JAVA 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!

以下是Java代码实现：

（1）Operation抽象类

public abstract class Operation {
    protected double numberA;
    protected double numberB;
    
    public abstract double getResult();
}

（2）Operation子类

public class OperationAdd extends Operation {
    public double getResult() {
        return numberA + numberB;
    }
}

public class OperationSub extends Operation {
    public double getResult() {
        return numberA - numberB;
    }
}

public class OperationMul extends Operation {
    public double getResult() {
        return numberA * numberB;
    }
}

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

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

（3）OperationFactory类

public class OperationFactory {
    public static Operation createOperate(String operate) {
        Operation operation = null;
        
        switch (operate) {
            case "+":
                operation = new OperationAdd();
                break;
            case "-":
                operation = new OperationSub();
                break;
            case "*":
                operation = new OperationMul();
                break;
            case "/":
                operation = new OperationDiv();
                break;
            case "avg":
                operation = new OperationAvg();
                break;
            default:
                throw new IllegalArgumentException("Invalid operator!");
        }
        
        return operation;
    }
}

（4）测试类

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.print("请输入表达式：");
        double numberA = scanner.nextDouble();
        String operator = scanner.next();
        double numberB = scanner.nextDouble();
        
        Operation operation = OperationFactory.createOperate(operator);
        operation.numberA = numberA;
        operation.numberB = numberB;
        
        double result = operation.getResult();
        System.out.printf("Result:%.3f", result);
    }
}

注意：以上代码需要在同一个包中，并且运行时需要手动输入数据和运算符。