Core Java Interview Questions

Here you can find out a list of interview questions for core java. These questions are often asked by the interviewer for core java job interview. We put our  maximum effort to make this answers error free. But still there might be some errors. If you feel any answer given for any  question is wrong or if you have a more perfect answer to give, please, please inform us by clicking on report bug button provided below.

In this section we are offering interview questions for core java only. if you need interview questions for any other java related technologies , please check the relevant sections.

1 Q

Why threads block or enters to waiting state on I/O?

A Threads enters to waiting state or block on I/O because other threads can execute while the I/O operations are performed.
2 Q What are transient variables in java?
A

Transient variables are variable that cannot be serialized.

3 Q How Observer and Observable are used?
  A

Subclass of Observable class maintain a list of observers. Whenever an Observable object is updated, it invokes the update() method of each of its observers to notify the observers that it has a changed state. An observer is  any object that implements the interface Observer.  

4 Q What is synchronization
  A

Synchronization is the ability to control the access of multiple threads to shared resources. Synchronization stops multithreading. With synchronization , at  a time only one thread will be able to access a shared resource.

5 Q What is List interface ?
A List is an ordered collection of objects.
6 Q What is a Vector
A Vector is a grow able array of objects.
7 Q What is the difference between yield() and sleep()?
  A

When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not ready state.

8 Q What are Wrapper Classes ?
A They are wrappers to primitive data types. They allow us to access primitives as objects.
9 Q Can we call finalize() method ?
A Yes.  Nobody will stop us to call any method , if it is accessible in our class. But a garbage collector cannot call an object's finalize method if that object is reachable.
10 Q

What is the difference between  time slicing and  preemptive scheduling ?

A

In preemptive scheduling, highest priority task continues execution till it enters a not running state or a higher priority task comes into existence. In time slicing, the task continues its execution for a predefined period of time and reenters the pool of ready tasks. 

11 Q What is the initial state of a thread when it is created and started?
A The thread is in ready state.
12 Q Can we declare an anonymous class as both extending a class and implementing an interface?
A

No. An anonymous class can extend a class or implement an interface, but it cannot be declared to do both

13 Q What are the differences between boolean & operator and  & operator
A

When an expression containing the & operator is evaluated, both operands are evaluated. And the & operator is applied to the operand. When an expression containing && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then only the second operand is evaluated otherwise the second part will not get executed.  && is also called short cut and.

14 Q What is the use of the finally block?
A

Finally is the block of code that executes always. The code in finally block will execute even if an exception is occurred. finally will not execute when the user calls System.exit().

15 Q What is an abstract method ?
A

An abstract method is a method that don't have a body. It is declared with modifier abstract.

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