关于堆、栈、引用

class DataWarp{
    int a;
    int b;
}

public class ReferenceTransferTest {

    public static void swap(DataWarp dw){
        int temp = dw.a;
        dw.a = dw.b;
        dw.b = temp;
        System.out.println(dw);
        System.out.println(dw.a + "," + dw.b);
        dw = null;
        
    }
   
    public static void main(String[] args){
        DataWarp dw = new DataWarp();
        dw.a = 6;
        dw.b = 9;
        swap(dw);
        System.out.println(dw.a + "," + dw.b);
        System.out.println(dw);
    }
}

输出结果为什么是9，6和9，6，而不是9，6和6，9?