Java4210: getting started with generics  (Page 3/12)

Discussion and sample code

Listings of the programs

Complete listings of all the programs discussed in this module are provided in Listing 5 through Listing 9 near the end of the module.

Collection behavior prior to Java version 1.5

The main purpose of the program named Generics01 is to establish a baseline against which to compare the other programs. A secondary purposeis to illustrate the warnings produced by the Java version 1.5 (or later) compiler when the syntax of the source code doesn't take generics into account.

A complete listing of the program named Generics01 is shown in Listing 5 near the end of the module.

As you will see later, this program was written using the program syntax and style that was common prior to the release of Java version 1.5. Inparticular, this program does not include the syntax necessary to take generics into account.

Notes at compile time

When this program is compiled using the Java version 1.5 or later compiler, the text shown in Figure 1 appears on the screen.

Note that the text in Figure 1 is not identified as either an error or a warning. Rather, the text is identified simply as notes. Thesenotes provide instructions on how to recompile and get more information regarding a potential problem.

Recompiling with the Xlint switch

When the program is recompiled using the Xlint switch shown in Figure 1 , the compiler produces the text shown in Figure 2 . Note that this text is identified as a warning.

Get used to it

The text in Figure 1 and Figure 2 is similar to what you can expect to see any time that you use the Java version 1.5 or later compiler to compile a program thatincludes the Java Collections Framework and doesn't use the required syntax to take generics into account. This probably includes many of the programsthat you wrote, compiled, and executed successfully prior to the release of Java version 1.5.

The program code

The code for the program named Generics01 is shown in its entirety in Listing 5 .

The main method

As you can see in Listing 5 , the main method instantiates a new object of the class named Generics01 , and calls the method named runIt on that object. It is the behavior of the runIt method that interests us. More particularly, it is the expression contained in theargument list for the println method that interests us the most.

A baseline program

As mentioned earlier, the main purpose of this program is to establish a baseline against which we can compare the other programs to be discussed later.This program was written exactly as it would have been written prior to the release of generics in Java version 1.5.

An ArrayList object

The program declares and initializes an instance variable named var1 with a reference to an object instantiated from the class named ArrayList . ArrayList is one of the concrete implementations of the interfaces provided in the Java Collections Framework. Briefly, an object of the ArrayList class is an object that implements the List interface, providing a convenient place to store references to other objects.