根据传入参数不同调用不同的方法：规则引擎重构

抽象规则

public interface Rule {

    boolean evaluate(Expression expression);

    Result getResult();
}

实现规则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：返回结果

public class Result {
    int value;

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

    public int getValue() {
        return value;
    }
}

表达式

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;
        }
}

规则引擎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();
    }
}

测试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());
}