Java 如何消除太多的 if else 判断?

1.简介

if判断语句是很多编程语言的重要组成部分。但是,若我们最终编写了大量嵌套的if语句,这将使得我们的代码更加复杂和难以维护。

让我们看看能否使用别的方式来做呢。

设计模式是为了更好的代码重用性,可读性,可靠性,可维护性,它有六大原则

1)单一职责原则(Single Responsibility Principle,简称SRP):该原则是针对类来说的,即一个类应该只负责一项职责.
2)开放–封闭原则(The Open-Closed Principle简称OCP):是说软件实体(类、模块、函数等等)应该可以扩展,但是不可以修改。
3)依赖倒转原则(Dependence Inversion Principle :针对接口编程,不要对实现编程
4)里氏代换原则(Liskov Substitution Principle,简称LSP):里氏代换原则,子类型必须能够替换掉他们的父类型
5)迪米特法则(Law of Demeter):如果两个类不必彼此直接通信,那么这两个类就不应当发生直接的相互作用
6)合成/聚合复用原则(Composition/Aggregation Principle],简称CARP):尽量使用合成/聚合,尽量不使用类继承。合成聚合是“has ?a”的关系,而继承是“is ?a”的关系。

2.示例

if..else

public int calculate(int a, int b, String operator) {
        int result = Integer.MIN_VALUE;

        if ("add".equals(operator)) {
            result = a + b;
        } else if ("multiply".equals(operator)) {
            result = a * b;
        } else if ("divide".equals(operator)) {
            result = a / b;
        } //其他减 除 等略
        return result;
    }

case-switch

public int calculateUsingSwitch(int a, int b, String operator) {
        int result = 0;
        switch (operator) {
        case "add":
            result = a + b;
            break;
        case "multiply":
            result = a * b;
            break;
        //其他减 除 等略
        default:
            result = Integer.MIN_VALUE;
        }
        return result;
    }

3.重构

3.1 工厂方式重构

抽象层Operation.java

public interface Operation {
    int apply(int a, int b);
}

加法实现Addition.java:

public class Addition implements Operation {
    @Override
    public int apply(int a, int b) {
        return a + b;
    }
}

乘法实现Multiplication.java

public class Multiplication implements Operation {
        @Override public int apply(int a, int b) {
                return a*b;
        }
}
工厂类OperatorFactory.java
import java.util.HashMap;
import java.util.Map;
import java.util.Optional;

public class OperatorFactory {

    static Map<String, Operation> operationMap = new HashMap<>();
    static {
        operationMap.put("add", new Addition());
        operationMap.put("divide", new Division());
        operationMap.put("multiply", new Multiplication());
        operationMap.put("subtract", new Subtraction());
        operationMap.put("modulo", new Modulo());
    }

    public static Optional<Operation> getOperation(String operation) {
        return Optional.ofNullable(operationMap.get(operation));
    }
}

使用示例

public int calculateUsingFactory(int a, int b, String operator) {
    Operation targetOperation = OperatorFactory
      .getOperation(operator)
      .orElseThrow(() -> new IllegalArgumentException("Invalid Operator"));
    return targetOperation.apply(a, b);
}

3.2 枚举方式重构

枚举实现Operator.java

public enum Operator {

ADD {
@Override
public int apply(int a, int b) {
return a + b;
}
},

MULTIPLY {
@Override
public int apply(int a, int b) {
return a * b;
}
},

public abstract int apply(int a, int b);
}

封装Operator到Calculator.java

public int calculate(int a, int b, Operator operator) {
        return operator.apply(a, b);
    }

使用示例

@Test
public void whenCalculateUsingEnumOperator_thenReturnCorrectResult() {
    Calculator calculator = new Calculator();
    int result = calculator.calculate(3, 4, Operator.valueOf("ADD"));
    assertEquals(7, result);
}

3.3 命令模式

抽象的接口

public interface Command {
    Integer execute();
}

实现类

package com.baeldung.reducingIfElse;

public class AddCommand implements Command {

    private int a;
    private int b;

    public AddCommand(int a, int b) {
        this.a = a;
        this.b = b;
    }

    @Override
    public Integer execute() {
        return a + b;
    }
}

其它略

包装

public int calculate(Command command) {
        return command.execute();
    }

测试demo

@Test
public void whenCalculateUsingCommand_thenReturnCorrectResult() {
    Calculator calculator = new Calculator();
    int result = calculator.calculate(new AddCommand(3, 7));
    assertEquals(10, result);
}

4.规则引擎重构

抽象规则

public interface Rule {

    boolean evaluate(Expression expression);

    Result getResult();
}

实现规则AddRule.java 其它略

public class AddRule implements Rule {

    private int result;

    @Override
    public boolean evaluate(Expression expression) {
        boolean evalResult = false;
        if (expression.getOperator() == Operator.ADD) {
            this.result = expression.getX() + expression.getY();
            evalResult = true;
        }
        return evalResult;
    }

    @Override
    public Result getResult() {
        return new Result(result);
    }
}

其中:返回结果

public class Result {
    int value;

    public Result(int value) {
        this.value = value;
    }

    public int getValue() {
        return value;
    }
}

表达式

public class Expression {

        private Integer x;
        private Integer y;
        private Operator operator;

        public Expression(Integer x, Integer y, Operator operator) {
                this.x = x;
                this.y = y;
                this.operator = operator;
        }

        public Integer getX() {
                return x;
        }

        public Integer getY() {
                return y;
        }

        public Operator getOperator() {
                return operator;
        }
}

规则引擎RuleEngine.java

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

public class RuleEngine {

    private static List<Rule> rules = new ArrayList<>();

    static {
        rules.add(new AddRule());
    }

    public Result process(Expression expression) {

        Rule rule = rules.stream()
            .filter(r -> r.evaluate(expression))
            .findFirst()
            .orElseThrow(() -> new IllegalArgumentException("Expression does not matches any Rule"));
        return rule.getResult();
    }
}

测试demo

@Test
public void whenNumbersGivenToRuleEngine_thenReturnCorrectResult() {
    Expression expression = new Expression(5, 5, Operator.ADD);
    RuleEngine engine = new RuleEngine();
    Result result = engine.process(expression);
 
    assertNotNull(result);
    assertEquals(10, result.getValue());
}

4.比较

重构方式 SRP OCP DIP LSP LD CARP
IF/ELSE N N N N N N
工厂方法 Y Y Y Y Y Y
枚举方法 N Y Y Y Y Y
命令模式 Y Y Y Y Y Y
规则引擎 Y Y Y Y Y Y

5.小结

为了更好的代码重用性,可读性,可靠性,可维护性,我们会尝试将IF/ELSE或者case-switch进行改造,使用工厂方法,枚举方法,命令模式,规则引擎方式不同方法进行尝试,最后使用设计模式的六大原则对代码进行评估

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