J.E.D.I.
13 An Introduction to Generics
13.1 Objectives
Javas latest release provides the biggest leap forward in Java programming compared to its other versions. It includes significant extensions to the source language syntax. The
most visible of which is the addition of generic types.
This module introduces you to the basic concepts related to Java generic types. After completing this lesson, you should be able to:
1. Enumerate the benefits of generic types 2. Declare a generic class
3. Use constrained generics 4. Declare generic methods
5. Use Java collections with generic types
13.2 Why Generics?
One of the most significant causes of bugs in the Java programming language is the need to continually typecast or downcast expressions to more specific data types than
their static types. For example, an ArrayList object allows us to add any reference type object to the list but when we retrieve these elements, we need to typecast the objects
to a specific reference type appropriate for our needs. Downcasting is a potential hotspot for ClassCastException. It also makes our codes wordier, thus, less readable. Moreover,
downcasting also effectively destroys the benefits of a strongly typed language since it nullifies the safety that accompanies built-in type checking.
The main goal of adding generics to Java is to solve this problem. Generic types allow a single class to work with a wide variety of types. It is a natural way of eliminating the
need for casting.
Lets first consider an ArrayList object and see how generic types would help in improving our code. As you already know, an ArrayList object has the ability to store
elements of any reference type to this list. An ArrayList instance, however, has always forced us to downcast the objects we retrieve out of the list. Consider the following
statement:
String myString = String myArrayList.get0; The generic version of the ArrayList class is designed to work natively with any type of
class. At the same, it also preserves the benefits of type checking. We can do away with the need of having to typecast the element we get from the list and have the following
statement instead of the previous one:
String myString = myArrayList.get0; Although downcasting was already removed, this doesnt mean that you could assign
anything to the return value of the get method and do away with typecasting altogether. If you assign anything else besides a String to the output of the get method, you would
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encounter a compile time type mismatch such as this message: found: java.lang.String
required: java.lang.Integer Integer data = myArrayList.get0;
For you to just have an idea how generic types are used before digging into more details, consider the following code fragment:
ArrayList String genArrList = new ArrayList String; genArrList.addA generic string;
String myString = genArrList.get0; JoptionPane.showMessageDialogthis, myString;
Browsing through the statements, you probably observed the word String appearing immediately after the reference data type ArrayList. You can interpret the first statement
as instantiating a generic version of the ArrayList class and this generic version contains objects of type String. genArrList is bound to String type. Hence, binding an Integer or
some other non-String type to the result of the get function would be illegal. The next statement is illegal.
int myInt = genArrList.get;
13.3 Declaring a Generic Class
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