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Thread: having problem declaring a generic type array in OrderSet class

  1. #1
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    Default having problem declaring a generic type array in OrderSet class

    guys, I have declare a generic array in OrderSet class; however, during compilation is giving me an error:

    Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable;
    	at wordset.OrderSet.<init>(OrderSet.java:17)
    	at wordset.Main.main(Main.java:20)
    Java Result: 1

    main

    public class Main {
     
        public static void main(String[] args) throws IOException
        {
            OrderSet<String> s1 = new OrderSet<>();
     
            String fileName;
            Scanner in = new Scanner(System.in);
     
            System.out.println("Enter the name of the input file");
            fileName = in.next();
     
            File inFile = new File(fileName);
     
            Scanner input;
            try {
                input = new Scanner(inFile);
                while (input.hasNext()) 
                {
                    String w = input.next();
                    s1.add(w);                
     
                }
            } catch (FileNotFoundException ex) {
               Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);            
            }
     
            s1.show();
     
        } 
     
    }

    public interface Set<T> {
     
        public void add(T x);
     
        public void show();
     
    }


    import java.util.Arrays;
     
     
    public class OrderSet<T extends Comparable> implements Set<T> {
     
        private T[] items;
        private int size;
     
        public OrderSet()
        {
            items = (T[]) new Object[5];        
        }
     
        @Override
        public void add(T s)
        {
     
            if(size >= items.length)
            {
                items = grow(items);
            }
     
            for (int i = 0; i < items.length; i++) {
                if(items[i] == null)
                {
                    items[i] = s;
                    size++;
                    break;
                }
            }      
     
        }
     
        @Override
        public void show()
        {
            int i = 0;
            while(i < size)
            {            
                System.out.print(items[i]+", ");
                i++;
            }
        }
     
        public T[] grow(T[] a)
        {
            T[] newA = (T[]) new Object[a.length+5];
            System.arraycopy(a, 0, newA, 0, a.length);
            return newA;
        }
     
    }
    Last edited by mia_tech; July 10th, 2012 at 12:30 PM.


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    Default Re: having problem declaring a generic type array in OrderSet class

    I don't understand that error, but I can tell you that this isn't correct:

    OrderSet<String> s1 = new OrderSet<>();

    That you can't use empty generic parameter brackets. Better to do:


    OrderSet<String> s1 = new OrderSet<String>();

    Edit: Please ignore this drivel as it was posted in ignorance. Listen to helloworld922.
    Last edited by Fubarable; July 10th, 2012 at 06:42 PM.

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    Default Re: having problem declaring a generic type array in OrderSet class

    Quote Originally Posted by Fubarable View Post
    I don't understand that error, but I can tell you that this isn't correct:

    OrderSet<String> s1 = new OrderSet<>();

    That you can't use empty generic parameter brackets. Better to do:


    OrderSet<String> s1 = new OrderSet<String>();
    That's actually a new featured of Java 7 (type inferencing for generic object creation). See: Type Inference for Generic Instance Creation

    The problem is actually here:

    items = (T[]) new Object[5];

    You can't cast an Object[] to a T[] since Object is not a T or a subclass of T.
    Last edited by helloworld922; July 11th, 2012 at 02:34 AM.

  4. The Following User Says Thank You to helloworld922 For This Useful Post:

    Fubarable (July 10th, 2012)

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