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Wrapper Classes

To put an int into an ArrayList, put the int inside an of an Integer object. Mostly, the job of the wrapper class Integer is to provide objects that contain primitive data. Now the objects (and the primitive data they contain) can be put into an ArrayList or other data structure. The following program builds a list of integers and then writes them out.

import java.util.* ;
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.


Would you expect the following to work?

    data.add( 44 );