java程序员面试宝典----线程

设计4个线程，两个线程每次对j增加1，另外两个对j减少1

package thread;

public class ThreadTest {
	private int j;

	private synchronized void inc() {
		j++;
		System.out.println(Thread.currentThread().getName() + "-inc:" + j);
	}

	private synchronized void dec() {
		j--;
		System.out.println(Thread.currentThread().getName() + "-dec:" + j);
	}

	class Inc implements Runnable { // 内部类，线程
		public void run() {
			for (int i = 0; i < 100; i++) {
				inc();
			}
		}
	}

	class Dec implements Runnable {
		public void run() {
			for (int i = 0; i < 100; i++) {
				dec();
			}
		}
	}

	public static void main(String[] args) {
		ThreadTest t = new ThreadTest();
		Inc inc = t.new Inc();
		Dec dec = t.new Dec();

		for (int i = 0; i < 2; i++) {
			Thread t1 = new Thread(inc);
			t1.start();
			Thread t2 = new Thread(dec);
			t2.start();

			// Thread t3 = new Thread(inc);
			// t3.start();
			// Thread t4 = new Thread(dec);
			// t4.start();
		}

	}

}

执行结果是根据线程之间的推进次序的不同而不同
加上同步关键字，则变量j的改变就是在前一个线程改变后的基础上改变的。
不加同步会出现：Thread-3-dec:28
Thread-3-dec:28
Thread-1-dec:-76
Thread-3-dec:23
Thread-3-dec:-77
Thread-3-dec:-78