用java写一个能计算俩个数的加减乘除的程序

在Java中，你可以创建一个名为`Calculator`的类，然后定义一个抽象方法`performOperation`，让各个具体的运算类来实现这个方法。这是一个简单的例子：

abstract class Calculator {
    public abstract double calculate(double num1, double num2, String operation);
}

class AddCalculator extends Calculator {
    @Override
    public double calculate(double num1, double num2, String operation) {
        if ("add".equals(operation)) {
            return num1 + num2;
        }
        throw new IllegalArgumentException("Unsupported operation for addition.");
    }
}

class SubtractCalculator extends Calculator {
    @Override
    public double calculate(double num1, double num2, String operation) {
        if ("subtract".equals(operation)) {
            return num1 - num2;
        }
        throw new IllegalArgumentException("Unsupported operation for subtraction.");
    }
}

class MultiplyCalculator extends Calculator {
    @Override
    public double calculate(double num1, double num2, String operation) {
        if ("multiply".equals(operation)) {
            return num1 * num2;
        }
        throw new IllegalArgumentException("Unsupported operation for multiplication.");
    }
}

class DivideCalculator extends Calculator {
    @Override
    public double calculate(double num1, double num2, String operation) {
        if ("divide".equals(operation) && num2 != 0) {
            return num1 / num2;
        } else {
            throw new IllegalArgumentException("Cannot divide by zero.");
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Calculator add = new AddCalculator();
        Calculator subtract = new SubtractCalculator();
        Calculator multiply = new MultiplyCalculator();
        Calculator divide = new DivideCalculator();

        double resultAdd = add.calculate(5, 3, "add");
        double resultSubtract = subtract.calculate(5, 3, "subtract");
        double resultMultiply = multiply.calculate(5, 3, "multiply");
        double resultDivide = divide.calculate(5, 3, "divide");

        System.out.println("Add: " + resultAdd);
        System.out.println("Subtract: " + resultSubtract);
        System.out.println("Multiply: " + resultMultiply);
        System.out.println("Divide: " + resultDivide);
    }
}

在这个例子中，用户可以在`main`函数中选择不同的计算器对象来进行相应的操作。