 <p><code>double[]</code> array is object. So, you get address and entire array is added at index 0.</p> <p>You may use <code>Arrays.toString(x.get(0))</code> to get readable array print.</p> <p>toString() for an array is to print [, followed by a character representing the data type of the array's elements, followed by @ then the memory address.</p>