Java Interview Questions


Question: Is Empty .java file a valid source file?
Question: Can a .java file contain more than one java classes? 
Question: Is String a primitive data type in Java?
Question: Is main a keyword in Java?  
Question: Is next a keyword in Java?
Question: Is delete a keyword in Java?
Question: Is exit a keyword in Java?
Question: What happens if you dont initialize an instance variable of any of the primitive types in Java?
Question: What will be the initial value of an object reference which is defined as an instance variable?
Question: What are the different scopes for Java variables?
Question: What is the default value of the local variables?
Question: How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
Question: Can a public class MyClass be defined in a source file named YourClass.java?
Question: Can main method be declared final?
Question: What will be the output of the following statement?
System.out.println ("1" + 3);
Question: What will be the default values of all the elements of an array defined as an instance variable?
   
   

Q:

Is Empty .java file a valid source file?

A: Yes, an empty .java file is a perfectly valid source file.
 
[Received from Sandesh Sadhale]

Q:

Can a .java file contain more than one java classes?

A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.
 
[ Received from Sandesh Sadhale]

Q:

Is String a primitive data type in Java?

A: No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.
 
[ Received from Sandesh Sadhale]

Q:

Is main a keyword in Java?

A: No, main is not a keyword in Java.
 
[ Received from Sandesh Sadhale]

Q:

Is next a keyword in Java?

A: No, next is not a keyword.
 
[ Received from Sandesh Sadhale]

Q:

Is delete a keyword in Java?

A: No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.
 
[ Received from Sandesh Sadhale]

Q:

Is exit a keyword in Java?

A: No. To exit a program explicitly you use exit method in System object.
 
[ Received from Sandesh Sadhale]

Q:

What happens if you dont initialize an instance variable of any of the primitive types in Java?

A: Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.
 
[ Received from Sandesh Sadhale]

Q:

What will be the initial value of an object reference which is defined as an instance variable?

A: The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.
 
[ Received from Sandesh Sadhale]

Q:

What are the different scopes for Java variables?

A: The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.
 
[ Received from Sandesh Sadhale]

Q:

What is the default value of the local variables?

A: The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..
 
[ Received from Sandesh Sadhale]

Q:

How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();

A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.
 
[ Received from Sandesh Sadhale]

Q:

Can a public class MyClass be defined in a source file named YourClass.java?

A: No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.
 
[ Received from Sandesh Sadhale]

Q:

Can main method be declared final?

A: Yes, the main method can be declared final, in addition to being public static.
 
[ Received fromSandesh Sadhale]

Q:

What will be the output of the following statement?
System.out.println ("1" + 3);

A: It will print 13.
 
[ Received from Sandesh Sadhale]

Q:

What will be the default values of all the elements of an array defined as an instance variable?

A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.
 
[ Received from Sandesh Sadhale]


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