3、 用实现接口Runable方式写一个三线程的程序，分别求出0的和，0+1的和，0+1+2的和，以及0+1+2+…+100的和，给线程加同步方法

这是一个经典的多线程问题，可以用实现接口Runable的方式来实现。

下面是一个示例代码，实现了三个线程分别计算0-99，0-100和0-101的和，最后将结果打印出来。

public class ThreeThreadSum implements Runnable {
    private int start;
    private int end;
    private int sum;

    public ThreeThreadSum(int start, int end) {
        this.start = start;
        this.end = end;
        this.sum = 0;
    }

    public synchronized void add(int num) {
        sum += num;
    }

    public int getSum() {
        return sum;
    }

    @Override
    public void run() {
        for (int i = start; i <= end; i++) {
            add(i);
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ThreeThreadSum t1 = new ThreeThreadSum(0, 99);
        ThreeThreadSum t2 = new ThreeThreadSum(0, 100);
        ThreeThreadSum t3 = new ThreeThreadSum(0, 101);

        Thread thread1 = new Thread(t1);
        Thread thread2 = new Thread(t2);
        Thread thread3 = new Thread(t3);

        thread1.start();
        thread2.start();
        thread3.start();

        thread1.join();
        thread2.join();
        thread3.join();

        int sum1 = t1.getSum();
        int sum2 = t2.getSum();
        int sum3 = t3.getSum();

        System.out.println("0-99的和：" + sum1);
        System.out.println("0-100的和：" + sum2);
        System.out.println("0-101的和：" + sum3);
        System.out.println("0-100的和：" + (sum1 + sum2 + sum3));
    }
}

在每个线程中使用同步方法add()来计算和，保证了线程安全。最后通过join()方法等待所有线程的执行完成，再将结果加起来输出。