1.簡介

if判斷語句是很多編程語言的重要組成部分。但是，若我們最終編寫了大量嵌套的if語句，這將使得我們的代碼更加復雜和難以維護。

讓我們看看能否使用別的方式來做呢。

設計模式是為了更好的代碼重用性，可讀性，可靠性，可維護性，它有六大原則:

單一職責原則（Single Responsibility Principle，簡稱SRP）:該原則是針對類來說的，即一個類應該只負責一項職責. 開放--封閉原則（The Open-Closed Principle簡稱OCP）:是說軟件實體（類、模塊、函數等等）應該可以擴展，但是不可以修改。 依賴倒轉原則（Dependence Inversion Principle :針對接口編程，不要對實現編程 里氏代換原則（Liskov Substitution Principle，簡稱LSP）:里氏代換原則，子類型必須能夠替換掉他們的父類型 迪米特法則（Law of Demeter）:如果兩個類不必彼此直接通信，那么這兩個類就不應當發生直接的相互作用 合成/聚合復用原則（Composition/Aggregation Principle]，簡稱CARP）:盡量使用合成/聚合，盡量不使用類繼承。合成聚合是“has a”的關系，而繼承是“is a”的關系。

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.重構

3.1 工廠方式重構

抽象層Operation.java

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

加法實現Addition.java：

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

減法實現Subtraction.java

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

乘法實現Multiplication.java

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

除法實現Division.java

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

求余實現Modulo.java

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

工廠類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 枚舉方式重構

枚舉實現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);}

封裝Operator到Calculator.java

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

使用示例

@Testpublic 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();}

實現類

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; }}

其它略包裝

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

測試demo

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

3.4 規則引擎重構

抽象規則

public interface Rule { boolean evaluate(Expression expression); Result getResult();}

實現規則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：返回結果

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

表達式

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; }}

規則引擎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(); }}

測試demo

@Testpublic 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.比較

重構方式 SRP OCP DIP LSP LD CARP IF/ELSE N N N N N N 工廠方法 Y Y Y Y Y Y 枚舉方法 N Y Y Y Y Y 命令模式 Y Y Y Y Y Y 規則引擎 Y Y Y Y Y Y

5.小結

　為了更好的代碼重用性，可讀性，可靠性，可維護性，我們會嘗試將IF/ELSE或者case-switch進行改造，使用工廠方法，枚舉方法，命令模式，規則引擎方式不同方法進行嘗試，最后使用設計模式的六大原則對代碼進行評估。

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