去除if-else，讓你的代碼更優雅

01 臃腫示例

1.1 if-elsepublic 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;
}
1.2 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;
}

02 重構

2.1 工廠方式重構

（1）抽象接口 Operation.java

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

（2）加法實現 Addition.javapublic class Addition implements Operation {
@Override
public int apply(int a, int b) {
return a + b;
}
}

（3）減法實現 Subtraction.javapublic class Subtraction implements Operation {
@Override
public int apply(int a, int b) {
return a - b;
}
}

（4）乘法實現 Multiplication.javapublic class Multiplication implements Operation {
@Override
public int apply(int a, int b) {
return a*b;
}
}

（5）除法實現 Division.javapublic class Division implements Operation {
@Override
public int apply(int a, int b) {
return a / b;
}
}

（6）求餘實現 Modulo.javapublic class Modulo implements Operation {
@Override
public int apply(int a, int b) {
return a % b;
}
}

（7）工廠類 OperatorFactory.javaimport 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));
}
}

（8）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);
}

2.2 枚舉方式重構

（1）枚舉實現Operator.javapublic 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);
}

（2）封裝Operator到Calculator.java public int calculate(int a, int b, Operator operator) {
return operator.apply(a, b);
}

（3）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);
}

2.3 命令模式

（1）抽象的接口

public interface Command {
 Integer execute();
}

（2）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;
 }
}

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

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

2.4 規則引擎重構

（1）抽象規則

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

（2）實現規則AddRule.javapublic 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;
}
}

（3）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;
}
}

（4）規則引擎RuleEngine.javaimport 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();
}
}

（5）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());
}

03 小結

爲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，最後使用設計模式的六大原則對代碼進行評估。