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 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 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

@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

@Test

public 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 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。

參考資料

總結

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