[LeetCode] 1115. Print FooBar Alternately

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本文探讨了在多线程环境中如何确保正确并发执行的方法。通过使用synchronized关键字和wait/notify机制，实现两个线程间foo和bar的交替打印，确保了线程安全和预期的输出顺序。

Suppose you are given the following code:

class FooBar {
  public void foo() {
    for (int i = 0; i < n; i++) {
      print("foo");
    }
  }

  public void bar() {
    for (int i = 0; i < n; i++) {
      print("bar");
    }
  }
}

The same instance of FooBar will be passed to two different threads. Thread A will call foo() while thread B will call bar(). Modify the given program to output "foobar" n times.

Example 1:

Input: n = 1
Output: "foobar"
Explanation: There are two threads being fired asynchronously. One of them calls foo(), while the other calls bar(). "foobar" is being output 1 time.

Example 2:

Input: n = 2
Output: "foobarfoobar"
Explanation: "foobar" is being output 2 times.

我们通过同步synchronized(lock())来进行阻塞操作，用isfooPrint来标记，如果满足while循环，则锁等待，不满足while循环，则进行下一步操作，进入打印程序，改写isfooPrint的值，唤醒其他进程。

class FooBar {
    private int n;
    Object lock=new Object();
    boolean isfooPrint;
    public FooBar(int n) {
        this.n = n;
    }

    public void foo(Runnable printFoo) throws InterruptedException {
        
        for (int i = 0; i < n; i++) {
            synchronized(lock){
                while(isfooPrint==true){
                    lock.wait();
                }
        	// printFoo.run() outputs "foo". Do not change or remove this line.
        	printFoo.run();
            isfooPrint=true;
            lock.notify();
            }
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        
        for (int i = 0; i < n; i++) {
            synchronized(lock){
                while(isfooPrint==false){
                    lock.wait();
                }
            // printBar.run() outputs "bar". Do not change or remove this line.
        	printBar.run();
            isfooPrint=false;
            lock.notify();
            }
        }
    }
}

也可以通过volatile关键字和其他锁机制进行结合，volatile关键字简介可参考：

https://blog.youkuaiyun.com/liuwenyou/article/details/100690719