java如何消除太多的if else判断示例代码

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:

• 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.

• 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。

• 依赖倒转原则（Dependence Inversion Principle :针对接口编程，不要对实现编程

• 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

• 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

• 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;
} else if ("subtract".equals(operator)) {
result = a - b;
} else if ("modulo".equals(operator)) {
result = a ％ b;
}
return result;
}

switch-case

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;
case "divide":
result = a / b;
break;
case "subtract":
result = a - b;
break;
case "modulo":
result = a ％ b;
break;
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;
}
}

减法实现Subtraction.java

public class Subtraction 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;
}
}

除法实现Division.java

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

求余实现Modulo.java

public class Modulo 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;
}
},
SUBTRACT {
@Override
public int apply(int a, int b) {
 return a - b;
}
},
DIVIDE {
@Override
public int apply(int a, int b) {
 return a / b;
}
},
MODULO {
@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);
}

3.4 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

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。

来源：https://segmentfault.com/a/1190000022604899