home Puzzling Through Erasure III

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2004/09/26 23:17 EST (via web):
Two points:

  1. When Neal suggested that you declare the array as an 'Object[]' then I think he meant that rep returns an 'Object[]' not a 'T[]', thats my reading of his comments anyway. The reason is that you will be less surprised by the ClassCastException particularly since the documentation presumably says that it returns an array of 'Object's and not 'T's.
  2. The method suggested of using java.lang.reflect.Array.newInstance( int ) works OK for an array because all arrays have a one-argument constructor. However in general you can't rely on a type having a no-argument constructor, i.e. java.lang.Class.newInstance() might fail. The general solution is to make an abstract-instance-factory method in GenericArray, i.e. 'public abstract T[] instance( int size );', and thus make GenericArray abstract. Then to use GenericArray you must sub-class it and provide a correctly typed instance-factory method, for e.g.: 
    abstract class GenericArray< T > {
    private final T[] array;
    public GenericArray( final int size ) {
        array = instance( size );
    }
    public abstract T[] instance( final int size );
    public void set( final int index, final T value ) {
        array[ index ] = value;
    }
    public T get( final int index ) {
        return array[ index ];
    }
    public T[] getArray() {
        return array;
    }
}
public class IntegerArray extends GenericArray< Integer > {
    public IntegerArray( final int size ) {
        super( size );
    }
    public Integer[] instance( final int size ) {
        return new Integer[ size ];
    }
    public static void main( final String[] notUsed ) {
        final int size = 3;
        final IntegerArray ia = new IntegerArray( size );
        for ( int i = 0; i < size; i++ ) ia.set( i, i );
        for ( int i = 0; i < size; i++ ) System.out.print( ia.get( i ) + " " );
        System.out.println();
        final Integer[] pia = ia.getArray();
        for ( int i = 0; i < size; i++ ) System.out.print( pia[ i ] + " " );
        System.out.println();
    }
}

a

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