Java - Threads - Interview Questions

What are the different ways to define threads in Java programming language?

FAQKey Concept

There are two ways to define and run threads in Java programming language.

First way to define threads is to extend from java.lang.Thread class, and override its run() method. You instantiate and start this thread by creating a new instance of your class and calling the start() method

Second way to define threads is to implement java.lang.Runnable interface, and implement its run method. You can instantiate and start this thread by creating a creating a new instance of your class that implements Runnable interface, and passing this instance of Runnable to a new Instance of Thread.

Defining a thread by implementing the Runnable is the preferred way, since if you make your class extend from the Thread class, you will not be able to extend from any other class.

/** Extending from java.lang.Thread */
//Define
Class MyThread extends Thread { 
	public void run() {...}
}
//Instantiate
MyThread t = new MyThread();
//Start
t.start();

/** Implementing java.lang.Runnable interface */
//Define 
Class MyRunnable implements Runnable {
	public void run() {...}
}
//Instantiate
MyRunnable r = new MyRunnable();
Thread t = new Thread(r);
//Start
t.start()

Describe the life-cycle of threads. What are the different states of threads? How does the thread transition from one state to the other?

FAQKey Concept

A thread can be in one of five states

New - A thread is in 'New' state when it has been instantiated, but the start() method is not yet called on the thread instance. In this state the thread is a Thread object, but it is not yet a thread of execution.

Runnable - A thread is in Runnable state when the start() method has been called on the thread instance but it is not yet picked up by the thread scheduler for execution.

Running - A thread is in Running state when it is currently executing . The thread scheduler picks up the thread from the thread pool and executes it.

Waiting, Blocked, Sleeping - In this state the thread is not eligible for execution and is not in the running pool. A thread may be ineligible for execution for a variety of reasons. The thread may be blocked waiting for an IO resource or on a lock. The thread may be sleeping since the code tells it to sleep for a specific period of time. Or the thread may be waiting on another thread to send it a notification.

Dead - A thread is in dead state when the execution of its run() method is complete. Once a thread is dead it cannot be run again, hence a thread cannot transition from dead state to any other state.

How do you pause a thread from executing for a certain period of time?

FAQKey Concept

You can pause the execution of a thread for a certain period of time by calling its sleep() method and passing the time that the thread has to stop executing.

Though you specify the time period that the thread should sleep for, there is no guarantee that the thread will pause executing exactly for that period of time.

The thread may start executing sooner than the specified time - if it gets an interrupt from another thread. Or the thread may start executing later than the specified time - After the specified sleep period of time has elapsed, the thread is put into the runnable pool of threads from where it will be picked up by the scheduler. The scheduler may pick up this thread for execution immediately or if there are higher priority threads then it will be executed later.

What is the function of join() method in Thread class in Java programming language?

FAQKey Concept

The join() method allows one thread to wait for the completion of another thread. For example - If t is a Thread instance, then calling t.join() in the current thread, say the main thread, will cause the main thread to wait until the t thread has completed its execution.