3．编程实现运算类—工厂设计模式 【问题描述】 (1)定义抽象类Operation，有double型数据成员numberA、numberB，有抽象方法getResult() (2)定义Operation的4个子类，分别实现加、减、除运算，二个数的平均数运算 (3)定义类OperationFactory：有静态方法Operation createOperate(String operate); 根据形参operate的值创建相应加、减、除、平均值类的对象，赋给抽象类引用后返回 (4)定义测试类及主方法：用户从键盘输入运算数及运算符，根据运算符调用OperationFactory类的静态方法，创建相应实例，设置运算数，输出运行结果 【输入形式】 一个表达式（数据与运算符之间空格间隔） 【输出形式】 表达式运算的结果，保留3位小数 【样例输入】 表4.3实验题目3测试用例 序号 样例输入 样例输出 1 45.8 + 80.9 Result:126.700 2 100 / 3 Result:33.333 3 123 / 0 Devide by zero!

抽象类Operation的代码实现：

abstract class Operation {
    protected double numberA;
    protected double numberB;

    public void setNumberA(double numberA) {
        this.numberA = numberA;
    }

    public void setNumberB(double numberB) {
        this.numberB = numberB;
    }

    public abstract double getResult();
}

加、减、除、平均值类的代码实现：

class AddOperation extends Operation {
    @Override
    public double getResult() {
        return numberA + numberB;
    }
}

class SubtractOperation extends Operation {
    @Override
    public double getResult() {
        return numberA - numberB;
    }
}

class DivideOperation extends Operation {
    @Override
    public double getResult() {
        if (numberB == 0) {
            throw new IllegalArgumentException("Devide by zero!");
        }
        return numberA / numberB;
    }
}

class AverageOperation extends Operation {
    @Override
    public double getResult() {
        return (numberA + numberB) / 2;
    }
}

OperationFactory类的代码实现：

public class OperationFactory {
    public static Operation createOperate(String operate) {
        Operation operation = null;
        switch (operate) {
            case "+":
                operation = new AddOperation();
                break;
            case "-":
                operation = new SubtractOperation();
                break;
            case "/":
                operation = new DivideOperation();
                break;
            case "avg":
                operation = new AverageOperation();
                break;
        }
        return operation;
    }
}

测试类及主方法的代码实现：

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double a = scanner.nextDouble();
        String operate = scanner.next();
        double b = scanner.nextDouble();
        Operation operation = OperationFactory.createOperate(operate);
        operation.setNumberA(a);
        operation.setNumberB(b);
        try {
            double result = operation.getResult();
            System.out.printf("Result:%.3f", result);
        } catch (IllegalArgumentException e) {
            System.out.println(e.getMessage());
        }
    }
}

这里的实现使用了工厂模式，根据输入的运算符创建相应的运算类对象，然后设置运算数并计算结果。如果是除法运算时，如果除数为0则会抛出IllegalArgumentException异常。输出结果时保留3位小数。