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The Collections Framework is defined by a set of interfaces and associated contracts, and provides concrete implementations of the interfaces for the mostcommon data structures. In addition, the framework also provides several abstract implementations, which are designed to make it easier for you to createnew and different implementations while still maintaining the structural polymorphic integrity of the framework.
The code in this series of modules is written with no thought given to Generics . As a result, if you copy and compile the code, you will probably get warnings about unchecked or unsafe operations .
While you will ultimately need to understand how to use Generics, that is a very complex topic. An understanding of Generics is beyond the scope of thiscourse. Therefore, for purposes of this course, you can simply ignore those warnings.
Let's begin with a little quiz to establish your baseline knowledge of the Collections Framework. Take a look at the program in Listing 6 near the end of this module. Which of the following is the output produced by that program?
If your answer was 1234 (and it wasn't a guess) then you may already know quite a lot about the use of the Collections Framework. If not, keep reading to begin learning aboutthe framework.
This simple introductory program is not intended to do anything useful. Instead, it was designed to illustrate several important features of theframework, including the ease with which elements of the framework can be reused in your programs.
As many of you already know, I am a college professor. I specialize in teaching OOP using Java. In the past, many college courses in Data Structures (often referred to as CS2 courses) have emphasized the concept of reinventing the wheel . Students were required to learn how to reinvent a variety of complex data structures in order to successfully complete the course.
Hopefully, with the conversion of these CS2 courses to Java OOP, the emphasis will change to reuse instead of reinvent .
The Java Collections Framework is designed to encourage programmers to reuse existing interfaces and classes instead of inventing new ones. In the event thatit is necessary to invent a new class or interface, the programmer is encouraged to integrate it into the framework in a polymorphic manner.
I am going to provide a brief discussion of the sample program (shown in Listing 6 ) in this module. Later, I will provide more detailed discussions of many of the features used in that program.
I will break this program down and discuss it in fragments.
The code fragment in Listing 1 instantiates an object of the TreeSet class and stores the object's reference in a reference variable of type Collection named ref .
| Listing 1 . A new object of the TreeSet class. |
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class Worker{
public void doIt(){Collection ref = new TreeSet(); |
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