package com.lewang;

import java.util.HashMap;
import java.util.Map;

abstract class Operator {
    public final String op;

    public Operator(String op) {
        this.op = op;
    }

    public double op(String lhs, String rhs) {
        double a = Double.parseDouble(lhs);
        double b = Double.parseDouble(rhs);
        return opImpl(a, b);
    }

    protected abstract double opImpl(double lhs, double rhs);
}

class OperatorAdd extends Operator {
    public OperatorAdd() {
        super("+");
    }

    @Override
    public double opImpl(double lhs, double rhs) {
        return 0;
    }
}

class OperatorSub extends Operator {
    public OperatorSub() {
        super("-");
    }

    @Override
    public double opImpl(double lhs, double rhs) {
        return 0;
    }
}

class Calculator {
    private Map<String, Operator> operatorMap = new HashMap<>();

    public Calculator() {
    }

    public Calculator addOperator(Operator operator) {
        this.operatorMap.put(operator.op, operator);
        return this;
    }

    public double calculate(String op, String lhs, String rhs) throws IllegalArgumentException {
        if (!this.operatorMap.containsKey(op)) throw new IllegalArgumentException("unknown operator");
        return this.operatorMap.get(op).op(lhs, rhs);
    }
}

public class Main {
    public static void main(String[] args) {
        Calculator calculator = new Calculator();
        calculator.addOperator(new OperatorAdd() {
            @Override
            public double opImpl(double lhs, double rhs) {
                return lhs + rhs;
            }
        }).addOperator(new OperatorSub() {
            @Override
            public double opImpl(double lhs, double rhs) {
                return lhs - rhs;
            }
        });
        System.out.println(calculator.calculate("+", "1.0", "2.0"));
    }
}

《面向对象的计算器》