public double get ( int i , int j ) { if ( i < 0 || i >= N ) throw new IllegalArgumentException ( "Illegal index " + i + " should be > 0 and < " + N ) ; if ( j < 0 || j >= N ) throw new IllegalArgumentException ( "Illegal index " + j + " should be > 0 and < " + N ) ; return rows [ i ] . get ( j ) ; }
access a value at i j