Wrapper Classes

To put an int into an ArrayList use the Integer wrapper class and autoboxing. (See chapter 11 for a discussion of autoboxing.) 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( 1 );  // add an Integer object containing 1
    data.add( 3 );
    data.add( 17 );
    data.add( 29 );

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

    System.out.println( );
  }
}

It looks as if the int 1 is directly added to the list. But this is NOT what happens.

The statement data.add( 1 ) first creates an Integer object by using autoboxing. Then the reference to that object is added to the list at index 0.

The program writes out:

1 3 17 29

The picture shows 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.

data.add( new Integer(1) );