消除 if else 判断

    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;
    }

    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;
    }

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

加法实现Addition.java：

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

减法实现Subtraction.java

public class Subtraction implements Operation {

        @Override 
　　　　 public int apply(int a, int b) {
                return a - b;
        }
}

乘法实现Multiplication.java

public class Multiplication implements Operation {

        @Override
　　　　 public int apply(int a, int b) {
                return a*b;
        }
}

除法实现Division.java

public class Division implements Operation {

        @Override
　　　　 public int apply(int a, int b) {
                return a / b;
        }
}

求余实现Modulo.java

public class Modulo implements Operation {

        @Override
　　　　　public int apply(int a, int b) {
                return a % b;
        }
}

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));
    }
}

public int calculateUsingFactory(int a, int b, String operator) {
    Operation targetOperation = OperatorFactory
      .getOperation(operator)
      .orElseThrow(() -> new IllegalArgumentException("Invalid Operator"));
    return targetOperation.apply(a, b);
}

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

@Test
public void whenCalculateUsingEnumOperator_thenReturnCorrectResult() {
    Calculator calculator = new Calculator();
    int result = calculator.calculate(3, 4, Operator.valueOf("ADD"));
    assertEquals(7, result);
}

public interface Command {
    Integer execute();
}

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;
    }
}

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

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

public interface Rule {

    boolean evaluate(Expression expression);

    Result getResult();
}

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);
    }
}

public class Result {
    int value;

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

    public int getValue() {
        return value;
    }
}

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;
        }
}

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();
    }
}

@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());
}