Question

I am working on an application that uses Swing. I have a JTabbedPane, and each tab is considered a ‘page.’ Each page contains 4 normal panels (I refer to them as ‘views’) that are arranged according to 2×2 GridLayout.

I want to minimize the amount of pages, so every time a view is removed, I want to re-sort all of the views across all of the pages (think two-dimensional arrays if it makes more sense) so that the views near the last page and removed from there, and are added to a page nearer the front.

Consider this example:

Object[][] array = new Object [][] {

    { new Object(), null, new Object(), new Object() },
    { null, null, new Object(), new Object() },
    { new Object(), new Object(), new Object(), new Object() }

};

How can I sort that array so that it looks more like:

Object[][] array = new Object[][] {

    { new Object(), new Object(), new Object(), new Object() },
    { new Object(), new Object(), new Object(), new Object() },
    { new Object(), null, null, null },

};

At first, I thought of using two loops, one going from 0 to array.length, and one going from array.length to 0. The idea was that: as the one going from length to 0 approaches 0, it would check whether or not the indices of the array going from 0 to length are empty. If so, it would place the non-null element in the index that contains null.

This approach gave me a headache because of all the looping so I asked a close friend of mine for a suggestion. He suggested a much more elegant solution: Arrays.sort(Object[][], Comparator).

This code was the result:

    Object[][] array = new Object[][] { { new Object(), null, new Object(), new Object() }, { null, null, new Object(), new Object() }, { new Object(), new Object(), new Object(), new Object() } };

    for (int i = 0; i < 3; i++)
    {
        for (int j = 0; j < 4; j++)
        {
            System.out.println("Before sorting: (i = " + i + " j = " + j + " null = " + (array[i][j] == null) + ")");
        }
    }

    Arrays.sort(array, new Comparator<Object>()
    {

        public int compare(Object a, Object b)
        {
            return a == null ? (b == null ? 0 : -1) : (b == null ? 1 : 0);
        }

    });

    for (int i = 0; i < 3; i++)
    {
        for (int j = 0; j < 4; j++)
        {
            System.out.println("After sorting: (i = " + i + " j = " + j + " null = " + (array[i][j] == null) + ")");
        }
    }

The output is:

Before sorting: (i = 0 j = 0 null = false)
Before sorting: (i = 0 j = 1 null = true)
Before sorting: (i = 0 j = 2 null = false)
Before sorting: (i = 0 j = 3 null = false)
Before sorting: (i = 1 j = 0 null = true)
Before sorting: (i = 1 j = 1 null = true)
Before sorting: (i = 1 j = 2 null = false)
Before sorting: (i = 1 j = 3 null = false)
Before sorting: (i = 2 j = 0 null = false)
Before sorting: (i = 2 j = 1 null = false)
Before sorting: (i = 2 j = 2 null = false)
Before sorting: (i = 2 j = 3 null = false)
After sorting: (i = 0 j = 0 null = false)
After sorting: (i = 0 j = 1 null = true)
After sorting: (i = 0 j = 2 null = false)
After sorting: (i = 0 j = 3 null = false)
After sorting: (i = 1 j = 0 null = true)
After sorting: (i = 1 j = 1 null = true)
After sorting: (i = 1 j = 2 null = false)
After sorting: (i = 1 j = 3 null = false)
After sorting: (i = 2 j = 0 null = false)
After sorting: (i = 2 j = 1 null = false)
After sorting: (i = 2 j = 2 null = false)
After sorting: (i = 2 j = 3 null = false)

Exactly the same.
I have also tried replacing the compare(Object, Object) implementation with:

        public int compare(Object a, Object b)
        {
            if (a == null && b != null)
            {
                return -1;
            }
            if (b == null && a != null)
            {
                return 1;
            }
            return 0;
        }

… and have achieved the same results.
I am kind of at a loss. This is not something that I do not have the knowledge to do, I just cannot wrap my head around how to actually create a solution for a problem like this.

I’d appreciate any help. Whichever way you prefer approaching it, the loop method or the Comparator method, I’d love to see it!

Thanks!

Answer 1

Did you mean

Object[][] array = new Object [][] { .. };

In your case, you need to convert 2-D array to array (1-D array). After sorting new array, you fill the 2-D array with the sorted array.

// convert to 1-D array
Object[] all = new Object[12];
int k = 0;
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 4; j++) {
        all[k++] = array[i][j];
    }
}

// then sort the new array
Arrays.sort(all, yourComparator);

// then fill the 2-D array with the sorted array
k = 0;
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 4; j++) {
        array[i][j] = all[k++];
    }
}