Working with Java Collections - Java in a Nutshell - page 256

List < Integer > primes = new ArrayList < Integer >();
List < Integer > readonly = Collections . unmodifiableList ( primes );
// We can modify the list through primes
primes . addAll ( Arrays . asList ( 2 , 3 , 5 , 7 , 11 , 13 , 17 , 19 ));
// But we can't modify through the read-only wrapper
readonly . add ( 23 ); // UnsupportedOperationException
The java.util.Collections class also defines methods to operate on collections.
Some of the most notable are methods to sort and search the elements of
collections:
Collections . sort ( list );
// list must be sorted first
int pos = Collections . binarySearch ( list , "key" );
Here are some other interesting Collections methods:
// Copy list2 into list1, overwriting list1
Collections . copy ( list1 , list2 );
// Fill list with Object o
Collections . fill ( list , o );
// Find the largest element in Collection c
Collections . max ( c );
// Find the smallest element in Collection c
Collections . min ( c );
Collections . reverse ( list ); // Reverse list
Collections . shuffle ( list ); // Mix up list
It is a good idea to familiarize yourself fully with the utility methods in Collections
and Arrays as they can save you from writing your own implementation of a com‐
mon task.
Special-case collections
In addition to its wrapper methods, the java.util.Collections class also defines
utility methods for creating immutable collection instances that contain a single ele‐
ment and other methods for creating empty collections. singleton() , singleton
List() , and singletonMap() return immutable Set , List , and Map objects that con‐
tain a single specified object or a single key/value pair. These methods are useful
when you need to pass a single object to a method that expects a collection.
The Collections class also includes methods that return empty collections. If you
are writing a method that returns a collection, it is usually best to handle the no-
values-to-return case by returning an empty collection instead of a special-case
value like null :
Set < Integer > si = Collections . emptySet ();
List < String > ss = Collections . emptyList ();
Map < String , Integer > m = Collections . emptyMap ();