 36- Show two of the shortcut assignment operators and explain how they behave by comparing
them with the regular (nonshortcut) versions. Hint: The (^=) operator is a shortcut assignment
operator.
A - Java supports the following list of shortcut assignment operators. These operators allow you to perform an assignment and another operation with a single operator. += -= *= /= %= &= |= ^= <<= >>= >>>=
For example, the two statements which follow perform the same operation.
x += y; x = x + y;
The behavior of all the shortcut assignment operators follows this same pattern.
Q 37 - Write a Java application illustrates the difference between the prefix and the postfix versions of the increment operator.
class prog3{
static public void main(String[] args){
int x = 3;
int y = 3;
int z = 10;
System.out.println("Prefix version gives " + (z + ++x));
System.out.println("Postfix version gives " + (z + y++));
}//end main
}//end class
Q38 Write a Java application that illustrates the use of the following relational operators: < > <= >= == !=
class Prog4 { //define the controlling class
public static void main(String[] args){ //define main method
System.out.println("The relational 6<5 is " + (6<5 ) );
System.out.println("The relational 6>5 is " + (6>5 ) );
System.out.println("The relational 5>=5 is " + (5>=5 ) );
System.out.println("The relational 5<=5 is " + (5<=5 ) );
System.out.println("The relational 6==5 is " + (6==5 ) );
System.out.println("The relational 6!=5 is " + (6!=5 ) );
}//end main
}//End prog4 class. Note no semicolon required
Q39 - Write a Java application that illustrates the use of the following logical or conditional operators:
&& || !
class prg5 { //define the controlling class
public static void main(String[] args){ //define main method
System.out.println("true and true is " + (true && true) );
System.out.println("true and false is " + (true && false) );
System.out.println("true or true is " + (true || true) );
System.out.println("true or false is " + (true || false) );
System.out.println("false or false is " + (false || false) );
System.out.println("not true is " + (! true) );
System.out.println("not false is " + (! false) );
}//end main
}
Q40 - Java supports a constant type: True or False. If false, explain why.
A - Java does not support a constant type. However, in Java, it is possible to achieve the same result by
declaring and initializing a variable and making it final.
Q41 Provide a code fragment that illustrates the syntax for creating a named constant in Java.
A - The syntax for creating a named constant in Java is as follows: final float PI = 3.14159;
Q42 - What is the common method of controlling the order of evaluation of expressions in Java?
A - you can control the order of evaluation by the use of parentheses.
Q43 - What are the three actions normally involved in the operation of a loop (in addition to executing
the code in the body of the loop)?
A - The operation of a loop normally involves the following three actions in addition to executing the code in the body of the loop:
Initialize a control variable.
Test the control variable in a conditional expression.
Update the control variable.
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Q - The Java read() method reads and returns a single byte from the standard input device. It
stores that byte according to what type. What does the method return if the user enters an eof?
A - The Java read() method reads and returns a single byte from the standard input device and stores that byte
in an integer. It returns an integer value of -1 if the user enters an eof.
Q - What keystroke combination can be used to simulate an eof at the keyboard of a DOS system?
A - An eof can be simulated on a DOS system keyboard by holding down the ctrl key and pressing the z key.
Q - Provide a Java code fragment illustrating how you would read a stream of bytes from the
standard input device until encountering an eof and quit reading when the eof is encountered.
A - The following Java code fragment will read a stream of bytes from the standard input device until
encountering an eof.
while(System.in.read() != -1) { //do something }
This code fragment accesses the read()method of the object referred to by the class variable named in of the
class named System.
Q - Provide a Java code fragment illustrating two different ways to display a String argument on the
Java standard output device. Explain how your code works in object-oriented terms. Make certain
that you explain the difference between the two.
A - The following two code fragments will each display a string argument on the Java standard output device.
System.out.println("String argument")
System.out.print("String argument")
In the first case, the code fragment accesses the println() method of the object referred to by the class variable
named out of the class named System. In the second case, the print() method is accessed instead of the println() method.
The difference between the two is that the println() method automatically inserts a newlineat the end of the
string argument whereas the print() method leaves the display cursor at the end of the string argument.
===============
OOP
Q - In Object-Oriented Programming, an object is often said to be an ____________ of a class.
A - An object is often said to be an instance of a class.
Q - In Object-Oriented Programming, an object is often said to have s_______ and b_______.
Provide the missing words which begin with the letters shown.
A - In OOP, an object is often said to have state and behavior.
