Java utilities - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 339

generics. Generic features pervade all the Collection Framework classes as well
as several other APIs in J2SE 5.0.
Suppose we have a list of strings:
List list - of - strings = new ArrayList ();
Without generics, we can insert any Object type into the list - Strings ,
Integers ,anything. With generics, we declare the type of object the list is
allowed to contain at compile time. The syntax is as follows:
List < String > list - of - strings = new ArrayList < String > ();
The < String > notation indicates that this particular List object is only allowed
to contain String types. Any attempt to add an Integer (autoboxed or not) or
anything else is disallowed at compile time:
list - of - strings.add ( " this is legal " );
list - of - strings.add (new Integer (5)); // this is not allowed
list - of - strings.add (5); // neither is this
In the first illegal add() ,weexplicitly wrap the 5 into an Integer .Even though
Integer is an Object type, meaning it could be added to a plain List ,itis
not permitted into the List < String > object container. In the second attempt,
neither is the int 5 permitted. The compiler errors you will see look like this:
Generics.java:7: cannot find symbol
symbol: method add (java.lang.Integer)
location: interface java.util.List < java.lang.String >
list.add (new Integer (5));
^
Generics.java:8: cannot find symbol
symbol: method add (int)
location: interface java.util.List < java.lang.String >
list.add (5);
^
The compiler is saying that it cannot find an add() method that takes an Integer
type in the interface java.util.List < java.lang.String > . Neither is
there an add() that takes an int .Byadding the generic type notation < String >
(also known as a parameterized type) we have effectively created a new List
interface that permits only String inserts. (Note that the second illegal add()
did not autobox the int 5 into an Integer . Doing so would not have worked
either since the first illegal attempt already demonstrated that adding an Integer
is not permitted.)
Where generics becomes important and saves a lot of code is when iterating
over generic types and, in particular, in conjunction with autoboxing and unboxing
of primitive types. Recall that without generics we must cast returned Object
types into the desired type during an iteration. That was with J2SE 1.4 and below.