There are two ways to create a new class that can have a thread running within it. One way is to extend the Thread class. The other is to extend any class and implement the Runnable interface. For the sake of illustration, extending Thread is the simplest approach and is initially used in this book. Implementing the Runnable interface tends to work much better in the real world; this technique is introduced in Chapter 4, “Implementing Runnable Versus Extending Thread.”
两种创建支持线程类的方式：
实现Runnable接口，可继承自任何类继承Thread类显然第一种方式比较灵活
The steps to spawn a new thread in this chapter’s example are
• Extend the java.lang.Thread class.
• Override the run() method in this subclass of Thread.
• Create an instance of this new class.
• Invoke the start() method on the instance
创建一个线程的步骤：
继承java.lang.Thread类覆盖子类的run()方法创建一个新类的实例调用实例的start()方法A call to start() returns right away and does not wait for the other thread to begin execution. In start(), the parent thread asynchronously signals through the JavaVM that the other thread should be started as soon as it’s convenient for the thread scheduler. At some unpredictable time in the very near future, the other thread will come alive and invoke the run() method of the Thread object (or in this case, the overridden run() method implemented in TwoThread). Meanwhile, the original thread is free to continue executing the statements that follow the start() call.
子父线程异步运行，调用start()后子线程不确定时间开始执行
Important to note is that although the order in which each thread will execute its own statements is known and straightforward, the order in which the statements will actually be run on the processor is indeterminate, and no particular order should be counted on for program correctness.
线程内顺序执行，线程间执行顺序不确定
Listing 2.1 TwoThread.java—The Complete Code for the TwoThread Example
1: public class TwoThread extends Thread {
2: public void run() {
3: for ( int i = 0; i < 10; i++ ) {
4: System.out.println(“New thread”);
5: }
6: }
7:
8: public static void main(String[] args) {
9: TwoThread tt = new TwoThread();
10: tt.start();
11:
12: for ( int i = 0; i < 10; i++ ) {
13: System.out.println(“Main thread”);
14: }
15: }
16: }