Creating a Java Thread by implementing Runnable Interface

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This is an article on Creating a Java Thread by implementing Runnable Interface in Java.
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.

Code: Java
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:
Code:
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.
Code: Java
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:
Code:
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
Light Poster
29Jun2011,23:36   #2
TPrice's Avatar
Thank you for this thread. This sure seems simple enough. I have been trying to figure this out, and I can see from this thread that I was going about it in completely the wrong way. I think I got it now, thank goodness. I really appreciate all of the great information that I am finding in this forum.
Go4Expert Member
31Aug2011,15:24   #3
Tobiasgar's Avatar
Thanx for the script)) the code is admiring
Newbie Member
4Sep2011,16:32   #4
lyfzopen's Avatar
Hi dude , your code is good and simple but I HAVE A QUESTION , will there be any difference if i call the StringThread.run method directly ???

ie


Code: Java

Code:
class PrintString
{
    public static void main (String args [ ])
    {
        StringThread t = new StringThread ("Java",50);
       // new Thread(t). start ( );   instead of this i do :
        t.run();
    }
}   

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+" "); 
    }
}
If no , why are u doing so ??? please reply asap ! ..
Last edited by shabbir; 4Sep2011 at 20:42.. Reason: Code blocks
Newbie Member
12Feb2012,03:39   #5
prabhas4's Avatar
start() method ll call run method... if do so (instead of start() , calling run()) here would be any Error or Exception run() method will simply be executed in the same Thread and new Thread will not be created.

and also u can't call same start() twice ....
t. start ( );
t. start ( ); // IllegalThreadStateException............

Regards
FOSS