Q - An object's state is contained in its ________ and its behavior is implemented through its
________.
A - An object's state is contained in its member variables ( or data members) and its behavior is implemented through its methods ( or member functions).
Q - The member variables of an object can be either ____________ or _________ .
A - Its member variables can be either instance variables or class variables.
Q - What is generally meant by the terminology "sending a message to an object?"
A - We activate the behavior of an object by invoking one of its methods (sending it a message).
Q - What are the two things that can usually happen when an object receives a message?
A - When an object receives a message, it usually either performs an action, or modifies its state, or both.
Q - What happens to the memory occupied by an object in Java when the object is no longer
needed, and what do we normally do to make that happen?
A - When an object is no longer needed in Java, we simply forget it. Eventually, the garbage collector may (or may not) come by and pick it up for recycling.
Q - Identify as the stages of an object's life?
A - The stages of an Object's life are:
Creation
Use
Cleanup
Q - The creation of an object involves three steps (which are often combined). What are the three steps?
A - The three steps are:
declaration (providing a name for the object)
instantiation (setting aside memory for the object)
optional initialization (providing initial values for the object's instance variables)
Q - Java allows the instantiation of variables of primitive types in dynamic memory: True or False?
If false, explain why and what you might be able to do to achieve almost the same result.
A - False. Java does not allow the instantiation of primitive variables in dynamic memory. (However, there are wrapper classes for primitive types which can be used to turn them into objects for this purpose.)
Q - An array of objects in Java is instantiated as an array of reference variables where each
reference variable can then be used to instantiate an object pointed to by the reference variable:
True or False. If false, explain why and either provide a code fragment that illustrates your answer
A - True.
Q - In Java, it is always necessary to declare (give a name to) all new objects: True or False? If
false, explain why and either provide a code fragment that illustrates your answer
A - It is not always necessary in Java to declare an object (to give it a name). Consider, for example a case where a
new object is instantiated to be used in an expression and there is no requirement to be able to access that object outside of the expression.
Q - Instance variables and instance methods can be accessed using an object as the access
mechanism. What is the difference in the syntax used to access an instance variable and an
instance method.
A - None. There is essentially no difference in the syntax used to access a variable or a method.
Q - Once you have instantiated an object, it is always possible to access all of the instance
variables and instance methods of that object by joining the name of the object to the name of the
variable or method using a period: True or False? If false, explain why.
A - False. Sometimes variables or methods may be hidden so as to make it impossible to access them. It is very common to hide the variables and to provide methods which can be accessed to serve as a pathway to the variables.
Q - Given an object named obj that has a public instance method named myMethod(), provide a
code fragment that shows the proper syntax for accessing the method.
A - The proper syntax for accessing an instance method named myMethod() follows:
obj.myMethod()
Q - The object-oriented approach normally recommends hiding instance variables behind access
methods: True or False? If true, explain why.
A - True. The object-oriented approach normally recommends hiding instance variables behind access methods. There are a variety of reasons why. One important reason is that hiding the instance variables makes it possible to later modify the implementation of instance variables to improve the behavior of objects of the class, without a requirement for modifying code that uses the clsss, provided that the access methods are not modified.
Q - The returning of memory (to the operating system) occupied by objects that are no longer
needed is automatically accomplished in Java by a feature commonly known as the
____________________.
A - The returning of memory to the operating system is taken care of automatically by a feature of Java known as the garbage collector.
Q - All necessary cleanup in a Java program is performed automatically by the garbage collector:
True or False? If false, explain why.
A - False. Java does not support anything like a destructor that is guaranteed to be called whenever the object is no longer needed. Therefore, other than returning allocated memory, it is the responsibility of the programmer to explicitly perform any other required cleanup at the appropriate point in time.
Q - When does an object become eligible for garbage collection?
A - An object becomes eligible for garbage collection when there are no more references to that object.
Q - What can your program do to purposely make an object eligible for garbage collection.
A - Your program can make an object eligible for garbage collection by assigning null to all references to the object.
Q - The purpose of garbage collection in Java is to perform all necessary cleanup and the memory
occupied by objects that are no longer needed will always be reclaimed by the garbage collector:
True or False? If false, explain why.
A - False. The sole purpose of garbage collection is to reclaim memory occupied by objects that are no longer needed, and it has no other purpose relative to necessary cleanup. An object becomes eligible for garbage collection when there are no more references to that object. However, just because an object is eligible for garbage collection doesn't mean that it will be reclaimed. The garbage collector runs in a low-priority thread, and may not run at all unless a memory shortage is detected.
