Bag Implementations That Use Arrays - Data Structures and Abstractions with Java - page 42

Before we implement this private method, let's see if we can use it by revising the remove
method in Segment 2.21. Since that method removes and returns the last entry in the array bag , that is,
bag[numberOfEntries - 1] , its definition can make the call removeEntry(numberOfEntries - 1) .
Proceeding as if removeEntry were defined and tested, we can define remove as follows:
/** 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()
{

T result = removeEntry(numberOfEntries - 1);
return result;
} // end remove

This definition looks good; let's implement the second remove method.

2.25

The second remove method. The first remove method does not search for the entry to remove, as it
removes the last entry in the array. The second remove method, however, does need to perform a
search. Rather than thinking about the details of locating an entry in an array right now, let's dele-
gate that task to another private method, which we specify as follows:

// Locates a given entry within the array bag.
// Returns the index of the entry, if located, or -1 otherwise.
private int getIndexOf(T anEntry)
Assuming that this method is defined and tested, we can define our public method as follows:

/** Removes one occurrence of a given entry from this bag.
@param anEntry the entry to be removed
@return true if the removal was successful, or false if not */
public boolean remove(T anEntry)
{

int index = getIndexOf(anEntry);
T result = removeEntry(index);
return anEntry.equals(result);
} // end remove
Notice that removeEntry returns either the entry it removes or null . That is exactly what the first
remove method needs, but the second remove method has to return a boolean value. Thus, in the second
method we need to compare the entry we want to remove with the one removeEntry returns to get the
desired boolean value.

Question 14 Can the return statement in the previous definition of remove be written as
follows?
a. return result.equals(anEntry);
b. return result != null ;

Question 15 The array bag in ArrayBag contains the entries in the bag aBag . If bag con-
tains the strings "A" , "A" , "B" , "A" , "C" , why does aBag.remove("B") change the contents
of bag to "A" , "A" , "C" , "A", null instead of either "A" , "A" , "A" , "C", null or "A" , "A" ,
null , "A" , "C" ?

2.26

The definition of the private method removeEntry . Let's look back at the pseudocode we wrote
in Segment 2.23 for removing a particular entry from the bag. The private method removeEntry