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Thinking in Java
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Coding variations

In the simple examples that you’ve seen so far, the thread objects are all inherited from Thread. This makes sense because the objects are clearly only being created as threads and have no other behavior. However, your class may already be inheriting from another class, in which case you can’t also inherit from Thread (Java doesn’t support multiple inheritance). In this case, you can use the alternative approach of implementing the Runnable interface. Runnable specifies only that there be a run( ) method implemented, and Thread also implements Runnable.

This example demonstrates the basics:

//: c13:RunnableThread.java
// SimpleThread using the Runnable interface.

public class RunnableThread implements Runnable {
  private int countDown = 5;
  public String toString() {
    return "#" + Thread.currentThread().getName() +
      ": " + countDown;
  }
  public void run() {
    while(true) {
      System.out.println(this);
      if(--countDown == 0) return;
    }
  }
  public static void main(String[] args) {
    for(int i = 1; i <= 5; i++)
      new Thread(new RunnableThread(), "" + i).start();
    // Output is like SimpleThread.java
  }
} ///:~


The only thing required by a Runnable class is a run( ) method, but if you want to do anything else to the Thread object (such as getName( ) in toString( )) you must explicitly get a reference to it by calling Thread.currentThread( ). This particular Thread constructor takes a Runnable and a name for the thread.

When something has a Runnable interface, it simply means that it has a run( ) method, but there’s nothing special about that—it doesn’t produce any innate threading abilities, like those of a class inherited from Thread. So to produce a thread from a Runnable object, you must create a separate Thread object as shown in this example, handing the Runnable object to the special Thread constructor. You can then call start( ) for that thread, which performs the usual initialization and then calls run( ).

The convenient aspect about the Runnable interface is that everything belongs to the same class; that is, Runnable allows a mixin in combination with a base class and other interfaces. If you need to access something, you simply do it without going through a separate object. However, inner classes have this same easy access to all the parts of an outer class, so member access is not a compelling reason to use Runnable as a mixin rather than an inner subclass of Thread.

When you use Runnable, you’re generally saying that you want to create a process in a piece of code—implemented in the run( ) method—rather than an object representing that process. This is a matter of some debate, depending on whether you feel that it makes more sense to represent a thread as an object or as a completely different entity, a process.[68] If you choose to think of it as a process, then you are freed from the object-oriented imperative that “everything is an object.” This also means that there’s no reason to make your whole class Runnable if you only want to start a process to drive some part of your program. Because of this, it often makes more sense to hide your threading code inside your class by using an inner class, as shown here:

//: c13:ThreadVariations.java
// Creating threads with inner classes.
import com.bruceeckel.simpletest.*;

// Using a named inner class:
class InnerThread1 {
  private int countDown = 5;
  private Inner inner;
  private class Inner extends Thread {
    Inner(String name) {
      super(name);
      start();
    }
    public void run() {
      while(true) {
        System.out.println(this);
        if(--countDown == 0) return;
        try {
          sleep(10);
        } catch (InterruptedException e) {
          throw new RuntimeException(e);
        }
      }
    }
    public String toString() {
      return getName() + ": " + countDown;
    }
  }
  public InnerThread1(String name) {
    inner = new Inner(name);
  }
}

// Using an anonymous inner class:
class InnerThread2 {
  private int countDown = 5;
  private Thread t;
  public InnerThread2(String name) {
    t = new Thread(name) {
      public void run() {
        while(true) {
          System.out.println(this);
          if(--countDown == 0) return;
          try {
            sleep(10);
          } catch (InterruptedException e) {
            throw new RuntimeException(e);
          }
        }
      }
      public String toString() {
        return getName() + ": " + countDown;
      }
    };
    t.start();
  }
}

