設計4個線程，其中兩個線程每次對j增加1，另外兩個線程對j每次減少1。寫出程序。

本題並不難，實現方式有很多種，有很多種結構。

方法一：利用內部類實現，兩個實現加減的類實現Runnable接口，然后再實現4個具體線程。

代碼：

package mystudy;

public class ManyThreads {

    private int j;

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ManyThreads many = new ManyThreads();
        Inc inc = many.new Inc();
        Dec dec = many.new Dec();
        for (int i = 0; i < 2; i++) {
            Thread t = new Thread(inc);
            t.start();
            t = new Thread(dec);
            t.start();
        }
    }

    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 {

        @Override
        public void run() {
            // TODO Auto-generated method stub
            for (int i = 0; i < 20; i++) {
                inc();
            }
        }

    }

    class Dec implements Runnable {

        @Override
        public void run() {
            // TODO Auto-generated method stub
            for (int i = 0; i < 20; i++) {
                dec();
            }
        }

    }
}

第二種方式：具體加減操作寫在一個類的方法里，沒有內部類，用另外一個類去調用。

代碼：

public class ManyThreads2 {
    private int j = 0;

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

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

}

調用：

public class MyTest {

    private ManyThreads2 many = new ManyThreads2();

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        MyTest myTest = new MyTest();
        myTest.test();
    }

    public void test() {
        for (int i = 0; i < 2; i++) {
            new Thread(new Runnable() {

                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    for (int i = 0; i < 20; i++) {
                        many.inc();
                    }
                }

            }).start();
            new Thread(new Runnable() {

                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    for (int i = 0; i < 20; i++) {
                        many.dec();
                    }
                }

            }).start();
        }
    }
}

僅供參考。。