由于某些原因连接了2个分离的对象

Question

This is my homework(assignment). I really hope someone can help me figure out what I did wrong.

When I tried to create an object of State state = new State(tablesize, tb);

and then tried to make a copy State st2 = new State(state);

and then tried to modify the data in state state.placeNumber(1,1,3);

for some reason, the data in st2 is also changed.

Below is the code. I really hope someone can point out where my mistake is.

Thanks

public class State 
{

private int arraysize;
private int lastfilledx;
private int lastfilledy;
private int table[][];

//constructor

    public State()
    {

    }
    public State(State s)
    {
        arraysize = s.getsize();
        lastfilledx = s.lastindex_x();
        lastfilledy = s.lastindex_y();
        table = s.gettable();
    }
    public State(int size, int[][] tb)
    {
        arraysize = size;
        table = new int[size][size];        
        //copy the initial table which is a 2d array
        table = tb;     
        for (int i = 0 ; i < size; i++)
        {
            for(int j = 0 ; j < size ; j++)
            {
                 if ( table[i][j] == 1)
                 {

                    lastfilledx = i; 
                    lastfilledy =j;                   
                    break;
                 }
            }
        }
    }

public void placeNumber(int i, int j, int nextvalue)
{
    lastfilledx = i; 
    lastfilledy = j;    
    table[i][j] = nextvalue;
}   

public void printoutput()
{
        for (int i=0; i < arraysize; i++)
        {
            for (int j=0; j < arraysize; j++)
                System.out.print("  " + table[i][j]);

            System.out.println("");
        }

        System.out.println("last fill " + lastfilledx + " " + lastfilledy);
}

public int[][] gettable()
{
    return table;
}

public int getsize()
{
    return arraysize;
}
public int lastindex_x()
{
    return lastfilledx;
}
public int lastindex_y()
{
    return lastfilledy;
}
}




public class Dikuho extends State
{
    private static State state;

public static void main(String[] args) {

    int tablesize = 3;
    int tb[][] = new int[tablesize][tablesize];

    /*****HARDCODE the table data***/
    for (int i=0; i < tablesize; i++)
        for (int j=0; j < tablesize; j++)
            tb[i][j] = 0;


    //test case for 3x3
    tb[2][2] = 1;
    tb[0][0] = tablesize*tablesize;
    tb[0][1] = 7;
    tb[1][0] = 8;
    tb[2][1] = 2;       

    //initialize the state
    state = new State(tablesize, tb);


    **//Here is where the problem is. I only change the data in state but the data in st2 is also changed. I'm not sure what happen here.**
    State st2 = new State(state);   
    state.placeNumber(1,1,3);

    st2.printoutput();    **//These both printout same output which is not correct**    
    state.printoutput();
    }


}

Answer 1

Your copy constructor has made a shallow copy of table 2D array. Both the original object and the copy refer to the same original array, because you assign the array reference from the original to the new object. That's fine for the int values, because the values are copied. But that's not okay for objects, for which references to the obejct are copied.

Instead of just copying the reference to the array...

table = s.gettable();

You'll need to create a new, copied array:

table = new int[arraysize][arraysize];
// 2 nested for loops here to copy the contents

Answer 2

public int[][] gettable()
{
    return table;
}

So, both of your state objects are referencing the same array. you need to create a new array for each instance.