Thread Creation Concept Simplified in Java

Thread Creation Concept

Multithreading corresponds to multiple flows of control. It is a conceptual paradigm where we can divide a program into two or more processes which can run in parallel.Java allows us to have inbuilt processes where more than one process can be executed concurrently within a single program.Java threads are lightweight, meaning they can run in the same memory space. They can communicate themselves(one object of one thread class can call a method of another thread without any overhead of operating system). Threads can execute independently and simultaneously. So java handles multiple tasks simultaneously using multi threaded way.This feature enables to finish tasks before even beginning another.
Java coding is mostly sequential single flow execution(single threaded programming). A thread is a single flow of control having a beginning a body and end.A thread executes commands sequentially.
The ability to support multi threads is referred as concurrency.Since the threads in java are sub program of the main application and share the same memory space.Even though threads appears to run parallel , it is  actually sequential but very fast switching of control between threads.

Once initiated by main program, Thread A, Thread B, and Thread C run independently , concurrently and share the system resources.

Java provides two to way to create a thread.

• Sub classing the Thread class. Here we define a class and extends to Thread class, then override the run() method.
• Implementing the runnable interface. Here we implement the runnable interface and define the run() method.

Sub Classing to the Thread Class:

Example:

class MyThread extends Thread{
//MyThread is a new Thread which extends to Thread Class. This subclass will contain a method called run().run() 
//contains the actual task what thread should perform.
    private String whichThread;
    private int delay;
    MyThread(String myString,int timeOut)
    {
        whichThread=myString;
        delay=timeOut;
    }
//Main method to construct thread and the only method in which thread's behavior can be written. 
//Here we need to override the run() method.
    public void run()
    {
        try{
            sleep(delay);
        }
    catch(InterruptedException e)
    {
        System.out.println("A thread is interrupted");
    }
    finally{
        System.out.println("Hello "+ whichThread+" slept "+delay);
    }    
    }
}
public class TestThread{
    public static void main(String []args){
      MyThread t1,t2,t3;
//creating the object of the class we created earlier. The state of the threads will be newborn
      t1=new MyThread("Thread 1",10);
      t2=new MyThread("Thread 2",20);
      t3=new MyThread("Thread 3",30);
//start() method to start the threads that will invoke the run() method . This will cause the threads to go to runnable state.Java run time will schedule the threads to run by invoking it's run method.
      t1.start();
      t2.start();
      t3.start();
      try{
          t1.join();
          t2.join();
          t3.join();
      }
      catch(InterruptedException e)
      {
          
      }
     }
}
 

the output of the code:
$javac TestThread.java
$java -Xmx128M -Xms16M TestThread
Hello Thread 1 slept 10
Hello Thread 2 slept 20
Hello Thread 3 slept 30
Another Example:
Once we initiate a thread with new and started them, they are running concurrently on their own. The output are not specially sequential. They just do not follow any specific order. Once the thread has been started , we can not decide in which order they may execute the statements.

class A extends Thread{
public void run()
{
for(int i=1;i<5;i++)
{
System.out.println("From Thread A "+i);
}
System.out.println("Exit from Thread A ");
}
}
class B extends Thread{
public void run()
{
for(int j=1;j<5;j++)
{
System.out.println("From Thread B "+j);
}
System.out.println("Exit from Thread B ");
}
}
class C extends Thread{
public void run()
{
for(int k=1;k<5;k++)
{
System.out.println("From Thread C "+k);
}
System.out.println("Exit from Thread C ");
}
}
public class ThraedTest{
public static void main(String args[])
{
new A().start();
new B().start();
new C().start();
}
}
 

the output of the code:

Using Runnable Interface:

1. Declare the class as implementing the Runnable interface.
2. Implement the run() method.
3. Create a thread by defining an object that is instantiated from this “runnable” class as the target of the thread.
4. Call the thread’s start() method to run the thread.

Example:

class Pen implements Runnable{
    private int price;
    private String name;
    Pen(int price,String name)
    {
        this.price=price;
        this.name=name;
    }
    public void run()
    {
        try{
            Thread.sleep(price*1000);
            System.out.println("A pen "+name+" price is "+price);
        }
    catch(InterruptedException e)
    {
        System.out.println("Pen thread is interrupted");
    }
       
    }
}
class Pencil implements Runnable{
    private int price;
    private String name;
    Pencil(int price,String name)
    {
        this.price=price;
        this.name=name;
    }
    public void run()
    {
        try{
            Thread.sleep(price*2000);
            System.out.println("A Pencil "+name+" price is "+price);
        }
    catch(InterruptedException e)
    {
        System.out.println("Pencil thread is interrupted");
    }
       
    }
}
public class TestThreadRunnable{
    public static void main(String []args){
      Pen myPen=new Pen(2,"Lammy");
      Thread t1=new Thread(myPen);
      t1.start();
      Pencil myPencil=new Pencil(3,"Nataraj");
      Thread t2=new Thread(myPencil);
      t2.start();
      
      try{
          t1.join();
          t2.join();
          
      }
      catch(InterruptedException e)
      {
          
      }
     }
}
 

the output of the code:
$javac TestThreadRunnable.java
$java -Xmx128M -Xms16M TestThreadRunnable
A pen Lammy price is 2
A Pencil Nataraj price is 3

When to use subclass of thread and when to use runnable?

The Thread class provides a static method called currentThread() that provides a reference to thread object that controls the current thread of execution.