Q - Before the garbage collector reclaims the memory occupied by an object, it always calls the
object's _________ method. (Provide the name of the method.) Explain why this method is one of
the methods in all new classes that you define.
A - Before the garbage collector reclaims the memory occupied by an object, it calls the object's finalizemethod. The finalize method is a member of the Object class. Since all classes inherit from the Object class, your classes also contain the default finalize method.
Q - What must you do to make effective use of the finalize method? Explain why you might want to
do this.
A - In order to make use of the finalize method, you must override it, providing the code that you want to have executed before the memory is reclaimed.
Q - You can always be confident that the finalize method will be promptly executed to perform
necessary cleanup in a Java program when an object becomes eligible for garbage collection: True
or False? If false, explain why.
A - False. Although you can be confident that the finalize method will be called before the garbage collector reclaims the memory occupied by a particular object, you cannot be certain when, or if that memory will be reclaimed. There is no guarantee that the memory will be reclaimed by the garbage collector during the execution of your program.
Q - Provide a code fragment illustrating the method call that you can make to ask the garbage
collector to run. This guarantees that garbage collection will take place: True or False? If false,
explain why.
A - False. Campione and Walrath indicate that you can ask the garbage collector to run at any time by calling the method shown below. They further point out, however, that making the request does not guarantee that your objects will be reclaimed through garbage collection. System.gc();
=====================================================================
Q1 - Three keywords are used in Java to specify access control. What are they?
A - The three keywords used to specify access control in Java are public, private, and protected.
Q2 - In Java, special access privileges are afforded to other members of the same package: True or False? If false, explain your answer.
A - True. In Java, special access privileges are afforded to other members of the same package.
Q3 - In Java, class variables are often used with the __________ keyword to create variables that act like constants.
A - The final keyword.
Q4 - In Java, the ___________ keyword is used to declare a class variable.
A - The static keyword.
Q5 - In Java, the ___________ keyword is used to declare a class method.
A - The static keyword.
Q6 - When you include a method in a Java class definition without use of static keyword, this will result in objects of that class containing an instance method: True or False? If false, explain why.
A 7- True.
Q8 - Normally each object contains its own copy of each instance method: True or False?
A - False, multiple copies of the method do not normally exist in memory.
Q9- When you invoke an instance method using a specific object, if that method refers to instance variables of the class, that method is caused to refer to the specific instance variables of the specific object for which it was invoked: True or False? If false, explain why.
A - True.
Q10 - Instance methods are invoked in Java using the name of the object, the colon, and the name of the method as shown below: True or False? If false, explain why.
myObject:myInstanceMethod( )
A - False. Use the period or dot operator, not the colon.
Q11 - Instance methods have access to both instance variables and class variables in Java: True or False. If false, explain why.
A - True.
Q12 - Class methods have access to both instance variables and class variables in Java: True or False. If false, explain why.
A - False. Class method can only access other class members.
Q13 - What are the two most significant characteristics of class methods?
A - 1. Class methods can only access other class members. 2. Class methods can be accessed using only the name of the class. An object of the class is not required to access class methods.
Q14 - In Java, a class method can be invoked using the name of the class, the colon, and the name of the method as shown below: True or False? If false, explain why.
MyClass:myClassMethod()
A - False. You must use the period or dot operator, not the colon.
Q15 - What is meant by overloaded methods?
A - The term overloaded methods means that two or more methods may have the same name so long as they have different argument lists.
Q16 - If you overload a method name, the compiler determines at run time, on the basis of the arguments provided to the invocation of the method, which version of the method to call in that instance: True or False? If false, explain why.
A - False. The determination is made at compile time.
Q16 - A constructor is a special method which is used to construct an object. A constructor always has the same name as the class in which it is defined, and has no return type specified. True or False? If false, explain why.
A - True.
Q17 - Constructors may be overloaded, so a single class may have more than one constructor, all of which have the same name, but different argument lists: True or False. If false, explain why.
A - True
Q18 - What is the purpose of a parameterized constructor?
A - The purpose of a parameterized constructor is to initialize the instance variables of an object when the object is instantiated.
Q 19 - The same set of instance variables can often be initialized in more than one way using overloaded constructors: True or False? If false, explain why.
A - True.
Q20 - It is not necessary to provide a constructor in Java. True or False? If false, explain why.
A - True. .
Q 21 You can think of the default constructor as a constructor which doesn't take any parameters: True or False? If false, explain why.
A - True.
Q 22- In Java, if you provide any constructors, the default constructor is no longer provided automatically: True or False? If false, explain why.
