Singleton 和 Monostate 模式

怎樣才能使得兩個實例表現得像一個對象呢？很簡單，只要把所有的變量都變成靜態變量即可。

public class Monostate {private static int itsX = 0;public Monostate() {}public void setX(final int x) {itsX = x;}public int getX() {return itsX;} }

實現地鐵十字轉門的簡單的有限狀態機。十字轉門開始時處于Locked 狀態。如果投入一枚硬幣，它就遷移到Unlocked 狀態，開啟轉門，復位可能出現的任何告警狀態，并把硬幣放到收集箱柜中。如果此時乘客通過了轉門，轉門就遷移回Locked 狀態并把門鎖上。

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實現類

public class Turnstile {/*** 是否已鎖.*/private static boolean isLocked = true;/*** 是否報警中.*/private static boolean isAlarming = false;/*** 硬幣數.*/private static int itsCoins = 0;/*** 退款數.*/private static int itsRefunds = 0;protected static final Turnstile LOCKED = new Locked();protected static final Turnstile UNLOCKED = new Unlocked();protected static Turnstile itsState = LOCKED;public void reset() {lock(true);alarm(false);itsCoins = 0;itsRefunds = 0;itsState = LOCKED;}public boolean locked() {return isLocked;}public boolean alarm() {return isAlarming;}/*** 投幣.*/public void coin() {itsState.coin();}/*** 通過.*/public void pass() {itsState.pass();}/*** 設置鎖住.* @param shouldLock*/protected void lock(boolean shouldLock) {isLocked = shouldLock;}/*** 設置報警.* @param shouldAlarm*/protected void alarm(boolean shouldAlarm) {isAlarming = shouldAlarm;}/*** 獲取投幣數.* @return*/public int coins() {return itsCoins;}/*** 獲取退款數.* @return*/public int refunds() {return itsRefunds;}/*** 存錢.*/public void deposit() {itsCoins++;}/*** 退款.*/public void refund() {itsRefunds++;}static class Locked extends Turnstile {public void coin() {Turnstile.itsState = UNLOCKED;lock(false);alarm(false);deposit();}public void pass() {alarm(true);}}static class Unlocked extends Turnstile {public void coin() {refund();}public void pass() {lock(true);itsState = LOCKED;}} }

測試類

import org.junit.Assert; import org.junit.Before; import org.junit.Test;public class TurnstileTest {@Beforepublic void before() {// 每次@Test前執行.Turnstile t = new Turnstile();t.reset();}@Testpublic void whenInit() {Turnstile t = new Turnstile();Assert.assertTrue(t.locked());}@Testpublic void whenCoin() {Turnstile t = new Turnstile();t.coin();Turnstile t1 = new Turnstile();Assert.assertTrue(!t1.locked());Assert.assertEquals(1, t1.coins());}@Testpublic void whenCoinAndPass() {Turnstile t = new Turnstile();t.coin();t.pass();Turnstile t1 = new Turnstile();Assert.assertTrue(t1.locked());Assert.assertTrue(!t1.alarm());Assert.assertEquals(1, t1.coins());}@Testpublic void whenTowCoins() {Turnstile t = new Turnstile();t.coin();t.coin();Turnstile t1 = new Turnstile();Assert.assertTrue(!t1.locked());Assert.assertEquals(1, t1.coins());Assert.assertEquals(1, t1.refunds());Assert.assertTrue(!t1.alarm());}@Testpublic void whenPass() {Turnstile t = new Turnstile();t.pass();Turnstile t1 = new Turnstile();Assert.assertTrue(t1.alarm());Assert.assertTrue(t1.locked());}@Testpublic void whenCancelAlarm() {Turnstile t = new Turnstile();t.pass();t.coin();Turnstile t1 = new Turnstile();Assert.assertTrue(!t1.alarm());Assert.assertTrue(!t1.locked());Assert.assertEquals(1, t1.coins());Assert.assertEquals(0, t1.refunds());}@Testpublic void whenTwoOperations() {Turnstile t = new Turnstile();t.coin();t.pass();t.coin();Assert.assertTrue(!t.locked());Assert.assertEquals(2, t.coins());t.pass();Assert.assertTrue(t.locked());} }

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