Creating a Java Thread by implementing Runnable Interface

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We can create a thread in Java by extending Thread Class. A better way to create a thread in Java is to implement Runnable interface. A thread can be created by extending Java Thread class also. Now the question arises why implementing Runnable interface is a better approach? Answer is, if the thread class you are creating is to be subclass of some other class, it can’t extend from the Thread class. This is because Java does not allow a class to inherit from more than one class. In such a case one can use Runnable interface to implement threads.

Let us see an example of creating a thread by implementing Runnable interface.

class PrintString 
{
    public static void main (String args [ ]) 
    {
        StringThread t = new StringThread ("Java",50);
        new Thread(t). start ( );
    }
}   

class StringThread implements Runnable 
{
    private String str;
    private int num;

    StringThread(String s, int n) 
    {
        str  = new String (s);
        num =n; 
    }
    
    public void run ( ) 
    {
        for (int i=1; i<=num; i++)
            System.out.print (str+" "); 
    }
}

The output is:

Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java
Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java
Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java
Java Java

If you have had created a thread class by extending Thread class, you could have directly called start() method as t.start (), where t is a thread object. This is because thread class created by extending Thread class is a subclass of Thread class, so it has all functionalities of a thread. While creating a thread implementing Runnable, a Thread object will have to be explicitly created which is what PrintString class is doing. It then passes StringThread object as a parameter to this thread and runs it. This causes the run ( ) method of StringThread class to get executed.

The same program can be rewritten, by implementing Runnable interface, and creating a Thread object inside StringThread class rather than in the PrintString class as below.

class PrintString 
{
    public static void main (String args [] ) 
    {
        new StringThread("Java",50 );
    }
}

class StringThread implements Runnable 
{
    private String str;
    private int num;

    StringThread(String s, int n) 
    {
        str=new String (s);
        num=n;
        Thread t=new Thread (this);
        t. start ( );
    }
    
    public void run () 
    {
        for (int i=1; i<=num; i++)
            System.out.print (str+" ");
    }
}

The output is:

Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java
Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java
Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java Java
Java Java