Lc 1115.交替打印FooBar

我们提供一个类：

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。其中一个线程将会调用 foo() 方法，另一个线程将会调用 bar() 方法。

请设计修改程序，以确保 “foobar” 被输出 n 次。

class FooBar {

    private int n;

    private boolean isFooTurn = true;
    
    private Object obj = new Object();

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

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

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

优化

class FooBar {
    private int n;
    
    private final Lock lock = new ReentrantLock();
    private boolean allowedAProcess = true;
    private final Condition conditionA = lock.newCondition();
    private final Condition conditionB = lock.newCondition();
    
    public FooBar(int n) {
        this.n = n;
    }


    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            lock.lock();
            try {
                while (!allowedAProcess) {
                    conditionA.await();
                }
                // printFoo.run() outputs "foo". Do not change or remove this line.
                printFoo.run();
                allowedAProcess = false;
                conditionB.signalAll();
            } finally {
                lock.unlock();
            }
        }
    }

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

                allowedAProcess = true;
                conditionA.signalAll();
            } finally {
                lock.unlock();
            }
        }
    }
}