Generic Array

The constructor of ArrayIndexedList must instantiate and initialize data, the generic array. In a perfect world, this would be identical to using any other (primitive or reference) type:

public ArrayIndexedList(int size, T defaultValue) {
  data = new T[size];
  for (int i = 0; i < size; i++) {
    data[i] = defaultValue;
  }
}

But the statement new T[size] will not work! It results in a compiler error. You must instead do this:

data = (T[]) new Object[size];

So, you must create an array of Object and then cast it to a generic array. This latter strategy is a workaround to create generic arrays. However, it results in a compilation warning (Unchecked cast: java.lang.Object[] to T[]), which you may ignore.

The reasons why Java cannot create a generic array (as in new T[size]) is irrelevant to the study of data structures. (It has to do with how Java's compiler works).

Aside: Here is another syntax for creating generic arrays:

data = (T[]) Array.newInstance(Object.class, size);

There is no preference in using either syntax. The Array.newInstance under the hood does the same thing as the syntax presented earlier.