Generics - Java: A Beginner's Guide - page 635

This program contains several interesting things. First, a raw type of the generic Gen
class is created by the following declaration:
Notice that no type arguments are specified. In essence, this creates a Gen object whose
type T is replaced by Object .
A raw type is not type safe. Thus, a variable of a raw type can be assigned a reference
to any type of Gen object. The reverse is also allowed, in which a variable of a specific
Gen type can be assigned a reference to a raw Gen object. However, both operations are
potentially unsafe because the type checking mechanism of generics is circumvented.
This lack of type safety is illustrated by the commented-out lines at the end of the pro-
gram. Let's examine each case. First, consider the following situation:
In this statement, the value of ob inside raw is obtained, and this value is cast to Integer .
The trouble is that raw contains a Double value, not an integer value. However, this cannot
be detected at compile time because the type of raw is unknown. Thus, this statement fails
at run time.
The next sequence assigns to strOb (a reference of type Gen<String> ) a reference to a
raw Gen object:
The assignment itself is syntactically correct, but questionable. Because strOb is of type
Gen<String> , it is assumed to contain a String . However, after the assignment, the object
referred to by strOb contains a Double . Thus, at run time, when an attempt is made to as-
sign the contents of strOb to str , a run-time error results because strOb now contains a
Double . Thus, the assignment of a raw reference to a generic reference bypasses the type-
safety mechanism.
The following sequence inverts the preceding case:
Here, a generic reference is assigned to a raw reference variable. Although this is syntactic-
ally correct, it can lead to problems, as illustrated by the second line. In this case, raw now