テーマの出所:https://leetcode.com/problems/print-in-order/
問題の説明

1114. Print in Order

Easy
Suppose we have a class:

public class Foo {

  public void first() { print("first"); }

  public void second() { print("second"); }

  public void third() { print("third"); }

}

The same instance of Foo will be passed to three different threads. Thread A will call first(), thread B will call second(), and thread C will call third(). Design a mechanism and modify the program to ensure that second() is executed after first(), and third() is executed after second().
 
Example 1:
Input: [1,2,3]
Output: "firstsecondthird"
Explanation: There are three threads being fired asynchronously. The input [1,2,3] means thread A calls first(), thread B calls second(), and thread C calls third(). "firstsecondthird"is the correct output.
Example 2:
Input: [1,3,2]
Output: "firstsecondthird"
Explanation: The input [1,3,2] means thread A calls first(), thread B calls third(), and thread C calls second(). "firstsecondthird"is the correct output.
 
Note:
We do not know how the threads will be scheduled in the operating system, even though the numbers in the input seems to imply the ordering. The input format you see is mainly to ensure our tests' comprehensiveness.
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に言及
3つの並列実行スレッドが一定の順序で出力されるように、スレッド間同期コードを記述します.
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構想
2つの共有変数first_を使用flagとsecond_flagは、2番目のスレッドと3番目のスレッドの実行をそれぞれ制御し、wait()とnotifyAll()を使用してスレッド間通信を行う.注意notify/notifyAllが起動したスレッドは、同期ブロックのヘッダからではなくwaitの後から実行されます.
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コード#コード#

class Foo {
    private Boolean first_flag = false;
    private Boolean second_flag = false;

    public Foo() {
        
    }

    public synchronized void first(Runnable printFirst) throws InterruptedException {
        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        notifyAll();
        first_flag = true;
    }

    public synchronized void second(Runnable printSecond) throws InterruptedException {
        while (!first_flag) {
            wait();
        }
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        notifyAll();
        second_flag = true;
    }

    public synchronized void third(Runnable printThird) throws InterruptedException {
        while (!second_flag) {
            wait();
        }
        // printThird.run() outputs "third". Do not change or remove this line.
        printThird.run();
    }
}