Plus for the isValid method you should just use the method that is in BoundedGrid, which is what is below. You don't have to have that long if else statement. The computer will check what is below itself, and whether that is true or false that is what it will return.

public boolean isValid(Location loc)
    {
        return 0 <= loc.getRow() && loc.getRow() < getNumRows()
                && 0 <= loc.getCol() && loc.getCol() < getNumCols();
    }
 

also in your isValid method you have equals signs next to the less than row and col part you should remove those equals signs the equals for the 0 part though are suppose to stay.

First off I copied everything from the resource file for gridworld to my own packages and classes, but kept the same names. First thing I recommend is keep the Tester in the default package, but move the other two to info.gridworld.grid package. The second thing I did was I made a copy of the image BoundedGrid.gif and changed the copies name to SparseGrid.gif, so that instead of null appearing next to what is the SparseGrid command in the menu there is an image of a grid next to that command. The third thing I noticed is that for the UnboundedGrid and BoundedGrid they both use 2 values for the thing that holds the data, while yours only uses one. I'm currently working on changing the code, so I have yet to find the complete solution, but those three things could help.

Also I moved the addcommand for the Sparse grid directly into the World Class and below is what that looks like

 
public World(Grid<T> g){
    gr = g;
    gridClassNames = new TreeSet<String>();
    occupantClassNames = new TreeSet<String>();
    addGridClass("info.gridworld.grid.BoundedGrid");
    addGridClass("info.gridworld.grid.UnboundedGrid");
    addGridClass("info.gridworld.grid.SparseGrid");
}