A - True.
Q 23- In Java, if you need both a parameterized constructor and a constructor which doesn't take parameters (often called a default constructor), you must provide them both: True or False? If false,explain why.
A - True.
Q 24- In Java, you can instantiate objects in static memory at compile time, or you can use the new operator to request memory from the operating system at runtime and use the constructor to instantiate the object in that memory: True or False? If false, explain why.
A - False. In Java, objects can only be instantiated on the heap at runtime.
Q25 - Provide a code fragment consisting of a single statement that illustrates how the constructor is typically used in Java to declare, instantiate, and initialize an object. Assume a parameterized constructor with three parameters of type int.
A - MyClass myObject = new MyClass(1,2,3);
Q26 - Provide a code fragment consisting of a single statement that illustrates how the default constructor is typically used in Java to declare and instantiate an object.
A - MyClass myObject = new MyClass();
Q 27- What are the three actions performed by the following statement?
MyClass myObject = new MyClass(1,2,3);
A - This statement performs three actions in one.
The object is declared by notifying the compiler of the name of the object.
The object is instantiated by using the new operator to allocate memory space to contain the new object.
The object is initialized by making a call to the constructor named MyClass.
Q 28- In Java, if you attempt to instantiate an object and the Java Virtual Machine cannot allocate the
requisite memory, the system will: ________________________________________.
A - Throw an OutOfMemoryError.
Q 29- The following is a valid method call: True or False. If false, explain why. obj.myFunction(new myClassConstructor(1,2,3) );//Java version
A - True.
Q30 - In Java, when a method begins execution, all of the parameters are created as local automatic
variables: True or False? If false, explain why.
A - True.
Q31 - In the following statement, an object is instantiated and initialized and passed as a parameter to a
function. What will happen to that object when the function terminates? obj.myFunction(new myClassConstructor(1,2,3) );//Java version
A - It will become eligible for garbage collection.
Q 32- In Java, you declare and implement a constructor just like you would implement any other method in your class, except that: _______________________________________________
A - you do not specify a return type and must not include a return statement.
Q33 - The name of the constructor must be the same as the name of the ___________________.
A - class.
Q 34- Usually in cases of inheritance, you will want the subclass to cause the constructor for the superclass to
execute last to initialize those instance variables which derive from the superclass:True or False? If false, explain why.
A - False. You will want the subclass to cause the constructor for the superclass to execute first.
Q 35- Provide a code fragment that you would include at the beginning of your constructor for a subclass to cause the constructor for the superclass to be invoked prior to the execution of the body of the constructor.
A - super(optional parameters);
Q 36- Every object has a finalize method which is inherited from the class named ________________.
A - object.
Q 37- Before an object is reclaimed by the garbage collector, the _______________ method for the
object is called.
A - finalize
Q 38- In Java, the destructor is always called when an object goes out of scope: True or False? If false, explain why.
A - False. Java does not support the concept of a destructor.
Q 39- The class at the top of the inheritance hierarchy is the Object class and this class is defined in the package named java.Object: True or False? If false, explain why.
A - False. The Object class is defined in the package named java.lang.
Q40 - We say that an object has state and behavior: True or False? If false, explain why.
A - True.
Q 41- In Java, a method can be defined as an empty method, normally indicating that it is intended to be overridden: True or False? If false, explain why.
A - True.
Q 42- Including an empty method in a class definition will make it impossible to instantiate an object of that
class: True or False.
A - False.
Q 43- A subclass can invoke the constructor for the immediate superclass by causing the last line of of the subclass constructor to contain the keyword super followed by a parameter list as though calling a function named super() and the parameter list must match the method signature of the superclass constructor: True or False? If false, explain why.
A - False. This can only be accomplished by causing the first line of the constructor to contain the keyword super followed by a parameter list as though calling a function named super(). The parameter list must match the method signature of the superclass constructor.
Q 43 The equals() method is used to determine if two reference variables point to the same object: True or False? If false, explain why.
A- False. You can use the equals() method to compare two objects for equality. You can use the equality operator (==) to determine if two reference variables point to the same object.
Q 44- The equals() method is used to determine if two separate objects are of the same type and contain the same data. The method returns false if the objects are equal and true otherwise. True or False? If false, explain why.
A - False. The method returns true if the objects are equal and false otherwise.
Q 45 - The equals() method is defined in the Object class: True or False? If false, explain why.
A - True.
Q 46- Your classes can override the equals() method to make an appropriate comparison between two objects of a type that you define: True or False? If false, explain why.
