Effective Java

Item 30: Favor generic methods
Just as classes can be generic, so can methods. Static utility methods that operate
on parameterized types are usually generic. All of the “algorithm” methods in
Collections
(such as 
binarySearch
and 
sort
) are generic.
Writing generic methods is similar to writing generic types. Consider this
deficient method, which returns the union of two sets:
// Uses raw types - unacceptable! (Item 26)
public static Set union(Set s1, Set s2) {
Set result = new HashSet(s1);
result.addAll(s2);
return result;
}
This method compiles but with two warnings:
Union.java:5: warning: [unchecked] unchecked call to
HashSet(Collection) as a member of raw type HashSet
Set result = new HashSet(s1);
^
Union.java:6: warning: [unchecked] unchecked call to
addAll(Collection) as a member of raw type Set
result.addAll(s2);
^
To fix these warnings and make the method typesafe, modify its declaration to
declare a 
type parameter
representing the element type for the three sets (the two
arguments and the return value) and use this type parameter throughout the
method. 
The type parameter list, which declares the type parameters, goes
between a method’s modifiers and its return type. 
In this example, the type
parameter list is 

, and the return type is 
Set
. The naming conventions for
type parameters are the same for generic methods and generic types (Items 29, 68):
// Generic method
public static 

Set

union(Set

s1, Set

s2) {
Set

result = new HashSet
<>
(s1);
result.addAll(s2);
return result;
}
At least for simple generic methods, that’s all there is to it. This method com-
piles without generating any warnings and provides type safety as well as ease of

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CHAPTER 5
GENERICS
136
use. Here’s a simple program to exercise the method. This program contains no
casts and compiles without errors or warnings: