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A thread is a single sequential flow of control within a program.Some texts use the name lightweight process instead of thread. A thread is similar to a real process in that a thread and a running program are both a single sequential flow of control. However, a thread is considered lightweight because it runs within the context of a full-blown program and takes advantage of the resources allocated for that program and the program’s environment.


class SimpleThread extends Thread {
public SimpleThread(String str) {
public void run() {
for (int i = 0; i < 10; i++) { System.out.println(i + " " + getName()); try { sleep((int)(Math.random() * 1000)); } catch (InterruptedException e) {} } System.out.println("DONE! " + getName()); } }

Commonly asked interview questions on Constructors

1) What is Thread in Java?

Answer :The thread is an independent path of execution. It's way to take advantage of multiple CPU available in a machine.Java provides excellent support for multithreading at the language level, and it's also one of the strong selling points.

2) What is the difference between Thread and Process in Java?

Answer :The thread is a subset of Process, in other words, one process can contain multiple threads. Two process runs on different memory space, but all threads share same memory space. Don't confuse this with stack memory, which is different for the different thread and used to store local data to that thread.

3) How do you implement Thread in Java?

Answer :At the language level, there are two ways to implement Thread in Java. An instance of java.lang.Thread represent a thread but it needs a task to execute, which is an instance of interface java.lang.Runnable. Since Thread class itself implement Runnable, you can override run() method either by extending Thread class or just implementing Runnable interface

4) When to use Runnable vs Thread in Java?

Answer :Main difference is that when program calls start() method a new Thread is created and code inside run() method is executed in new Thread while if you call run() method directly no new Thread is created and code inside run() will execute on current Thread. Most of the time calling run() is bug or programming mistake because caller has intention of calling start() to create new thread and this error can be detect by many static code coverage tools like findbugs. If you want to perform time consuming task than always call start() method otherwise your main thread will stuck while performing time consuming task if you call run() method directly. Another difference between start vs run in Java thread is that you can not call start() method twice on thread object. once started, second call of start() will throw IllegalStateException in Java while you can call run() method twice

5) What is Java Memory model?

Answer :Java Memory model is set of rules and guidelines which allows Java programs to behave deterministically across multiple memory architecture, CPU, and operating system. It's particularly important in case of multi-threading. Java Memory Model provides some guarantee on which changes made by one thread should be visible to others, one of them is happens-before relationship.

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Reference Books

1. Head First Java

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