Wrapper Classes

To put an int into an ArrayList, put the int inside of an Integer object. Now the object can be put into an ArrayList. The following program builds a list of integers and then writes them out.

import java.util.* ;
public class WrapperExample
{
  public static void main ( String[] args)
  {
    ArrayList<Integer> data = new ArrayList<Integer>();

    data.add( new Integer(1) );
    data.add( new Integer(3) );
    data.add( new Integer(17) );
    data.add( new Integer(29) );

    for ( Integer val : data )
      System.out.print( val + " " );

    System.out.println( );
  }
}

The program writes out:

1 3 17 29

The picture emphasizes that the ArrayList contains an array of object references, as always. It shows the ints each contained in a little box that represents the wrapper object.