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(fù)雜和難以維護。

讓我們看看能否使用別的方式來做呢。

設(shè)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

　　　　　　1)單一職責(zé)原則（Single Responsibility Principle，簡稱SRP）:該原則是針對類來說的，即一個類應(yīng)該只負責(zé)一項職責(zé).
　　　　　　2)開放--封閉原則（The Open-Closed Principle簡稱OCP）:是說軟件實體（類、模塊、函數(shù)等等）應(yīng)該可以擴展，但是不可以修改。
　　　　　　3)依賴倒轉(zhuǎn)原則（Dependence Inversion Principle?:針對接口編程，不要對實現(xiàn)編程
　　　　　　4)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
　　　　　　5)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(yīng)當(dāng)發(fā)生直接的相互作用
　　　　　　6)合成/聚合復(fù)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(guān)系，而繼承是“is ?a”的關(guān)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;}

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;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(gòu)

3.1 工廠方式重構(gòu)

抽象層Operation.java

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

加法實現(xiàn)Addition.java：

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

減法實現(xiàn)Subtraction.java

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

乘法實現(xiàn)Multiplication.java

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

除法實現(xiàn)Division.java

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

求余實現(xiàn)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(gòu)

枚舉實現(xiàn)Operator.java

public enum Operator {ADD {@Overridepublic int apply(int a, int b) {return a + b;}},MULTIPLY {@Overridepublic int apply(int a, int b) {return a * b;}},SUBTRACT {@Overridepublic int apply(int a, int b) {return a - b;}},DIVIDE {@Overridepublic int apply(int a, int b) {return a / b;}},MODULO {@Overridepublic 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

@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(); }

實現(xiàn)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;}@Overridepublic Integer execute() {return a + b;} }

其它略

包裝

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

測試demo

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

?

4.規(guī)則引擎重構(gòu)

抽象規(guī)則

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

實現(xiàn)規(guī)則AddRule.java 其它略

public class AddRule implements Rule {private int result;@Overridepublic boolean evaluate(Expression expression) {boolean evalResult = false;if (expression.getOperator() == Operator.ADD) {this.result = expression.getX() + expression.getY();evalResult = true;}return evalResult;}@Overridepublic Result getResult() {return new Result(result);} }

?

其中：返回結(jié)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(guī)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

@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.比較

重構(gòu)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(guī)則引擎	Y	Y	Y	Y	Y	Y

?

5.小結(jié)

?　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(guī)則引擎方式不同方法進行嘗試，最后使用設(shè)計模式的六大原則對代碼進行評估。

參考資料

【1】https://www.cnblogs.com/davidwang456/p/3641369.html

【2】https://www.baeldung.com/java-replace-if-statements

轉(zhuǎn)載于:https://www.cnblogs.com/davidwang456/p/10831546.html

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