Creating a thread in Java

What is a Process

A process is an instance of a computer program that is being executed. A process can be executed by a processor or a set of processors. A process has a virtual address space, executable code, open handles to system objects, a security context, a unique process identifier , environment variables, a priority class, minimum and maximum working set sizes, and at least one thread of execution.

What is a Thread

A thread is the entity within a process that can be scheduled for execution. It is a basic unit of CPU utilization , consisting of a program counter, a stack, and a set of registers, ( and a thread ID. ). All threads of a process share its virtual address space and system resources. Threads are different from processes. A thread is a context of execution, while a process is a bunch of resources associated with a computation. A process can have one or many threads. The implementation of threads and processes differs between operating systems , but in most cases a thread is a component of a process.

Threads in Java

A thread is an independent path of execution within a program. That means, it is a sequence of instructions within a program that can be executed independently of other code. Java Threads are a relatively lightweight way to implement multiple paths of execution inside of an application. A thread is similar to the sequential programs, that is, a single thread has a beginning, an end, a sequence, and at any given time during the runtime of the thread there is a single point of execution. However, a thread itself is not a program. It cannot run on its own, but runs within a program. Many threads can run concurrently within a program. All Java programs have at least one thread, known as the main thread , which is created by the JVM (Java Virtual Machine) at the program’s start, when the main() method is invoked with the main thread. It is an object that has its own registers, stack and code segment that can run parallel with other threads in a process (a process is a collection of threads). Every thread in Java is created and controlled by the java.lang.Thread class. A Java program can have many threads, and these threads can run concurrently, either asynchronously or synchronously.

How to create thread in Java

There are two ways to create a thread in Java:

  1. Extending Thread class
  2. Implementing Runnable interface

Extending Thread class in Java

You can extend java.lang.Thread class to create your own java thread class and override run() method. Then you can create it's object and call start() method to execute our custom java thread class run method. Example
public class TestClass extends Thread{ public void run(){ System.out.println("Print this....."); } public static void main(String[] args) { TestClass tc = new TestClass(); tc.start(); } }

Implementing Runnable interface in Java

You can implement java.lang.Runnable interface and provide implementation in public void run() method. To use this class as Thread, you need to create a Thread object by passing object of this runnable class and then call start() method to execute the run() method in a separate thread. Example
public class TestClass implements Runnable{ public void run(){ System.out.println("Print this....."); } public static void main(String[] args) { TestClass tc = new TestClass(); Thread tr = new Thread(tc); tr.start(); } }
It is important to note that, when you extend Thread class , you can't extend any other class which you require because Java does not allow multiple inheritance . When you implement Runnable, you can save a space for your class to extend any other class in future or now.