Java4210: getting started with generics  (Page 8/12)

The bottom line

The bottom line on generics (when used with collections) seems to be that references to objects are still stored in the collection as type Object . However, when we notify the compiler of the type of data to be stored in thecollection using angle-bracket syntax, and the program compiles without warnings, the compiler will do at least the following:

• Ensure that only references to objects of the specified type are stored in the collection, and used consistently throughout the program, thuseliminating the possibility of a ClassCastException at runtime.
• Automatically cast the reference to the specified type when it is later retrieved by program code.

No explicit cast is required

That brings us back to a discussion of the code in Listing 7 . Note that unlike the code in Listing 5 , the print statement in Listing 7 does not contain an explicit cast to type Date , (at least not in the code that I wrote).

As described above, having been notified that the collection can contain only references to objects of type Date , the compiler automatically inserted a cast to type Date at the appropriate place in the code, therebyguaranteeing that the reference is converted from type Object to type Date before the getTime method is called on the reference.

There is still a cast involved. However, the cast is automatically inserted by the compiler. This eliminates the requirement for me (the programmer ) to insert the cast, and also eliminates the possibility of me inserting an incorrect cast.

Once again, all of this assumes that the program compiles without warnings.

Program output

The program in Listing 7 compiles and executes correctly, producing the following output for one particular run.

1377995720768

Compile-time type safety provided by the use of generics

The program named Generics04 shown in Listing 8 illustrates the ability of generics to prevent the storing of the wrong type of references in a collection. This in turn can preventruntime errors.

A new ArrayList object

Listing 8 instantiates a new object of type ArrayList capable of storing only references to objects of type Date . This object's reference is stored in the instance variable named var1 .

Other types cannot be stored in the collection

Once the ArrayList has been constrained to contain only references to objects of type Date , the compiler will not allow a reference to an object of any other type (other than types that are assignment compatible with Date , such as subclasses of Date ) to be stored in the collection.

A compiler error rather than a runtime error

The first statement in the runIt method in Listing 8 attempts to add a new element to the ArrayList object. The new element is a reference to a literal String object that encapsulates the string "abcd". This results in the compiler error shown in Figure 4 .