Threads - The Thread Class

• the easiest way to create a thread is by subclassing java.lang.Thread

      class BasicThread extends Thread {
          char c;
          
          BasicThread(char c) {
              this.c = c;
          }        
      }
  

• to actually start the thread running you must invoke its start() method

    BasicThread bt  = new BasicThread('!');
    BasicThread bt1 = new BasicThread('*');
    bt.start();   
    bt1.start();

• the start() method allocates system resources required for a thread, schedules the thread to run and invokes the run() method
• the above code will execute but nothing will happen
• if you want your thread to do something you need to override the run() method
• the run() method is actually defined in the Runnable interface which the class Thread implements

    public void run() {
        for(int i=0; i<100; i++) {
            System.out.print(c);
        }
    }

• if the above code is added and the threads started you see something like:

    
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*
!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!****
***********************************************    

• the output is intermingled because the threads are running concurrently and are interleaved

• you can alter thread processing with program control mechanisms
• one way is to use the sleep() method which is defined in the Thread class
• the sleep() method stops the execution of a thread for a given number of milliseconds
• it also throws an InterruptedException so you need to wrap it in a try-catch block
• adding sleep() to the run method can alter the threads execution

New run() method:

    public void run() {
        for(int i=0; i<100; i++) {
            System.out.print(c);
            
            try{ 
                sleep((int)(Math.random() * 10));
            } catch( InterruptedException e ) {
                System.out.println("Interrupted");
            }
        }
    }    

Example output:

*!*!!*!*!**!!!*!!*!***!*!!***!*!*!*!*!!!!***!!*!*
!!*!**!*!!*!**!*!**!!*!**!!*!!*!*!**!*!***!!!!*!*
!!*!**!*!*!*!*!*!*!*!!*!*!*!!!*!*!!!*!*!!*!*!*!*!
!*!**!*!*!**!**!!*!***!!!****!*!!****!*!**!!**!!!
****

• you can give a thread a name by creating it with a String argument

      Thread t = new Thread("Thread1");
  

• if a thread is created without a name, one is automatically generated in the form Thread-n, where n is an integer
• the following is output from TwoThreadsTest which creates two SimpleThread's and displays their automatically generated names using the getName() method of the Thread class.

    0 Thread-0
    0 Thread-1
    1 Thread-0
    1 Thread-1
    2 Thread-0
    2 Thread-1
    3 Thread-0
    3 Thread-1
    4 Thread-1
    4 Thread-0
    DONE! Thread-0
    DONE! Thread-1

ThreadGroup

• you can group threads using the ThreadGroup class
• this allows multiple threads to be handled as one unit ie for setting priority, destroying, etc
• threads in the same group can access information about other threads in the group but not about the parent thread or threads in other groups
• a ThreadGroup can have both daemon and nondaemon threads

Example Code

• BasicThread.java
• Bouncing Ball Applet: UpDown.java
• PrimeNumbers Applet: Ex1.java
• Java Is Hot: Thread1.java
• Traffic Simulation: Traffic.java and SetOfLights.java