Section 11.5. Collections Interfaces

   

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11.5 Collection s Interfaces

While you can always look in the API under java.util (which is always a great idea), the following tables will help give you a concise reference of what is available and what each is used for.

Table 11.2 overviews the List , Map , and Set interfaces. It shows you what they are used for, what classes in the API implement them, and how long they've been in the language.

Table 11.2. Collection s Interfaces in java.util

Interface	Purpose
List	An ordered collection, also known as a sequence. Like a CF list in that they allow precise control over the location of the ordinal placement of an item in the list. Duplicate elements are allowed. Provides a special iterator called a ListIterator that allows insertion and replacement of items. Implementing classes are AbstractList , ArrayList , LinkedList , Vector . Since Java 1.2.
Map	Maps keys to values, as a CF structure. Cannot contain multiple keys with same name , and each key can map to only one value. Implementing classes are AbstractMap , Attributes , HashMap , Hashtable , IdentityHashMap , RenderingHints , TreeMap , WeakHashMap . Subinterface: SortedMap . Since Java 1.2.
Set	A collection that contains no duplicate elements. Implementing classes are AbstractSet , HashSet , LinkedHashSet , TreeSet . Subinterface: SortedSet . Since Java 1.2.

These interfaces, as we can see from the number of classes implementing each, are the parents of many related collections. So now that we have a sense of the collections landscape, let's look at some of the original collections in greater detail.

11.5.1 The Collection Life Cycle

The fundamental interface from which these derive is the Collection interface. Its methods were outlined above. Once you have created a collection of a certain type, then you can add objects to it using the add() method. Note that add() returns a boolean specifying whether the collection item was added. If the item is a duplicate, it will not be added to the collection and will return false .

You can loop over the collection to read, write, or modify each element one by one using an Iterator or an Enumeration .

11.5.1.1 Iterator AND Enumeration

A class implementing the Iterator interface can step through a collection item by item. The Iterator was introduced in Java 1.2 to replace the Enumeration . There are two chief ways in which an Iterator improves upon the Enumeration :

• The caller can remove elements from the collection in a clear, well-defined manner. This was not easily possible with Enumeration .

• Method names have been improved.

Preferring Iterator to Enumeration is recommended. The methods of an Iterator are as follows :

• hasNext() returns true if the Iterator has more elements

• next () returns the next element in the iteration

• remove() removes the last element the Iterator returned

You can call an iterator's next() method to look up the elements in the collection one by one. Use caution when employing the remove() method, because it will remove the element that the last call to next() returned. So, you must call next() and only then call remove() .

11.5.2 AbstractCollection

java.util.AbstractCollection is an abstract class. It was added in JDK 1.2 to aid developers implementing the Collection interface. Recall that interfaces can contain no method implementations ; they just define what must be implemented. This meant that developers had to redefine certain useful methods over and over again. This was a lot of work for little return.

So the makers of the JDK created AbstractCollection as a partial implementation. It is an abstract class and as such it contains implementations for routine methods, but leaves the core methods abstract for you. Because the class is abstract, you can override the implementations it provides if you have a specialized need, but otherwise you aren't forced to do all of that work. The advantage to this choice is that it achieves an excellent balance of power, flexibility, and efficiency.

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