题目描述

给你一个类：

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");
    }
  }
}

两个不同的线程将会共用一个 FooBar 实例：

线程 A 将会调用 foo() 方法，而
线程 B 将会调用 bar() 方法
请设计修改程序，以确保 "foobar" 被输出 n 次。

题解

class FooBar {
    private int n;
    private volatile boolean flag = true;

    public FooBar(int n) {
        this.n = n;
    }

    public void foo(Runnable printFoo) throws InterruptedException {
        
        for (int i = 0; i < n; i++) {
            while(!flag) {
                ;
            }
            // printFoo.run() outputs "foo". Do not change or remove this line.
            printFoo.run();
            flag = false;
        }
    }

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

结果：

image.png

改进方案：

class FooBar {
    private int n;
    private volatile boolean flag = true;

    public FooBar(int n) {
        this.n = n;
    }

    public void foo(Runnable printFoo) throws InterruptedException {
        
        for (int i = 0; i < n; i++) {
            while(!flag) {
                Thread.yield();
            }
            // printFoo.run() outputs "foo". Do not change or remove this line.
            printFoo.run();
            flag = false;
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        
        for (int i = 0; i < n; i++) {
            while(flag) {
                Thread.yield();
            }
            // printBar.run() outputs "bar". Do not change or remove this line.
            printBar.run();
            flag = true;
        }
    }
}

结果：AC