java面向对象报错

Question

import java.awt.dnd.DropTarget;

public class ValueTransferTest {
    public static void main(String[] args) {
        Data data=new Data();
        data.m=10;
        data.n=20;
        ValueTransferTest test=new ValueTransferTest();
        test.swap(data);
    }
    public void swap(Data data){
            int temp=data.m;
            data.m=data.n;
            data.n=temp;
        }
class Data{
    int m;
    int n;
    }
}

![img](https://img-mid.csdnimg.cn/release/static/image/mid/ask/590186897056156.png "#left")

Answer 1

public static void main(String[] args) {
        ValueTransferTest test=new ValueTransferTest();
        Data data=test.new Data();
        data.m=10;
        data.n=20;
        test.swap(data);
    }