List Implementations That Use Arrays - Data Structures and Abstractions with Java - page 325

13.10

The method remove . Removing a list entry at an arbitrary position is like a student leaving room

A in our example in Segment 13.3. We need to shift existing entries to avoid a gap in the array,
except when removing the list's last entry. The following implementation uses a private method
removeGap to handle the details of moving data within the array. Like the method add , remove is
responsible for checking the validity of the given position. Note how this check also ensures that
the list is not empty.
public T remove( int givenPosition)
{

VideoNote

Completing the class AList

T result = null ; // return value
if ((givenPosition >= 1) && (givenPosition <= numberOfEntries))
{

assert !isEmpty();
result = list[givenPosition - 1]; // get entry to be removed
// move subsequent entries toward entry to be removed,
// unless it is last in list
if (givenPosition < numberOfEntries)
removeGap(givenPosition);
numberOfEntries--;
} // end if
return result; // return reference to removed entry, or
// null if either list is empty or givenPosition
// is invalid

} // end remove

Question 7 Since the method remove does not explicitly check for an empty list, why is
the assertion given in the method true?

Question 8 When a list is empty, how does remove return null ?

13.11

The private method removeGap . The following private method removeGap shifts list entries within
the array, as Figure 13-5 illustrates. Beginning with the entry after the one to be removed and con-
tinuing until the end of the list, removeGap moves each entry to its next lower position.

// Shifts entries that are beyond the entry to be removed to the
// next lower position.
// Precondition: 1 <= givenPosition < numberOfEntries;
// numberOfEntries is list's length before removal.
private void removeGap( int givenPosition)
{

assert (givenPosition >= 1) && (givenPosition < numberOfEntries);
int removedIndex = givenPosition - 1;
int lastIndex = numberOfEntries - 1;
for ( int index = removedIndex; index < lastIndex; index++)
list[index] = list[index + 1];
} // end removeGap