问题

我有一个任务

创建一个名为 Math 的类，具有以下方法：
int calculate(int a, int b) - 执行计算操作
以及
int returnPrev() - 返回之前计算的结果。


创建一个名为 Addition 的类来表示加法操作。重写这些方法。
我已经创建了一个抽象类 Math

```java
    abstract class Math {
        abstract int calculate(int a, int b);
        abstract int returnPrev();
    }
    
    
    public class Addition extends Math {
        int result;
        int prevResult;
    
        
        @Override
        int calculate(int a, int b) {
            result = a + b;
            prevResult = result; // 保存上一次计算的结果
            return result;
        }
        
        @Override
        int returnPrev() {
            return prevResult;
        }
    }

对于我来说，不清楚如何创建 returnPrev 方法？我不明白应该如何保存第一次的结果。

对于任何解释，我会非常感激

<details>
<summary>英文:</summary>

I have a task

Create a class Math with methods:
int calculate (int a, int b) - the operation itself
and
int returnPrev () - returns the previous calculated result.


Create class Addition to represent addition operations. Override the methods.
I&#39;ve created abstract class Math

abstract class Math {
    abstract int calculate(int a,int b);
    abstract int returnPrev();
}


public class Addition extends Math {
    int result;
    int prevresult;

    
    @Override
    int calculate(int a, int b) {
       result = a + b;
        return result   }
    @Override
    int returnPrevious() {
     }
}

For me is unclear how should i create a method returnPrevious ?? I do not understand how should i save the first result.



For any explanation would be very grateful





</details>


# 答案1
**得分**: 0

```java
public class Addition extends Math {
    int result;
    int prevresult;

    @Override
    int calculate(int a, int b) {
        this.prevresult = result;
        result = a + b;
        return result;
    }

    @Override
    int returnPrev() {
        return this.prevresult;
    }
}

public class Addition extends Math {
    int result;
    int prevresult;


    @Override
    int calculate(int a, int b) {
        this.prevresult = result;
        result = a + b;
        return result ;
    }

    @Override
    int returnPrev() {
        return this.prevresult;
    }

}

答案2

得分: 0

1. 不要使用原生的名称，例如，你应该使用 MyMath 来避免与原生的 Math 发生冲突。
2. 最好将 MyMath 声明为一个接口。请注意，一个类可以实现多个接口，但只能继承一个类。
3. 在将新计算的值放入 result 之前，将 result 的值赋给 prevresult。

interface MyMath {
    public int calculate(int a, int b);
    public int returnPrev();
}

class Addition implements MyMath {
    int result;
    int prevresult;

    @Override
    public int calculate(int a, int b) {
        prevresult = result;
        result = a + b;
        return result;
    }

    @Override
    public int returnPrev() {
        return prevresult;
    }
}

public class Demo {
    public static void main(String[] args) {
        Addition addition = new Addition();
        System.out.println(addition.calculate(10, 20));
        System.out.println(addition.returnPrev());
        System.out.println(addition.calculate(20, 30));
        System.out.println(addition.returnPrev());
    }
}

输出:

30
0
50
30

如有任何疑问或问题，请随时提问。

interface MyMath {
	public int calculate(int a, int b);
	public int returnPrev();
}

class Addition implements MyMath {
	int result;
	int prevresult;

	@Override
	public int calculate(int a, int b) {
		prevresult = result;
		result = a + b;
		return result;
	}

	@Override
	public int returnPrev() {
		return prevresult;
	}
}

public class Demo {

	public static void main(String[] args) {
		Addition addition = new Addition();
		System.out.println(addition.calculate(10, 20));
		System.out.println(addition.returnPrev());
		System.out.println(addition.calculate(20, 30));
		System.out.println(addition.returnPrev());
	}
}

30
0
50
30