4.4 Java4030: purpose of framework interfaces

Designing a framework

In theory, it should be possible (but perhaps not very practical) to define a framework consisting solely of interface definitions and associatedcontracts for methods and algorithms. Then each user could implement the interfaces however they see fit, provided that they comply with the contracts. (This might not be very practical, however, because every user of the framework would then be required to implement the interfaces, which would entail a lot of work.)

Concrete implementations

Fortunately, Oracle didn't stop work after defining the interfaces and contracts for the Java Collections Framework. Rather, they also provided us with severaluseful classes that implement the interfaces in the framework. Thus, we can instantiate and use objects of those classes immediately without having todefine them ourselves. Here is a list of some of the concrete implementation classes in the Java Collections Framework:

• HashSet
• TreeSet
• LinkedList
• Vector
• ArrayList
• HashMap
• Hashtable
• WeakHashMap
• TreeMap

In addition, Oracle provided us with several partial implementation classes including AbstractSet , AbstractList , and AbstractMap , which are intended to serve a starting point for new implementations that we choose to define. According toOracle, these classes provide a skeletal implementation of the Set, List, and Map interfaces to minimize the effort required to implement thoseinterfaces.

Summary

There are eight core interfaces in the Collections Framework.

As is always the case, each of the core interfaces defines a data type. Each interface declares several methods. In addition, each interface provides acontract that applies to each declared method. The contracts become morespecific as we traverse down the interface inheritance hierarchy.

Objects instantiated from classes that implement the interfaces can be considered to be of the interface type or any ancestor interface in theinterface's hierarchical family tree.

The method declarations and their associated contracts in the interfaces specify the general behavior of matching methods in the classes that implementthe interfaces.

The framework provides several concrete implementations of the interfaces that we can use to instantiate new objects to use as data structures or datacontainers.

The framework also provides several abstract implementations that we can use as a starting point for defining our own implementations.

Miscellaneous

This section contains a variety of miscellaneous information.

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