题目：有一个盘子，盘子只能放一个水果，爸爸每次生产一个苹果，妈妈每次生产一个橘子，儿子每次消费一个苹果，女儿每次消费一个橘子，使用信号量实现；

分析：因为盘子是有容量的，一个信号量并不能保证盘子的容量是否超过一个（除非重复轮训盘子数量，put时若盘子数量不等于0则wait，直到盘子数量等于0打破轮训），这里通过使用两个信号量notEmpty，notFull分别表示盘子是否为空，是否已满来实现；

Plate类：

package proandconusesemaphore;

import java.util.concurrent.Semaphore;

import proandconusesync.Apple;
import proandconusesync.Fruit;
import proandconusesync.Orange;

public class Plate {
	Semaphore notFull = new Semaphore(1);
	Semaphore notEmpty = new Semaphore(0);
	// Semaphore mutex = new Semaphore(1);
	Fruit fruit;

	public void putApple(Apple apple) {
		try {
			notFull.acquire();
			this.fruit = apple;
			System.out.println("爸爸放了一个苹果:  " + apple + "  " + apple.id);
			notEmpty.release();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

	public void putOrange(Orange orange) {
		try {
			notFull.acquire();
			this.fruit = orange;
			System.out.println("妈妈放了一个橘子:  " + orange + "  " + orange.id);
			notEmpty.release();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

	public Apple getApple() {
		try {
			notEmpty.acquire();
			if (this.fruit instanceof Apple) {
				Apple apple = (Apple) this.fruit;
				System.out.println("儿子吃了一个苹果:  " + apple + "  " + apple.id);
				this.fruit = null;
				notFull.release();
				return apple;
			} else {
				notEmpty.release();
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		return null;
	}

	public Orange getOrange() {
		try {
			notEmpty.acquire();
			if (this.fruit instanceof Orange) {
				Orange orange = (Orange) this.fruit;
				this.fruit = null;
				System.out.println("女儿吃了一个橘子:  " + orange + "  " + orange.id);
				notFull.release();
				return orange;
			}else{
				notEmpty.release();
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		return null;
	}
}

Main类：

package proandconusesemaphore;

import java.util.concurrent.atomic.AtomicInteger;

import proandconusesync.Apple;
import proandconusesync.Orange;

public class Main {
	public static Plate plate = new Plate();
	public static AtomicInteger id = new AtomicInteger(0);

	public static void main(String[] args) {

		new Thread(new Runnable() {

			public void run() {
				while (true) {
					Apple apple = new Apple(id.getAndIncrement());
					plate.putApple(apple);
					try {
						Thread.sleep(10);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}
		}).start();

		new Thread(new Runnable() {

			public void run() {
				while (true) {
					Orange orange = new Orange(id.getAndIncrement());
					plate.putOrange(orange);
					try {
						Thread.sleep(10);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}
		}).start();

		new Thread(new Runnable() {
			public void run() {
				while (true) {
					plate.getApple();
					try {
						Thread.sleep(10);
					} catch (InterruptedException e) {
					}
				}
			}
		}).start();

		new Thread(new Runnable() {
			public void run() {
				while (true) {
					plate.getOrange();
					try {
						Thread.sleep(10);
					} catch (InterruptedException e) {
					}
				}
			}
		}).start();
	}
}