lock锁的问题求解--->半天了 没看懂

public class Test15 {
    private int num = 0;
    public Test15(int num){
        this.num=num;
    }
    private Lock lock4Add = new ReentrantLock();
    public void add(){
        for (int i = 0; i < 10000; i++) {
            lock4Add.lock();
            num=num+1;
            lock4Add.unlock();   
            try {
                Thread.sleep((long)0.7);
            } catch (InterruptedException e){
            e.printStackTrace();
            }
        }
        System.out.println(num);
    }

}
上面红色的3段代码不是很懂  lock  0的时候是开  1的时候是关  为毛num=1了  它就打开
本来等于0   确是关闭呢？？？？？？？
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没看明白问题
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