produce a matrix table using toString()

I need to display Matrix tables using toString(). My code only displays the value in the last column of the last row. I cant work my way through it. It needs to come out like this:
1 2 3
4 5 6
7 8 9
It must have spaces after each digit and "\n" after each row, and has to be in a String toString() method. This is what I have so far:

public class Matrix {
	int numRows;
	int numColumns;
	int data[][];
	@Override // instruct the compiler that we do indeed intend for this method to override the superclass' (Object) version
	public String toString()
	{
		String output = null;
		/*
		 * TODO: replace the below return statement with the correct code, you must return a String that represents this 
		 * 			matrix, as specified on the assignment page
		 */
		for (int i=0; i<numRows; i++)
		
			for (int j=0; j<numColumns; j++)
				output = data[i][j] + " ";
			
		output +="\n";
		
		
		return output; // placeholder
	}
}