定義：策略模式定義了一系列的演算法，並將每一個演算法封裝起來，而且使他們可以相互替換，讓演算法獨立於使用它的客戶而獨立變化。

分析下定義，策略模式定義和封裝了一系列的演算法，它們是可以相互替換的，也就是說它們具有共性，而它們的共性就體現在策略介面的行為上，另外為了達到目的，也就是說讓演算法獨立於使用它的客戶而獨立變化，我們需要讓客戶端依賴於策略介面。

策略模式的優缺點:
優點：
1、結構清晰，把策略分離成一個個單獨的類「替換了傳統的 if else」
2、程式碼耦合度降低，安全性提高「各個策略的細節被遮蔽」
缺點：
1、客戶端必須要知道所有的策略類，否則你不知道該使用那個策略，所以策略模式適用於提前知道所有策略的情況下
2、增加了類的編寫，本來只需要 if else 即可「但是這是所有模式和架構的通病」

策略模式類圖:

下面來看兩個經典的案例:
1.headfirst設計模式一書中的鴨子例項
Duck.java

package StrategyPattern;

/**
 * 鴨子超類
 */

public abstract class Duck {
    protected FlyBehavior flyBehavior;
    protected QuackBehavior quackBehavior;

    public Duck() {
    }

    public abstract void display();

    public void performFly
 
() {
        flyBehavior.fly();
    }

    public void perormQuack() {
        quackBehavior.quack();
    }

    public void swim() {
        System.out.println("All ducks float,even decoys!");
    }

    public void setFlyBehavior(FlyBehavior fb) {
        flyBehavior = fb;
    }

    public void setQuackBehavior
 
(QuackBehavior qb) {
        quackBehavior = qb;
    }
}

FlyBehavior.java

package StrategyPattern;

/**
 * 實現飛行介面的類
 */
public interface FlyBehavior {
    void fly();
}

FlyNoWay.java

package StrategyPattern;

/**
 * 不能飛
 */
public class FlyNoWay implements FlyBehavior {

    @Override
    public void fly() {
        System.out.println("I can't fly!");
    }

}

FlyRocketPowered.java

package StrategyPattern;

/**
 * 用火箭飛
 */
public class FlyRocketPowered implements FlyBehavior {

    @Override
    public void fly() {
        System.out.println("I'm flying with a rocket!");
    }
}

FlyWithWings.java

package StrategyPattern;

/**
 * 用翅膀飛
 */
public class FlyWithWings implements FlyBehavior {
    @Override
    public void fly() {
        System.out.println("I'm flying!");
    }

}

MallardDuck.java

package StrategyPattern;

/**
 * 綠頭鴨
 */
public class MallardDuck extends Duck {
    public MallardDuck(){
        flyBehavior = new FlyWithWings();
        quackBehavior = new Quack();
    }
    @Override
    public void display() {
        System.out.println("I'm a real Mallard duck!");
    }
}

ModelDuck.java

package StrategyPattern;

/**
 * 模型鴨
 */
public class ModelDuck extends Duck {

    public ModelDuck() {
        flyBehavior = new FlyNoWay();
        quackBehavior = new Quack();
    }

    @Override
    public void display() {
        System.out.println("I'm a model duck!");
    }
}

MuteQuack.java

package StrategyPattern;

/**
 * 不會叫
 */
public class MuteQuack implements QuackBehavior{
     @Override
     public void quack()
    {
        System.out.println("<< Silence >>");
    }
}

Quack.java

package StrategyPattern;

/**
 * 呱呱叫
 */
public class Quack implements QuackBehavior{
    @Override
    public void quack()
    {
        System.out.println("Quack");
    }
}

QuackBehavior.java

package StrategyPattern;

/**
 * 實現發聲介面的類
 */
public interface QuackBehavior {
    void quack();
}

Squeak.java

package StrategyPattern;

/**
 * 吱吱叫
 */
public class Squeak implements QuackBehavior{
    @Override
    public void quack()
    {
        System.out.println("Squeak");
    }
}

Test.java

package StrategyPattern;

/**
 * 測試類
 */
public class Test {
    public static void main(String[] args) {

        //綠頭鴨
        Duck mallard = new MallardDuck();
        mallard.perormQuack();
        mallard.performFly();

        //模型鴨
        Duck model = new ModelDuck();
        model.performFly();
        model.setFlyBehavior(new FlyRocketPowered());
        model.performFly();
    }
}

2.headfirst設計模式一書中策略模式課後練習之冒險遊戲
類圖設計:

AxeBehavior.java

package AdventureGame;

public class AxeBehavior implements WeaponBehavior {
    @Override
    public void useWeaPon() {
        System.out.println("使用斧頭");
    }
}

BowAndArrowBehavior.java

package AdventureGame;

public class BowAndArrowBehavior implements WeaponBehavior{
    @Override
    public void useWeaPon() {
        System.out.println("使用弓箭");
    }

}

King.java

package AdventureGame;

public class King extends Role {
    public King(){
        weapon=new AxeBehavior();
    }
    @Override
    void display() {
        System.out.println("這是國王");
    }
}

KnifeBehavior.java

package AdventureGame;

public class KnifeBehavior implements WeaponBehavior{
    @Override
    public void useWeaPon() {
        System.out.println("使用寶劍");
    }
}

knight.java

package AdventureGame;

public class knight extends Role {
    public knight(){
        weapon=new SwordBehavior();
    }
    @Override
    void display() {
        System.out.println("這是騎士");
    }
}

Queen.java

package AdventureGame;

public class Queen extends Role{
    public Queen(){
        weapon=new KnifeBehavior();
    }
    @Override
    void display() {
        System.out.println("這是皇后");
    }
}

Role.java

package AdventureGame;

public abstract class Role {
    WeaponBehavior weapon;
    public Role(){
    }
    public void setWeapon(WeaponBehavior weapon) {
        this.weapon = weapon;
    }
    abstract void display();
    public void weaponWay(){
        weapon.useWeaPon();
    }
}

SwordBehavior.java

package AdventureGame;

public class SwordBehavior implements WeaponBehavior{
    @Override
    public void useWeaPon() {
        System.out.println("使用匕首");
    }
}

Troll.java

package AdventureGame;

public class Troll extends Role{
    public Troll(){
        weapon=new SwordBehavior();
    }
    @Override
    void display() {
        System.out.println("這是妖怪");
    }
}

WeaponBehavior.java

package AdventureGame;

public interface WeaponBehavior {
    void useWeaPon();
}

Test.java

package AdventureGame;

public class Test {
public static void main(String[] args) {
        King king = new King();
        king.display();
        king.weaponWay();
        System.out.println("更換武器以後");
        king.setWeapon(new SwordBehavior());
        king.weaponWay();
        }
}

執行效果圖:

策略模式就先介紹到這,以後有更深的理解時再繼續補充相關內容!