 <p><code>map[i][j]</code> needs to be set to a new object, like <code>map[i][j] = new Node('a')</code> (well, if you had a constructor in <code>Node</code> which worked like that: it would be written <code>Node(char a) { this.content = a; }</code>).</p> <p>You cannot do <code>map[i][j].setContent('a')</code> because it is not a preexisting <code>Node</code> object.</p>