A - True.
Q 47- You must override the equals() method to determine if two string objects contain the same data: True or False? If false, explain why.
A - False. The system already knows how to apply the equals() method to all of the standard classes and objects of which the compiler has knowledge. For example, you can already use the method to test two String objects or two array objects for equality.
Q 48- Given an object named obj1, provide a code fragment that shows how to obtain the name of the class from which obj1 was instantiated and the name of the superclass of that class.
A - See code fragment below:
System.out.println("Name of class for obj1: " + obj1.getClass().getName());
System.out.println("Name of superclass for obj1: " + obj1.getClass().getSuperclass());
Q 49- Given an object named obj2, provide a code fragment that shows how to use the newInstance() method to create a new object of the same type
A - See code fragment below:
obj2 = obj1.getClass().newInstance();
Q 50- By overriding the getClass() method, you can use that method to determine the name of the class from which an object was instantiated: True or False? If false, explain why.
A - False. The getClass() method is a final method and cannot be overridden.
Q 51- You must use the new operator to instantiate an object of type Class: True or False? If false,explain why.
A - False. There is no public constructor for the class Class. Class objects are constructed automatically by the
Java Virtual Machine as classes are loaded and or by calls to the defineClass method in the class loader.
Q 52- The Class class provides a toString() method which can be used to convert all objects known to the compiler to some appropriate string representation. The actual string representation depends on the type of object: True or False? If false, explain why.
A - False. The Object class (not the Class class) provides a toString() method which can be used to convert all objects known to the compiler to some appropriate string representation. The actual string representation depends on the type of object.
Q 53- You can override the toString() method of the Class class to cause it to convert objects of your design to strings: True or False? If false, explain why.
A - False. The toString() method is a method of the Object class, not the Class class.
Q 54- By default, all classes in Java are either direct or indirect descendants of the Class class which is at the top of the inheritance hierarchy: True or False? If false, explain why.
A - False. By default, all classes in Java are either direct or indirect descendants of the Object class (not the Class class) which is at the top of the inheritance hierarchy.
Q 55 - To a limited extent, the interface concept allows you to treat a number of objects, instantiated from different classes, as if they were all of the same type: True or False? If false, explain why.
A - True.
Q 56- At its simplest level, an interface definition has a name, and declares one or more methods: True or False? If false, explain why.
A - True.
Q57- In an interface definition, both the method signatures and the actual implementations (bodies) of the methods are provided: True or False? If false, explain why.
A - False. Only the method signatures are provided. The actual implementations (bodies) of the methods are not provided.
Q 58- An interface definition can contain only method declarations: True or False? If false, explain why.
A - False. In addition to method declarations, an interface can also declare constants. Nothing else may be included inside the body of an interface definition.
Q59 - If classes P, D, and Q all implement interface X, a reference variable for an object of class P, D, or Q could be assigned to a reference variable of type X: True or False? If false, explain why.
A - True.
Q 60- If classes P, D, and Q all implement interface X, then all of the methods declared in X must be exactly the same in classes P, D, and Q: True or False? If false, explain why.
A - False. The interface simply declares the signatures for methods. Classes that implement the interface are free to provide a body for those methods which best suits the needs of the class.
Q 61- If classes P, D, and Q all implement interface X a reference variable for an object of class P, D, or Q could be assigned to a reference variable of type X and that reference variable could be used to access all of the methods of the class (which are not excluded using public, private, or protected): True or False? If false, explain why.
A - False. If one or more of the classes P, D, and Q, define instance methods which are not declared in the interface X, then a variable of type X cannot be used to access those instance methods. Those methods can only be accessed using a reference variable of the class in which the method is defined. Reference variables of the type X can only be used to access methods declared in the interface X (or one of its superinterfaces).
Q 61 - The new operator must be used to instantiate an object which is of the type of an interface: True or False? If false, explain why.
A - False. Even though you can consider the interface name as a type for purposes of storing references to objects, you cannot instantiate an object of the interface type itself.
Q 63- One of the difficulties of implementing interfaces is the requirement to coordinate the definition of interface methods among the classes that implement the interface: True or False? If false, explain why.
A - False. In defining interface methods, each class defines the methods in a manner that is appropriate to its own class without concern for how it is defined in other classes.
Q 64- As with classes, multiple interface definitions can be combined into the same source file: True or False? If false, explain why.
A- False. The compiler requires interface definitions to be in separate files.
