GENERICS - Introduction to Java Programming: Comprehensive Version - page 746

19.6 Raw Types and Backward Compatibility
A generic class or interface used without specifying a concrete type, called a raw type,
enables backward compatibility with earlier versions of Java.

Key
Point

You can use a generic class without specifying a concrete type like this:

GenericStack stack = new GenericStack(); // raw type

This is roughly equivalent to

GenericStack<Object> stack = new GenericStack<Object>();

A generic class such as GenericStack and ArrayList used without a type parameter is
called a raw type . Using raw types allows for backward compatibility with earlier versions of
Java. For example, a generic type has been used in java.lang.Comparable since JDK 1.5,
but a lot of code still uses the raw type Comparable , as shown in Listing 19.5:

raw type
backward compatibility

L ISTING 19.5

Max.java
1 public class Max {
2 /** Return the maximum of two objects */

3 public static Comparable max(Comparable o1, Comparable o2) { raw type

4 if (o1.compareTo(o2) > 0 )

5 return o1;

6 else

7 return o2;

8 }
9 }

Comparable o1 and Comparable o2 are raw type declarations. Be careful: raw types are
unsafe . For example, you might invoke the max method using

Max.max( "Welcome" , 23 ); // 23 is autoboxed into new Integer(23)

This would cause a runtime error, because you cannot compare a string with an inte-
ger object. The Java compiler displays a warning on line 3 when compiled with the option
-Xlint:unchecked , as shown in Figure 19.5.

Xlint:unchecked

F IGURE 19.5

The unchecked warnings are displayed using the compiler option
-Xlint:unchecked .

A better way to write the max method is to use a generic type, as shown in Listing 19.6.

L ISTING 19.6

MaxUsingGenericType.java
1 public class MaxUsingGenericType {
2 /** Return the maximum of two objects */

3 public static <E extends Comparable<E>> E max(E o1, E o2) { bounded type

4 if (o1.compareTo(o2) > 0 )

5 return o1;

6 else