ClassCastException when casting Object[] array to generic type array in Java

Question

Hi I'm very new to Java and in this code, I think I'm not creating the Bag correctly in the Main? Please help thanks!

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable; at mid.Bag.(Bag.java:12) at mid.Bag.main(Bag.java:91)

        public class Bag<T extends Comparable<T>> implements Iterable<T> {
      private int MAX_ITEMS = 10; // initial array size
      private int size;
      private T[] data;

      public Bag( ) {
        data = (T []) new Object[MAX_ITEMS];
        size = 0;
      }

      public void add(T newItem) {
        // check if it's full, then extend (array resizing)
        if (size == data.length) {
          T[ ] temp = (T [ ] ) new Object[data.length*2];
          for (int i = 0; i < size; ++i)
            temp[i] = data[i];
          // reassign data to point to temp
          data = temp;
        }
        // then do the assignment
        data[size++] = newItem; // assign newItem in the next-available slot
      }

public Iterator<T> iterator() {
    return new BagIterator();
  }

 /***************************
  * nested class BagIterator
  ***************************/
   class BagIterator implements Iterator<T> {
    // instance member
    private int index;

    // (0) constructor
    public BagIterator() {
      index = 0;
    }
    // (1)
    public boolean hasNext() {
      return (index < size); // size in the outer Bag<E>
    }
    // (2)
    public T next() {
      /*
      T temp = data[index]; // save the element value
      index++; // increment index
      return temp;
      */
      return data[index++];
    }
      public static void main(String[ ] args) {
          Bag<String> bag1=new Bag<String>();

          bag1.add("good");
          bag1.add("fortune");
          bag1.add("billionarie");
          for (String x: bag1)
              System.out.println(x);

      }

You need to update your code for iterator() as the code does not compile. — akhil_mittal
– akhil_mittal, Commented May 7, 2015 at 9:32

Jon Skeet · Accepted Answer · 2015-05-07 09:17:17Z

7

Yes, you're creating an Object[] and then trying to cast it to T[], which the compiler is converting to a cast to Comparable[] (using the raw Comparable type) due to your constraint on T.

Arrays and generics don't work terribly nicely together, basically.

It would probably be simpler to make your data field just an Object[] and cast individual values where necessary.

answered May 7, 2015 at 9:17

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Robert Over a year ago

Hi Jon, thanks for your reply, is there a way to keep my field and adjust my bag type in the main?

Tagir Valeev Over a year ago

@Robert, it would be nice to see your iterator() method to answer to this. Please add it to your question.

Tagir Valeev Over a year ago

So you just need to use return (T)data[index++]; in your next() method and keep the data as Object[]. Your main method will not notice anything. Also it's easier to use ((List<T>)Arrays.asList(data)).iterator() instead of implementing your own iterator.

Jon Skeet Over a year ago

@Robert: I don't really know what you mean by that... and your response of "it's not working" to Tagir doesn't really give enough information to help you. I suspect you should create a new question for your ongoing problems - your initial question was about a ClassCastException, and that's now solved. Stack Overflow is geared towards one question per post - don't forget that we're trying to create a repository of information for future readers, not just solving all your problems.

khelwood · Accepted Answer · 2015-05-07 09:21:12Z

2

Here:

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

you are constructing an Object array and trying to cast it to T[]. But you have declared that T inherits from Comparable. So use:

data = (T []) new Comparable[MAX_ITEMS];

answered May 7, 2015 at 9:21

khelwood

59.7k14 gold badges91 silver badges116 bronze badges

2 Comments

Robert Over a year ago

Hi khelwood, thanks for your reply, is there a way to keep my field and adjust my bag type in the main?

khelwood Over a year ago

You can still keep your field. Just construct it as an array of Comparable instead of an array of Object. Or take out <T extends Comparable<T>> and replace it with <T>.

akhil_mittal · Accepted Answer · 2015-05-07 09:30:37Z

0

You can probably rewrite your constructor as well:

public Bag(Class<T> c, int s) {
    // Use Array native method to create array of a type only known at run time
    @SuppressWarnings("unchecked")
    final T[] dataArray = (T[]) Array.newInstance(c, s);
    this.data = dataArray;
}

Then you can use it like:

Bag<String> bag1 = new Bag<>(String.class,10);

That should also work, IMO. The instances of T must be comparable in any case.

answered May 7, 2015 at 9:30

akhil_mittal

24.3k8 gold badges99 silver badges97 bronze badges

Collectives™ on Stack Overflow

ClassCastException when casting Object[] array to generic type array in Java

3 Answers 3

4 Comments

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related