Question

Goal

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I am making a Java class that will give enhanced usability to arrays, such as add, remove, and contains methods. I figured the best solution is to make a class (called ArrayPP) that has a type parameter T. This way, the user can interact with the ArrayPP object as easily as they can with an array of the same type.

Problem

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I quickly found that such methods as add will require the creation of a separate array, and end up changing the target array t from an array of Ts into an array of Objects. As you may guess, this totally destroys the usability, and when I try to do something like

File[] f = new File[0];
ArrayPP<File> appF = new ArrayPP(f);
appF.add(saveFile);
f = appF.toArray();

the program throws

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.io.File;

because the add method has to change the array into an array of Objects, as the Java compiler won’t let you make a generic array (T[] t = new T[0]; is bad, but T[] t = (T[]) new Object[0]; is okay). I know from line-by-line debugging that the above code keeps the array t, in this case, as a n array of Files UNTIL the 4th line of the add method is called. Does anyone have a solution that will keep the array t being an array of Ts and not an array of Objects?

Sample Code

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Below is a VERY watered-down version of my class.

public class ArrayPP<T>
{
  T[] t;

  /**
   * Creates a new Array++ to manage the given array.
   * <h3>Analogy:</h3>
   * <tt>ArrayPP&lt;String&gt; s = new ArrayPP(args);</tt><br/>
   * is analogous to<br/>
   * <tt>String s[] = args;</tt>
   * @param array The array to be managed
   */
  public ArrayPP(T[] array)
  {
    t = array;
  }

  /**
   * Appends a value to the end of the array
   * @param val the value to be appended
   * @return the resulting array.
   */
  public ArrayPP add(T val)
  {
    T[] temp = (T[]) new Object[t.length + 1];
    System.arraycopy(t, 0, temp, 0, t.length);
    temp[temp.length - 1] = val;
    t = (T[])temp;
    return this;
  }

  /**
   * Returns the array at the core of this wrapper
   * @return the array at the core of this wrapper
   */
  public T[] toArray()
  {
    return t;
  }
}

Possible Solution?

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After looking at other questions about generic arrays, I think I have a solution:

Instead of

  /**
   * Appends a value to the end of the array
   * @param val the value to be appended
   * @return the resulting array.
   */
  public ArrayPP add(T val)
  {
    T[] temp = (T[]) new Object[t.length + 1];
    System.arraycopy(t, 0, temp, 0, t.length);
    temp[temp.length - 1] = val;
    t = (T[])temp;
    return this;
  }

will this work?

  /**
   * Appends a value to the end of the array
   * @param val the value to be appended
   * @return the resulting array.
   */
  public ArrayPP<T> add(T val)
  {
    t = java.util.Arrays.copyOf(t, t.length + 1);
    t[t.length - 1] = val;
    return this;
  }

Answer 1

In principle you can’t easily create arrays of a generic type (or type variable).

If you have a class object, you can use reflection, or if you have an example array, the methods in the java.util.Arrays class to create a (longer/shorter) copy. But it is not elegant either way.

The ArrayList class internally simply uses an Object[] to store its elements, and converts only on get/set/add/toArray.
What would your class do better than ArrayList?

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Edit:

I would recommend either simply delegate to an ArraysList, or do the implementation like ArrayList does, using an Object[] internally, and converting on output where necessary.

If you really want to have an array of the right type internally, it is possible – but it gets ugly, as I said.

The add method is still the easiest case:

  /**
   * Appends a value to the end of the array
   * @param val the value to be appended
   * @return the resulting array.
   */
  public ArrayPP add(T val)
  {
     T[] temp = Arrays.copyOf(t, t.length+1);
     temp[t.length] = val;
     t = temp;
     return this;
  }

When you want to add in the middle or remove, you’ll have to combine this with your arraycopy.