Java 7 Pocket Guide, 2nd Edition

Chapter 14. Concurrency

Threads in Java allow the use of multiple processors or multiple cores in one processor more efficiently. On a single processor, threads provide for concurrent operations such as overlapping I/O with processing.

Java supports multithreaded programming features with the Thread class and the Runnable interface.

Creating Threads

Threads can be created two ways, either by extending java.lang.Thread or by implementing java.lang.Runnable.

Extending the Thread Class

Extending the Thread class and overriding the run() method can create a threadable class. This is an easy way to start a thread:

class Comet extends Thread {
  public void run() {
    System.out.println("Orbiting");
    orbit();
  }
}

Comet halley = new Comet();

Remember that only one superclass can be extended, so a class that extends Thread cannot extend any other superclass.

Implementing the Runnable Interface

Implementing the Runnable interface and defining its run() method can also create a threadable class. Creating a new Thread object and passing it an instance of the runnable class creates the thread:

class Asteroid implements Runnable {
  public void run() {
    System.out.println("Orbiting");
    orbit();
  }
}

Asteroid maja = new Asteroid();
Thread majaThread = new Thread(maja);

A single runnable instance can be passed to multiple thread objects. Each thread performs the same task:

Asteroid pallas = new Asteroid();
Thread pallasThread1 = new Thread(pallas);
Thread pallasThread2 = new Thread(pallas);

Thread States

Enumeration ...