Most Important Core Java Interview Questions Answers

 1.) What is serialization?

Quite simply, object serialization provides a program the ability to read or write a whole object to and from a raw byte stream. It allows Java objects and primitives to be encoded into a byte stream suitable for streaming to some type of network or to a file system, or more generally, to a transmission medium or storage facility. A serializable object must implement the Serializable interface. We use Object Output Stream to write this object to a stream and Object Input Stream to read it from the stream.

2.) Is synchronized a modifier? identifier?? what is it??

It's a modifier. Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

3.) What is a singleton class? where is it used?

Singleton is a design pattern meant to provide one and only one instance of an object. Other objects can get a reference to this instance through a static method (class constructor is kept private). Why do we need one? Sometimes it is necessary, and often sufficient, to create a single instance of a given class. This has advantages in memory management, and for Java, in garbage collection. Moreover, restricting the number of instances may be necessary or desirable for technological or business reasons--for example, we may only want a single instance of a pool of database connections.

4.) Why java does not have multiple inheritance?

The Java design team strove to make Java:

• Simple, object-oriented, and familiar

• Robust and secure

• Architecture neutral and portable

• High performance

• Interpreted, threaded, and dynamic

The reasons for omitting multiple inheritance from the Java language mostly stem from the "simple, object-oriented, and familiar" goal. As a simple language, Java's creators wanted a language that most developers could grasp without extensive training. To that end, they worked to make the language as similar to C++ as possible (familiar) without carrying over C++'s unnecessary complexity (simple).

In the designers' opinion, multiple inheritance causes more problems and confusion than it solves. So they cut multiple inheritance from the language (just as they cut operator overloading). The designers' extensive C++ experience taught them that multiple inheritance just wasn't worth the headache.

5.)Why java is not 100% oops?

Many people say this because Java uses primitive types such as int, char, double. But then all the rest are objects. Confusing question.

6.) What is a resource bundle?

In its simplest form, a resource bundle is represented by a text file containing keys and the text value for each key.

7.) What is a transient variable?

The transient variables can't be serialized. For example, if a variable is declared as transient in a Serializable class and the class is written to an Object Stream, the value of the variable can't be written to the stream instead when the class is retrieved from the Object Stream the value of the variable becomes null.

Top Most Multithreading Interview Questions and Answers

1.) What is multithreading?

Multithreading is a process of executing multiple threads simultaneously. Its main advantage is:

Threads share the same address space.

Thread is lightweight.

The cost of communication between process is low.

2) What is the thread?

A thread is a lightweight subprocess. It is a separate path of execution. It is called a separate path of execution because each thread runs in a separate stack frame.

3)What is the difference between preemptive scheduling and time slicing?

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

4) What does the join() method?

The join() method waits for a thread to die. In other words, it causes the currently running threads to stop executing until the thread it joins with completes its task.

5) What is the difference between the wait() and sleep() methods?

                            wait()                                                                       sleep()

1) The wait() method is defined in Object class.     The sleep() method is defined in the Thread class.

2) wait() method releases the lock.                             The sleep() method doesn't release the lock.

6) Is it possible to start a thread twice?

No, there is no possibility to start a thread twice. If we do, it throws an exception.

7) Can we call the run() method instead of start()?

Yes, but it will not work as a thread rather it will work as a normal object so there will not be context-switching between the threads.

8) What about the daemon threads?

The daemon threads are basically the low-priority threads that provide the background support to the user threads. It provides services to the user threads.

9)Can we make the user thread a daemon thread if the thread is started?

No, if you do so, it will throw IllegalThreadStateException

10)What is a shutdown hook?

The shutdown hook is basically a thread i.e. invoked implicitly before JVM shuts down. So we can use it to perform clean-up resources.

11)When should we interrupt a thread?

We should interrupt a thread if we want to break out of sleep or wait for the state of a thread.

12) What is synchronization?

Synchronization is the capability of control the access of multiple threads to any shared resource. It is used:

To prevent thread interference.

To prevent consistency problems.

13) What is the purpose of the Synchronized block?

The synchronized block is used to lock an object for any shared resource.

The scope of the synchronized block is smaller than the method.

14)Can Java objects be locked down for exclusive use by a given thread?

Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it.

15) What is static synchronization?

If you make any static method as synchronized, the lock will be on the class not on the object. more details...

16)What is the difference between notify() and notifyAll()?

The notify() is used to unblock one waiting thread whereas the notifyAll() method is used to unblock all the threads in the waiting state.

17)What is deadlock?

Deadlock is a situation when two threads are waiting on each other to release a resource. Each thread waiting for a resource which is held by the other waiting thread.