14. Removing an Element

Answer:
No. You are not allowed to create gaps between elements. Adding an element at index 5 in the above program would generate a runtime error.

To remove an element from a list use the remove() method. This method removes an element without leaving a hole. The object references above the removed element are moved down to fill the gap.

E remove(int index)  //  Delete the element at index. Each element with an index 
                     //  greater than index is shifted downward to have an index 
                     //  one smaller than its previous value. 
                     //  Returns a reference to the removed element.
                     //  Throws an IndexOutOfBoundsException if the index is out
                     //  of range.

The object reference at location index is removed from the list. Elements at locations index+1, index+2, ... , size()-1 are each moved down one to fill in the gap. This is like pulling out a book from the middle of a stack of books.

The object reference that was removed from the list is the return value of the method. This value can be assigned to a reference variable. As long as there is a reference to the object, the object is not lost. If the only reference to the object was the one in the list, and that reference is not assigned to a variable, then the object will be garbage collected.

Question 14:
Examine the following program. What will it print? 

    

        

        
        

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

    names.add( "Amy" );    
    names.add( "Bob" );
    names.add( "Chris" );  
    names.add( "Deb" );

    names.remove(2);

    for ( int j=0; j < names.size(); j++ )
      System.out.println( j + ": " + names.elementAt(j) ); 

  }
}