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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