Q 65- List four ways in which interfaces are useful:
A - See the following list:
To a limited extent, the interface concept allows you to treat a number of objects, instantiated from different classes, as if they were all of the same type
Capturing similarities between unrelated classes without forcing a class relationship
Declaring methods that one or more classes are expected to implement
Revealing an object's programming interface without revealing its class (objects such as these are called
anonymous objects and can be useful when shipping a package of classes to other developers)
Q 66- A minimum interface declaration contains the Java keyword interface, the name of the interface, and the name of the interface that it extends: True or False? If false, explain why.
A - False. A minimum interface declaration contains the Java keyword interface and the name of the interface. There is no requirement to specify the name of the interface that it extends, because it may not extend another interface.
Q 67 - An interface can extend any number of other interfaces: True or False? If false, explain why.
A - True.
Q 68- Just like a class definition can extend any number of other classes, an interface can extend any number
of other interfaces: True or False? If false, explain why.
A - False. A class can extend only one other class.
Q 69- An interface can extend any number of other interfaces but not more than one class: True or False? If false, explain why.
A - False. An interface cannot extend a class.
Q 70- An interface inherits all constants and methods from its superinterface: True or False? If false, explain why.
A - False. See reasons below:
An interface inherits all constants and methods from its superinterface unless:
the interface hides a constant with another of the same name, or redeclares a method with a new method declaration.
Q 71- The method declaration in an interface consists of the method signature followed by a pair of
empty curly braces: True or False? If false, explain why.
A - False. The method declaration is terminated by a semicolon and no body (no curly braces) is provided for the method.
Q 72- The keyword private is used to restrict access to the members of an interface only to classes within the same package: True or False? If false, explain why.
A - False. You may not use private in a member declaration in an interface.
Q 73- All methods declared in an interface are implicitly public and abstract: True or False? If false, explain why.
A - True.
Q 74- In addition to declaring methods, the body of the interface may also define constants. Constant values defined in an interface are implicitly public, static, and final: True or False? If false, explain why.
A - True.
Q 75- You use an interface by defining a class that extends the interface by name: True or False? If false, explain why.
A - False. You use an interface by defining a class that implements (not extends) the interface by name.
Q 76- When a class claims to implement an interface, it must provide a full definition for all the methods declared in the interface as well as all of the methods declared in all of the superinterfaces of that interface: True or False? If false, explain why.
A - True.
Q 77- A class can implement more than one interface by including several interface names in a comma-separated list of interface names, and by providing a full definition for all of the methods declared in all of the interfaces listed as well as all of the superinterfaces of those interfaces: True or False? If false, explain why.
A - True.
Q 78- Whenever a class implements an interface, it is allowed to define only those methods declared in the interface: True or False? If false, explain why.
A - False. Whenever a class implements an interface, the class must define all of the methods declared in the interface, but is also free to define other methods as well.
Q 79- The definition of an interface is a definition of a new reference data type. You can use interface names just about anywhere that you would use other type names, except that you cannot ____________________.
A - You cannot instantiate objects of the interface type.
Q 80- Explain in your own words the "bottom line" benefits of the use of an interface.
A - The interface makes it possible for a method in one class to invoke methods on objects of other classes,
without the requirement to know the true class of those objects, provided that those objects are all instantiated from classes that implement one or more specified interfaces. In other words, objects of classes that implement specified interfaces can be passed into methods of other objects as the generic type Object, and the methods of the other objects can invoke methods on the incoming objects by first casting them as the interface type.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Strings
Q - Java provides two different string classes from which string objects can be instantiated. What
are they?
A - The two classes are:
String
StringBuffer
Q - The StringBuffer class is used for strings that are not allowed to change. The String class is
used for strings that are modified by the program: True or False. If false, explain why.
A - False. This statement is backwards. The String class is used for strings that are not allowed to change. The
StringBuffer class is used for strings that are modified by the program.
Q - While the contents of a String object cannot be modified, a reference to a String object can be
caused to point to a different String object: True or False. If false, explain why.
A - True.
Q - The use of the new operator is required for instantiation of objects of type String: True or
False? If false, explain your answer.
A - False. A String object can be instantiated using either of the following statements:
String str1 = new String("String named str2");
String str2 = "String named str1";
Q - The use of the new operator is required for instantiation of objects of type StringBuffer: True
or False? If false, explain your answer.
A - True.
Q - Provide a code fragment that illustrates how to instantiate an empty StringBuffer object of a
default length and then use a version of the append() method to put some data into the object.
