Core Java Interview Questions Part-1

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

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 ?

List is an ordered collection of objects.

6 Q What is a Vector

Vector is a grow able array of objects.

7 Q What is the difference between yield() and sleep()?

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 ?

They are wrappers to primitive data types. They allow us to access primitives as objects.

9 Q Can we call finalize() method ?

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 ?

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?

The thread is in ready state.

12 Q Can we declare an anonymous class as both extending a class and implementing an interface?

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

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.