Interface Collection and Class Collections

Interface Collection is the root interface in the collection hierarchy from which interfaces Set, Queue and List are derived. Interface Set defines a collection that does not contain duplicates. Interface Queue defines a collection that represents a waiting linetypically, insertions are made at the back of a queue and deletions are made from the front, though other orders can be specified. We discuss Queue and Set in Section 19.8 and Section 19.9, respectively. Interface Collection contains bulk operations (i.e., operations performed on an entire collection) for adding, clearing, comparing and retaining objects (or elements) in a collection. A Collection can also be converted to an array. In addition, interface Collection provides a method that returns an Iterator object, which allows a program to walk through the collection and remove elements from the collection during the iteration. We discuss class Iterator in Section 19.5.1. Other methods of interface Collection enable a program to determine a collection's size and whether a collection is empty.

Software Engineering Observation 19.1

Collection is used commonly as a method parameter type to allow polymorphic processing of all objects that implement interface Collection.

Software Engineering Observation 19.2

Most collection implementations provide a constructor that takes a Collection argument, thereby allowing a new collection to be constructed containing the elements of the specified collection.

Class Collections provides static methods that manipulate collections polymorphically. These methods implement algorithms for searching, sorting and so on. Chapter 16, Searching and Sorting, discussed and implemented various searching and sorting algorithms. Section 19.6 discusses more about the algorithms that are available in class Collections. We also cover the wrapper methods of class Collections that enable you to treat a collection as a synchronized collection (Section 19.12) or an unmodifiable collection (Section 19.13). Unmodifiable collections are useful when a client of a class needs to view the elements of a collection, but should not be allowed to modify the collection by adding and removing elements. Synchronized collections are for use with a powerful capability called multithreading (discussed in Chapter 23). Multithreading enables programs to perform operations in parallel. When two or more threads of a program share a collection, there is the potential for problems to occur. As a brief analogy, consider a traffic intersection. We cannot allow all cars access to one intersection at the same timeif we did, accidents would occur. For this reason, traffic lights are provided at intersections to control access to the intersection. Similarly, we can synchronize access to a collection to ensure that only one thread manipulates the collection at a time. The synchronization wrapper methods of class collection return synchronized versions of collections that can be shared among threads in a program.