// Using a named Runnable implementation:
class InnerRunnable1 {
  private int countDown = 5;
  private Inner inner;
  private class Inner implements Runnable {
    Thread t;
    Inner(String name) {
      t = new Thread(this, name);
      t.start();
    }
    public void run() {
      while(true) {
        System.out.println(this);
        if(--countDown == 0) return;
        try {
          Thread.sleep(10);
        } catch (InterruptedException e) {
          throw new RuntimeException(e);
        }
      }
    }
    public String toString() {
      return t.getName() + ": " + countDown;
    }
  }
  public InnerRunnable1(String name) {
    inner = new Inner(name);
  }
}

// Using an anonymous Runnable implementation:
class InnerRunnable2 {
  private int countDown = 5;
  private Thread t;
  public InnerRunnable2(String name) {
    t = new Thread(new Runnable() {
      public void run() {
        while(true) {
          System.out.println(this);
          if(--countDown == 0) return;
          try {
            Thread.sleep(10);
          } catch (InterruptedException e) {
            throw new RuntimeException(e);
          }
        }
      }
      public String toString() {
        return Thread.currentThread().getName() +
          ": " + countDown;
      }
    }, name);
    t.start();
  }
}

// A separate method to run some code as a thread:
class ThreadMethod {
  private int countDown = 5;
  private Thread t;
  private String name;
  public ThreadMethod(String name) { this.name = name; }
  public void runThread() {
    if(t == null) {
      t = new Thread(name) {
        public void run() {
          while(true) {
            System.out.println(this);
            if(--countDown == 0) return;
            try {
              sleep(10);
            } catch (InterruptedException e) {
              throw new RuntimeException(e);
            }
          }
        }
        public String toString() {
          return getName() + ": " + countDown;
        }
      };
      t.start();
    }
  }
}

public class ThreadVariations {
  private static Test monitor = new Test();
  public static void main(String[] args) {
    new InnerThread1("InnerThread1");
    new InnerThread2("InnerThread2");
    new InnerRunnable1("InnerRunnable1");
    new InnerRunnable2("InnerRunnable2");
    new ThreadMethod("ThreadMethod").runThread();
    monitor.expect(new String[] {
      "InnerThread1: 5",
      "InnerThread2: 5",
      "InnerThread2: 4",
      "InnerRunnable1: 5",
      "InnerThread1: 4",
      "InnerRunnable2: 5",
      "ThreadMethod: 5",
      "InnerRunnable1: 4",
      "InnerThread2: 3",
      "InnerRunnable2: 4",
      "ThreadMethod: 4",
      "InnerThread1: 3",
      "InnerRunnable1: 3",
      "ThreadMethod: 3",
      "InnerThread1: 2",
      "InnerThread2: 2",
      "InnerRunnable2: 3",
      "InnerThread2: 1",
      "InnerRunnable2: 2",
      "InnerRunnable1: 2",
      "ThreadMethod: 2",
      "InnerThread1: 1",
      "InnerRunnable1: 1",
      "InnerRunnable2: 1",
      "ThreadMethod: 1"
    }, Test.IGNORE_ORDER + Test.WAIT);
  }
} ///:~


InnerThread1 creates a named inner class that extends Thread, and makes an instance of this inner class inside the constructor. This makes sense if the inner class has special capabilities (new methods) that you need to access in other methods. However, most of the time the reason for creating a thread is only to use the Thread capabilities, so it’s not necessary to create a named inner class. InnerThread2 shows the alternative: An anonymous inner subclass of Thread is created inside the constructor and is upcast to a Thread reference t. If other methods of the class need to access t, they can do so through the Thread interface, and they don’t need to know the exact type of the object.

The third and fourth classes in the example repeat the first two classes, but they use the Runnable interface rather than the Thread class. This is just to show that Runnable doesn’t buy you anything more in this situation, but is in fact slightly more complicated to code (and to read the code). As a result, my inclination is to use Thread unless I’m somehow compelled to use Runnable.

The ThreadMethod class shows the creation of a thread inside a method. You call the method when you’re ready to run the thread, and the method returns after the thread begins. If the thread is only performing an auxiliary operation rather than being fundamental to the class, this is probably a more useful/appropriate approach than starting a thread inside the constructor of the class.

Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire