Question

I’m trying to create a 2 dimensional array that is fairly large and I keep getting the error “java.lang.OutOfMemoryError: Java heap space”. I’ve tried the -Xmx command and I still get it.

Could there be an alternative I could use to 2D arrays? Or a way to increase heap size that works.

Answer 1

If not all elements of the matrix are occupied (or even share a very common default value), you can use a sparse matrix.

One of the simplest methods for doing this is using a HashMap:

Map<Point, Integer> matrix = new HashMap<Point, Integer>();

matrix.put(new Point(5, 2), 7);
Integer fiveTwo = matrix.get(new Point(5, 2));

You can iterate over all indices like this:

for ( int i = 0; i < ROW_COUNT; i++ ) {
    for ( int j = 0; j < COL_COUNT; j++ ) { 
       Integer val = matrix.get(new Point(i, j));
       //val is null here if not stored in the matrix.  Replace with a different
       //default as desired.  eg:
       //if ( val == null ) val = 0;
       process(val);
    }
}    

Iterating over only occupied indices is also easily possible:

for ( Entry<Point, Integer> entry : matrix ) {
    process(entry.getValue());
}

Though this may return some cells that have been explicitly set to the default. What is more difficult with this implementation is iterating over only occupied indices in a specific order.

This is an example of a “dictionary of keys” implementation as described in the Wikipedia article linked above.