探讨if…else的替代方案

大家在开发过程中，经常会用到if...else语句，尤其针对那些业务分支较多的情况下，if...else显得非常臃肿，大大影响代码的可读性，并且增加了维护的成本，一些大公司代码规范比较严格，单个类或者单个方法也有圈复杂度的最大限制。那么，针对以上问题，来和大家探讨下if...else的替代方案。假设我们现在要做一个计算器，实现加减乘除的功能：

令人头疼的if...else

public static int calculate(String operator, int a, int b) {
    if ("add".equals(operator)) {
        return a + b;
    } else if ("subtract".equals(operator)) {
        return a - b;
    } else if ("multiply".equals(operator)) {
        return a * b;
    } else if ("divide".equals(operator)) {
        return a / b;
    }
    return 0;
}

还不错的switch...case

public static int calculate(String operator, int a, int b) {
    switch (operator) {
        case "add":
            return a + b;
        case "subtract":
            return a - b;
        case "multiply":
            return a * b;
        case "divide":
            return a / b;
        default:
            return 0;
    }
}

使用枚举类

public static int calculate(String operator, int a, int b) {
    return Calculator.calculate(operator, a, b);
}
 
public enum Calculator {
 
    ADD {
        @Override
        public int apply(int a, int b) {
            return a + b;
        }
    },
    SUBTRACT {
        @Override
        public int apply(int a, int b) {
            return a - b;
        }
    };
 
    abstract int apply(int a, int b);
 
    public static int calculate(String operatorName, int a, int b) {
        for (Calculator calculator : Calculator.values()) {
            if (calculator.toString().equalsIgnoreCase(operatorName)) {
                return calculator.apply(a, b);
            }
        }
        return 0;
    }
}

工厂模式

public interface Operation {
    int apply(int a, int b);
}
 
public class Add implements Operation {
    @Override
    public int apply(int a, int b) {
        return a + b;
    }
}
 
public class Subtract implements Operation {
    @Override
    public int apply(int a, int b) {
        return a - b;
    }
}
 
public class Calculator {
    private static Map<String, Operation> calcutorMap = new HashMap<>();
    
    static {
        calcutorMap.put("add", new Add());
        calcutorMap.put("subtract", new Subtract());
    }
 
    public static int calculate(String operator, int a, int b) {
        return calcutorMap.get(operator).apply(a, b);
    }
}

jdk8的功能性接口（这里卖个关子，理解了下面的代码后，可自己动手哦^_^）

@FunctionalInterface
public interface VoidFunction {
 
    void apply();
 
}

public class IfElse {
 
    private Map<Boolean, VoidFunction> logicAndFunctionMaps = new LinkedHashMap<>();
 
    public static IfElse create() {
        return new IfElse();
    }
 
    public IfElse ifLogic(boolean condition, VoidFunction ifFunction) {
        return addLogic(condition, ifFunction);
    }
 
    public IfElse elseIfLogic(boolean condition, VoidFunction ifFunction) {
        return addLogic(condition, ifFunction);
    }
 
    public IfElse elseLogic(VoidFunction ifFunction) {
        return addLogic(true, ifFunction);
    }
 
    private IfElse addLogic(boolean condition, VoidFunction ifFunction) {
        this.logicAndFunctionMaps.put(condition, ifFunction);
        return this;
    }
 
    public boolean execute() {
        for (Map.Entry<Boolean, VoidFunction> entry : logicAndFunctionMaps.entrySet()) {
            if (entry.getKey()) {
                entry.getValue().apply();
                return true;
            }
        }
        return false;
    }
 
}

public static void calculate(String payType) {
    IfElse.create()
            .ifLogic("alipay".equals(payType), () -> System.out.println("支付宝支付"))
            .elseIfLogic("wechat".equals(payType), () -> System.out.println("微信支付"))
            .elseLogic(() -> System.out.println("无效的支付方式"))
            .execute();
}

综上所述，替代if...else的方案有很多种（包括策略模式、命令模式等等，偷个懒，这里就不一一列举了），有的是轻量级，有的则稍显笨重些，根据不同的业务场景选择合适的方案。