A - See code fragment below:
StringBuffer str5 = new StringBuffer();//accept default initial length
str5.append("StringBuffer named str5");//modify length as needed
Q - Without specifying any explicit numeric values, provide a code fragment that will instantiate an
empty StringBuffer object of the correct initial length to contain the string "StringBuffer named str6"
and then store that string in the object.
A - See the following code fragment:
StringBuffer str6 = new StringBuffer("StringBuffer named str6".length());
str6.append("StringBuffer named str6");
Q - Provide a code fragment consisting of a single statement showing how to use the Integer wrapper class to convert a string containing digits to an integer and store it in a variable of type int.
A - See code fragment below
int num = new Integer("3625").intValue();
Q - Explain the difference between the capacity() method and the length() methods of the StringBuffer class.
A - The capacity() method returns the amount of space currently allocated for the StringBuffer object. The
length() method returns the amount of space used.
Q - The following is a valid code fragment: True or False? If false, explain why.
StringBuffer str6 = new StringBuffer("StringBuffer named str6".length());
A - True.
Q - Which of the following code fragments is the most efficient, first or second?
String str1 = "THIS STRING IS NAMED str1";
String str1 = new String("THIS STRING IS NAMED str1");
A - The first code fragment is the most efficient.
System
Java provides the System class which provides a platform-dependent interface between your program and various system resources: True or False? If false, explain why.
A - False. Java provides the System class which provides a platform-independent interface between your program and those resources.
Q - You must instantiate an object of the System class in order to use it: True or False? If false, explain why.
A - False. You don't need to instantiate an object of the System class to use it, because all of its variables and methods are class variables and methods.
Q - The following code fragment can be used to instantiate an object of the System class: True or False? If false, explain why.
System mySystemObject = new System();
A - False. You cannot instantiate an object of the System class. It is a final class, and all of its constructors are private.
Q - What is the purpose of the write() method of the PrintStream class?
A - The write() method is used to write bytes to the stream. You can use write() to write data which is not intended to be interpreted as text (such as bit-mapped graphics data).
Exceptions
Q - The exception-handling capability of Java makes it possible for you to monitor for exceptional conditions within your program, and to transfer control to special exception-handling code which you design. List five keywords that are used for this purpose.
A - try, throw, catch, finally, and throws
Q - All exceptions in Java are thrown by code that you write: True or False? If false, explain why.
A - False. There are situations where an exceptional condition automatically transfers control to special exception-handling code which you write (cases where you don't provide the code to throw the exception object).
Q - When an exceptional condition causes an exception to be thrown, that exception is an object derived, either directly, or indirectly from the class Exception: True or False? If false, explain why.
A - False. When an exceptional condition causes an exception to be thrown, that exception is an object derived, either directly, or indirectly from the class Throwable.
Q - All exceptions other than those in the RuntimeException class must be either caught, or declared in a throws clause of any method that can throw them: True or False? If false, explain why.
A - True.
Q - What method of which class would you use to extract the message from an exception object?
A - The getMessage() method of the Throwable class.
Q - Normally, those exception handlers designed to handle exceptions closest to the root of the exception class hierarchy should be placed first in the list of exception handlers: True or False? If false, explain why.
A - False. The above statement has it backwards. Those handlers designed to handle exceptions furthermost from the root of the hierarchy tree should be placed first in the list of exception handlers.
Q - Explain why you should place exception handlers furthermost from the root of the exception hierarchy tree first in the list of exception handlers.
A - An exception hander designed to handle a specialized "leaf" object may be preempted by another handler whose exception object type is closer to the root of the exception hierarchy tree if the second exception handler appears earlier in the list of exception handlers.
Q - In addition to writing handlers for very specialized exception objects, the Java language allows you to write general exception handlers that handle multiple types of exceptions: True or False? If false, explain why.
A - True.
Q - Your exception handler can be written to handle any class that inherits from Throwable. If you write a handler for a node class (a class with no subclasses), you've written a specialized handler: it will only handle exceptions of that specific type. If you write a handler for a leaf class (a class with subclasses), you've written a general handler: it will handle any exception whose type is the node class or any of its subclasses. True or False? If false, explain why.
A - False. "Leaf" and "node" are reversed in the above statement. If you write a handler for a "leaf" class (a class with no subclasses), you've written a specialized handler: it will only handle exceptions of that specific type. If you write a handler for a "node" class (a class with subclasses), you've written a general handler: it will handle any exception whose type is the node class or any of its subclasses."
Q - Java's finally block provides a mechanism that allows your method to clean up after itself regardless of what happens within the try block. True or False? If false, explain why.
