Java Interview Questions

Why is the main method static?

So that it can be invoked without creating an instance of that class

What is the difference between class variable, member variable and automatic(local) variable ?

class variable is a static variable and does not belong to instance of class but rather shared across all the instances member variable belongs to

a particular instance of class and can be called from any method of the class automatic or local variable is created on entry to a method and has only method

scope

When are static and non static variables of the class initialized ?

The static variables are initialized when the class is loadedNon static variables are initialized just before the constructor is called.

When are automatic variable initialized?

Automatic variable have to be initialized explicitly

What is a modulo operator % ?

This operator gives the value which is related to the remainder of a divisione.g x=7%4 gives remainder 3 as an answer

What is garbage collection ?

The runtime system keeps track of the memory that is allocated and is able to determine whether that memory is still useable. This work is usually

done in background by a low-priority thread that is referred to as garbage collector. When the gc finds memory that is no longer accessible from any live thread it

takes steps to release it back to the heap for reuse

Does System.gc and Runtime.gc() guarantee garbage collection ?

No

What are different types of operators in java ?

Uniary ++, –, +, -, |, ~, ()
Arithmetic *, /, %,+, –
Shift <<, >>, >>>
Comparison =, instanceof, = =,!=Bitwise &, ^, |Short Circuit &&, ||Ternary ?:Assignment =

How does bitwise (~) operator work ?

It converts all the 1 bits in a binary value to 0s and all the 0 bits to 1s, e.g 11110000 coverts to 00001111
Can shift operators be applied to float types.
No, shift operators can be applied only to integer or long types

What happens to the bits that fall off after shifting?

They are discarded

What values of the bits are shifted in after the shift ?

In case of signed left shift >> the new bits are set to zero But in case of signed right shift it takes the value of most significant bit

before the shift, that is if the most significant bit before shift is 0 it will introduce 0, else if it is 1, it will introduce 1

What are access modifiers ?

These public, protected and private, these can be applied to class, variables, constructors and methods. But if you don’t specify an access

modifier then it is considered as Friendly

Can protected or friendly features be accessed from different packages ?

No when features are friendly or protected they can be accessed from all the classes in that package but not from classes in another package

How can you access protected features from another package ?

You can access protected features from other classes by subclassing the that class in another package, but this cannot be done for friendly

features

What are the rules for overriding ?

Private method can be overridden by private, friendly, protected or public methods
Friendly method can be overridden by friendly, protected or public methods
Protected method can be overridden by protected or public methods
Public method can be overridden by public method

Explain modifier ‘final’?

Final can be applied to classes, methods and variables and the features cannot be changed. Final class cannot be subclassed, methods cannot be

overridden

Can you change the reference of the final object ?

No the reference cannot be change, but the data in that object can be changed

Can abstract modifier be applied to a variable ?

No it is applied only to class and methods

Can abstract class be instantiated ?

No abstract class cannot be instantiated i.e you cannot create a new object of this class

When does the compiler insist that the class must be abstract ?

If one or more methods of the class are abstract.
If class inherits one or more abstract methods from the parent abstract class and no implementation is provided for that method.
If class implements an interface and provides no implementation for those methods

How is abstract class different from final class?

Abstract class must be subclassed and final class cannot be subclassed

Where can static modifiers be used ?

They can be applied to variables, methods and even a block of code, static methods and variables are not associated with any instance of

class

When are the static variables loaded into the memory ?

During the class load time

When are the non static variables loaded into the memory ?

They are loaded just before the constructor is called

How can you reference static variables ?

Via reference to any instance of the class
Computer comp = new Computer ();
comp.harddisk where hardisk is a static variable
comp.compute() where compute is a method
Via the class name
Computer.harddisk
Computer.compute()
Can static method use non static features of there class
No they are not allowed to use non static features of the class, they can only call static methods and can use static data