如何用卫语句、策略模式、状态模式重构if-else语句

《java开发手册》中:

况且日常开发时遇到一推的if-else着实让人头疼!

so，现在开始改造

一、卫语句

卫语句就是把复杂的条件表达式拆分成多个条件表达式，即代码逻辑先考虑失败、异常、中断、退出等直接返回的情况，以方法多个出口的方式，解决代码中判断分支嵌套的问题，这是逆向思维的体现

比如:

if (type == 0) {
     code...
}else if (type == 1) {
     code...
} else if (type == 2) {
     code...
} else {
    otherCode...
}  

可以改造为:

if (type == 0) {
　　 code...
 　　return;
}
if (type == 1) {
　　 code...
 　　return;
}
if (type == 2) {
    code...
    return;
}
otherCode....

二、策略模式　

如果不了解策略模式，可以去看下菜鸟教程中的例子，简单明了，地址:https://www.runoob.com/design-pattern/strategy-pattern.html

现在有个需求，根据用户的类型来展示不同的内容，我们将其具体化为一段代码:

if ("新客户".equals(userType)) {
　　showNewContent...;
}else if("VIP客户".equals(userType)){
　　showVIPContent...;
}else if ("白金客户".equals(userType)) {
　　showPlatinaContent...;
}else if ("黑卡用户".equals(userType)){
　　showBlackCardContent...;
}else if ("回归用户".equals(userType)){
　　showComeBackContent...;
}else {
　　showOtherContent...;
}

　　这样看是看不出来有何不妥的，但是想想，要推荐的内容何止这么点代码，如果不管不顾，就这么写，到时候写完代码回过头来，自己都得看老半天才能看懂，何论后期维护的人呢，而且要是后期又添加了一种新的用户类型，那是不是又要写一个else-if插在中间，然后又增量添加一推代码，这样岂不是太麻烦了?，反正强迫症肯定是受不了的

策略模式改造:

1.定义一个策略接口

public interface ShowStrategy{
    void showContentByUserType();
}

2.定义多个具体的策略实现

public class ShowNewContentStrategy implements ShowStrategy {
    @Override
    public void showContentByUserType() {
        System.out.println("展示新用户内容");
    }
}
public class ShowVIPContentStrategy implements ShowStrategy {
    @Override
    public void showContentByUserType() {
        System.out.println("展示VIP用户内容");
    }
}
public class ShowPlatinaContentStrategy implements ShowStrategy {
    @Override
    public void showContentByUserType() {
        System.out.println("展示白金用户内容");
    }
}    
public class ShowBlackCardContentStrategy implements ShowStrategy {
    @Override
    public void showContentByUserType() {
        System.out.println("展示黑卡用户内容");
    }
}
public class ShowComeBackContentStrategy implements ShowStrategy {
    @Override
    public void showContentByUserType() {
        System.out.println("展示回归用户内容");
    }
}

3.定义一个策略使用的client

public class ShowStrategyClient {
    //持有一个策略接口引用
    private ShowStrategy showStrategy;
    //注入
    public ShowStrategyClient(ShowStrategy showStrategy) {
        this.showStrategy = showStrategy;
    }
    //策略client的方法
    public void clientMethod(){
        showStrategy.showContentByUserType();
    }
}

4.外部使用

回到我们的需求，用写好的策略模式重构

public void test() {
　　//1.创建新用户的内容展示策略
　　ShowStrategy newContentStrategy = new ShowNewContentStrategy();
　　//2.创建策略客户端,注入新用户策略实现
　　ShowStrategyClient showStrategyClient = new ShowStrategyClient(newContentStrategy);
　　showStrategyClient.clientMethod();
}　　

三、状态模式

继续引用我们在“二、策略模式”中的例子

状态模式改造:

1.定义一个状态行为接口

interface UserState {
    void showNewContent(ShowStateClient client);
    void showVIPContent(ShowStateClient client);
    void showPlatinaContent(ShowStateClient client);
    void showBlackCardContent(ShowStateClient client);
    void showComeBackContent(ShowStateClient client);
}

2.定义多个具体的状态实现

public class ShowNewContentState implements UserState {
    @Override
    public void showNewContent(ShowStateClient client) {
        System.out.println("展示新用户内容");
    }
    @Override
    public void showVIPContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showPlatinaContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showBlackCardContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showComeBackContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
}
public class ShowVIPContentState implements UserState {
    @Override
    public void showNewContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showVIPContent(ShowStateClient client) {
        System.out.println("展示VIP用户内容");
    }
    @Override
    public void showPlatinaContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showBlackCardContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showComeBackContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
}
public class ShowPlatinaContentState implements UserState {
    @Override
    public void showNewContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showVIPContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showPlatinaContent(ShowStateClient client) {
        System.out.println("展示白金用户内容");
    }
    @Override
    public void showBlackCardContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showComeBackContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
}
public class ShowBlackCardContentState implements UserState {
    @Override
    public void showNewContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showVIPContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showPlatinaContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showBlackCardContent(ShowStateClient client) {
        System.out.println("展示黑卡用户内容");
    }
    @Override
    public void showComeBackContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
}
public class ShowComeBackContentState implements UserState {
    @Override
    public void showNewContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showVIPContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showPlatinaContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showBlackCardContent(ShowStateClient client) {
        System.out.println("用户状态错误");
    }
    @Override
    public void showComeBackContent(ShowStateClient client) {
        System.out.println("展示回归用户内容");
    }
}

3.定义一个状态使用的client

public class ShowStateClient {
    private UserState userState;
    public void setUserState(UserState userState){
        this.userState = userState;
    }
    public UserState getUserState(){
        return userState;
    }
    public void showNewContent() {
        userState.showNewContent(this);
    }
    public void showVIPContent() {
        userState.showVIPContent(this);
    }
    public void showPlatinaContent() {
        userState.showPlatinaContent(this);
    }
    public void showBlackCardContent() {
        userState.showBlackCardContent(this);
    }
    public void showComeBackContent() {
        userState.showComeBackContent(this);
    }
}

4.外部使用

public static void main(String[] args) {
    ShowStateClient client = new ShowStateClient();
    client.setUserState(new ShowNewContentState());
    client.showNewContent();
    client.showVIPContent();
    client.showPlatinaContent();
    client.showBlackCardContent();
    client.showComeBackContent();
}

　　

个人觉得状态模式过于臃肿，用处反而没有特别突出，不过也许是我没有遇到合适的场景或者理解有误吧!