前面的程序只创建了一个子线程,实际上一个程序可以创建任意多个线程.例如,下面的程序就创建了三个子线程:

public class test2
{
    // @param args
    public static void main(String args[])
    {
     MyThread mt1 = new MyThread("child#1");
     MyThread mt2 = new MyThread("child#2");
     MyThread mt3 = new MyThread("child#3");
     
     Thread newThrd1 = new Thread(mt1);
     newThrd1.start();
     Thread newThrd2 = new Thread(mt2);
     newThrd2.start();
     Thread newThrd3 = new Thread(mt3);
     newThrd3.start();
     
     for(int i=0;i<50;i++)
     {
         System.out.print(".");
         try {
             Thread.sleep(100);
         }catch(InterruptedException exc) {
             System.out.println("main thread  interrupted");
         }
     }
     System.out.println("main thread ending");
    }
}
class MyThread implements Runnable
{
    String thrdName;
    
    MyThread(String name)
    {
        thrdName = name;
    }
    
    public void run()
    {
        System.out.println(thrdName+" start");
        try {
            for(int count=0;count<10;count++)
            {
                Thread.sleep(400);
                System.out.println(thrdName+"  count is "+count);
            }
        }catch(InterruptedException exc) {
            System.out.println(thrdName+"  interrupted");
        }
    }
}

输出如下所示:

child#1 start
child#2 start
child#3 start
.....child#2 count is 0
child#3 count is 0
child#1 count is 0
....child#3 count is 1
child#2 count is 1
child#1 count is 1
....child#2 count is 2
child#3 count is 2
child#1 count is 2
...child#2 count is 3
child#1 count is 3
.child#3 count is 3
....child#2 count is 4
child#1 count is 4
child#3 count is 4
....child#3 count is 5
child#2 count is 5
child#1 count is 5
...child#3 count is 6
child#1 count is 6
.child#2 count is 6
...child#3 count is 7
.child#2 count is 7
child#1 count is 7
....child#2 count is 8
child#1 count is 8
child#3 count is 8
....child#3 count is 9
child#2 count is 9
child#1 count is 9
.........main thread ending

如你所见,一旦主线程启动,三个子线程都将共享CPU,注意线程按照被出罕见的顺序启动,然而,情况不总是这样,Java可以以自己的方式自由调度线程的执行.当然,由于时间或环境的不同,程序的具体输出可能会不尽相同,所以如果测试程序时发现有些许不同,也不必大惊小怪.

问:为什么在Java中创建子线程有两种方法(通过扩展线程或实现Tunnable接口),使用哪种方法更合适呢?

答:Thread类定义了几个派生类可以重写的方法.在这些方法中,必须被重写的一个方法是run().当然,在实现runnable接口时也需要重写该方法.一些Java程序员认为仅在通过某些方式增强或修改类时才应该扩展这些类,因此,如果不重写Thread的任何其他方法,最好实现Runnable接口.另外,实现tunnable接口可以让线程继承除Thread类外的其他类.