A - True.
Q - Explain how you would specify that a method throws one or more exceptions.
A - To specify that a method throws one or more exceptions, you add a throws clause to the method signature for the method. The throws clause is composed of the throws keyword followed by a comma-separated list of all the exceptions thrown by that method.
Q - Provide a code fragment that illustrates how you would specify that a method throws more than one exception.
A - See code fragment below.
void myMethod() throws InterruptedException, MyException,
HerException, UrException
{
//method body
}
Q - What type of argument is required by the throw statement?
A - The throw statement requires a single argument, which must be an object derived either directly or indirectly
from the class Throwable.
Q - Some exception objects are automatically thrown by the system. It is also possible for you to define your own exception classes, and to cause objects of those classes to be thrown whenever an exception occurs. True or False? If false, explain why.
A - True.
=======
Threads
Q - What is the definition of multi-threaded programming according to Patrick Naughton?
A - According to The Java Handbook, by Patrick Naughton,
"Multi-threaded programming is a conceptual paradigm for programming where you divide programs into two or more processes which can be run in parallel."
Q - Multithreading refers to two or more programs executing, "apparently" concurrently, under control of the operating system. The programs need have no relationship with each other, other than the fact that you want to start and run them all concurrently. True or False? If false, explain why.
A - False. That is a description of multiprocessing, not multithreading. Multithreading refers to two or more tasks executing, "apparently" concurrently, within a single program.
Q - According to current terminology, the term blocked means that the thread is waiting for something to happen and is not consuming computer resources. True or False? If false, explain why.
A - True.
Q - What are the two ways to create threaded programs in Java?
A - In Java, there are two ways to create threaded programs:
Implement the Runnable interface
Extend the Thread class
Q - What two steps are required to spawn a thread in Java?
A - The two steps necessary to spawn a thread in Java are:
instantiate an object of type Thread and invoke its run() method.
Q - How do you start a thread actually running in Java?
A - Invoke the start() method on object of the Thread class or of a subclass of the Thread class.
Q - It is always possible to extend the Thread class in your Java applications and applets. True or False? If false, explain why.
A - False. Sometimes it is not possible to extend the Thread class, because you must extend some other class and Java does not support multiple inheritance.
Q - Although multithreaded programming in Java is possible, it is also possible to write Java programs that do not involve threads: True or False? If false, explain why.
A - False. The main method itself runs in a thread which is started by the interpreter.
Q - What is the name of the method that can be used to determine if a thread is alive?
A - The name of the method is isAlive().
Q - Once you start two or more threads running, unless you specify otherwise, they run synchronously and independently of one another: True or False? If false, explain why.
A - False. Once you start two or more threads running, unless you specify otherwise, they run asynchronously and independently of one another.
Q - The process of keeping one thread from corrupting the data while it is being processed by another thread is known as synchronization: True or False? If false, explain why.
A - True.
Q - Java allows you to specify the absolute priority of each thread: True or False? If false, explain why.
A - False. Java allows you to specify the priority of each thread relative to other threads but not on an absolute basis.
Q - Thread synchronization can be achieved using wait(), notify(), and notifyAll() which are methods of the Thread class: True or False? If false, explain why.
A - False. wait(), notify(), and notifyAll() are not methods of the Thread class, but rather are methods of the Object class.
Q - When you implement a threaded program, you will always override the _____________ method of the Thread class and build the functionality of your threaded program into that method. What is the name of the method?
A - The run() method.
Q - In a multithreaded program, you will start a thread running by invoking the __________ method on your Thread object which will in turn invoke the ___________ method. What are the names of the missing methods, and what are the required parameters for each method?
A - In a multithreaded program, you will start a thread running by invoking the start() method on your Thread object which will in turn invoke the run() method. Neither method takes any parameters.
Q - What do Campione and Walrath list as the four possible states of a thread?
A -
New Thread
Runnable
Not Runnable
Dead
Q - What methods can be invoked on a thread object which is in the state that Campione and Walrath refer to as a New Thread and what will happen if you invoke any other method on the thread?
A - When a thread is in this state, you can only start the thread or stop it. Calling any method other than start() or stop() will cause an IllegalThreadStateException.
Q - What, according to Campione and Walrath, will cause a thread to become Not Runnable?
A - a thread becomes Not Runnable when one of the following four events occurs:
Someone invokes its sleep() method.
Someone invokes its suspend() method.
The thread uses its wait() method to wait on a condition variable.
The thread is blocking on I/O.
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