Question

I am considering using the Optional< T > class in Guava library to handle a matrix (2D grid) of objects, while avoiding the use of null references to denote empty cells.

I am doing something like:

class MatrixOfObjects() {
    private Optional<MyObjectClass>[][] map_use;

    public MatrixOfObjects(Integer nRows, Integer nCols) {
        map_use = (Optional<MyObjectClass>[][]) new Optional[nRows][nCols];
        // IS THIS CAST THE ONLY WAY TO CRETE THE map_use INSTANCE?
    }

    public MyObjectClass getCellContents(Integer row, Integer col) {
         return map_use[row][col].get();
    }

    public void setCellContents(MyObjectClass e, Integer row, Integer col) {
         return map_use[row][col].of(e);
         // IS THIS THE CORRECT USE OF .OF METHOD?
    }

    public void emptyCellContents(Integer row, Integer col) {
         map_use[row][col].set(Optional.absent());
         // BUT SET() METHOD DOES NOT EXIST....
    }

    public Boolean isCellUsed(Integer row, Integer col) {
         return map_use[row][col].isPresent();
    }
}

I have three questions about the code above:

1. How to create an instance of the Array of Arrays of Optional?
2. How to assign a MyObjectClass object to a cell (this should be correct I think)
3. How to assign to “empty” a cell such that it does not contains a reference anymore?

I think I am missing something essential about this Optional class.

Thanks

Answer 1

I fixed a few errors in your code, and added comments to explain:

class MatrixOfObjects { // class declaration should not have a "()"
    private Optional<MyObjectClass>[][] map_use;

    public MatrixOfObjects(Integer nRows, Integer nCols) {
        map_use = (Optional<MyObjectClass>[][]) new Optional[nRows][nCols];
    }

    public MyObjectClass getCellContents(Integer row, Integer col) {
         return map_use[row][col].get();
    }

    public void setCellContents(MyObjectClass e, Integer row, Integer col) {
         // removed "return" keyword, since you don't return anything from this method
         // used correct array assignement + Optional.of() to create the Optional
         map_use[row][col] = Optional.of(e); 
    }

    public void emptyCellContents(Integer row, Integer col) {
         // unlike lists, arrays do not have a "set()" method. You have to use standard array assignment
         map_use[row][col] = Optional.absent();
    }

    public Boolean isCellUsed(Integer row, Integer col) {
         return map_use[row][col].isPresent();
    }
}

Here are a few alternatives to create a generic array: How to create a generic array in Java?

Note that it’s hard to use arrays and generics together if you don’t have a good understanding of how Java treats generics. Using collections is often a better approach.

All that said, I would use Guava’s Table interface instead of your “MatrixOfObjects” class.