Java中大量if...else语句的消除替代方案
针对大量的if嵌套让代码的复杂性增高而且难以维护。本文将介绍多种解决方案。
案例
下面模拟业务逻辑，根据传入的条件作出不同的处理方式。
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;}return result;}
当然也可以用switch来实现。
public int calculateUsingSwitch(int a, int b, String operator) {switch (operator) {case "add":result = a + b;break;// other cases}return result;}
重构
public interface Operation {int apply(int a, int b);}
然后实现加减乘除四个方法
public class Addition implements Operation {@Overridepublic int apply(int a, int b) {return a + b;}}
然后通过操作工厂提供操作
public class OperatorFactory {static Map<String, Operation> operationMap = new HashMap<>();static {operationMap.put("add", new Addition());operationMap.put("divide", new Division());// more operators}public static Optional<Operation> getOperation(String operator) {return Optional.ofNullable(operationMap.get(operator));}}调用
public int calculateUsingFactory(int a, int b, String operator) {Operation targetOperation = OperatorFactory.getOperation(operator).orElseThrow(() -> new IllegalArgumentException("Invalid Operator"));return targetOperation.apply(a, b);}新增操作只需要维护操作工厂的operationMap即可。
2、使用枚举
public enum Operator {ADD, MULTIPLY, SUBTRACT, DIVIDE}
然而不同的操作对应的逻辑不一样，我们编写抽象方法
ADD {@Overridepublic int apply(int a, int b) {return a + b;}},// other operatorspublic abstract int apply(int a, int b);
调用时直接传入枚举值
public int calculate(int a, int b, Operator operator) {return operator.apply(a, b);}
@Testpublic void whenCalculateUsingEnumOperator_thenReturnCorrectResult() {Calculator calculator = new Calculator();int result = calculator.calculate(3, 4, Operator.valueOf("ADD"));assertEquals(7, result);}
3、命令模式
定义命令接口。
public interface Command {Integer execute();}
实现加法
public class AddCommand implements Command {// Instance variablespublic AddCommand(int a, int b) {this.a = a;this.b = b;}@Overridepublic Integer execute() {return a + b;}}
定义一个Calculator类，加入执行命令的方法。
public int calculate(Command command) {return command.execute();}
测试代码
@Testpublic void whenCalculateUsingCommand_thenReturnCorrectResult() {Calculator calculator = new Calculator();int result = calculator.calculate(new AddCommand(3, 7));assertEquals(10, result);}
4、规则引擎
定义规则接口
public interface Rule {boolean evaluate(Expression expression);Result getResult();}
实现规则引擎
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();}}定义表达式
public class Expression {private Integer x;private Integer y;private Operator operator;}
定义加法规则
public class AddRule implements Rule {@Overridepublic boolean evaluate(Expression expression) {boolean evalResult = false;if (expression.getOperator() == Operator.ADD) {this.result = expression.getX() + expression.getY();evalResult = true;}return evalResult;}}
给规则引擎传入表达式来调用
@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());}
英文原文：https://www.baeldung.com/java-replace-if-statements
其他参考资料： 策略设计模式：http://www.runoob.com/design-pattern/strategy-pattern.html
命令设计模式：http://www.runoob.com/design-pattern/command-pattern.html
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