Bag Implementations That Use Arrays - Data Structures and Abstractions with Java - page 39

Question 10 Revise the definition of the method clear so that it does not call isEmpty .
Hint : The while statement should have an empty body.

Question 11 Consider the following definition of clear :
public void clear()
{

numberOfEntries = 0;
} // end clear

What is a disadvantage of this definition as compared to the one shown in Segment 2.20?

2.21

Removing an unspecified entry. The method remove that has no parameter removes an unspecified
entry from a bag, as long as the bag is not empty. Recall from the method's specification given in the
interface in Listing 1-1 of the previous chapter that the method returns the entry it removes:

/** Removes one unspecified entry from this bag, if possible.
@return either the removed entry, if the removal was successful,
or null otherwise */
public T remove()
If the bag is empty before the method executes, null is returned.
Removing an entry from a bag involves removing it from an array. Although we can access
any entry in the array bag , the last one is easy to remove. To do so, we

Access the entry so it can be returned ●

Set the entry's array element to null ●

Decrement numberOfEntries ●

Decrementing numberOfEntries causes the last entry to be ignored, meaning that it is effectively
removed, even if we did not set its location in the array to null .
A literal translation of the previous steps into Java leads to the following definition of the method:

public T remove()
{

T result = null ;
if (numberOfEntries > 0)
{

result = bag[numberOfEntries - 1];
bag[numberOfEntries - 1] = null ;
numberOfEntries--;
} // end if
return result;
} // end remove
Note that this method computes numberOfEntries - 1 three times. The following refinement
avoids this repetition:

public T remove()
{

T result = null ;
if (numberOfEntries > 0)
{

numberOfEntries--;
result = bag[numberOfEntries];
bag[numberOfEntries] = null ;
} // end if