The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
The Java Collections Framework was designed to ensure complete interoperability between the core
collection interfaces and the types that were used to represent collections in the early versions of the Java platform:
Vector
,
Hashtable
,
array, and
Enumeration
. In this section, you'll learn how to transform old collections to the Java Collections Framework collections and vice versa.
Suppose that you're using an API that returns legacy collections in tandem with another API that requires objects implementing the collection interfaces. To make the two APIs interoperate smoothly, you'll have to transform the legacy collections into modern collections. Luckily, the Java Collections Framework makes this easy.
Suppose the old API returns an array of objects and the new API requires a Collection
. The Collections Framework has a convenience implementation that allows an array of objects to be viewed as a List
. You use
Arrays.asList
to pass an array to any method requiring a Collection
or a List
.
Foo[] result = oldMethod(arg); newMethod(Arrays.asList(result));
If the old API returns a Vector
or a Hashtable
, you have no work to do at all because Vector
was retrofitted to implement the List
interface, and Hashtable
was retrofitted to implement Map
. Therefore, a Vector
may be passed directly to any method calling for a Collection
or a List
.
Vector result = oldMethod(arg); newMethod(result);
Similarly, a Hashtable
may be passed directly to any method calling for a Map
.
Hashtable result = oldMethod(arg); newMethod(result);
Less frequently, an API may return an Enumeration
that represents a collection of objects. The Collections.list
method translates an Enumeration
into a Collection
.
Enumeration e = oldMethod(arg); newMethod(Collections.list(e));
Suppose you're using an API that returns modern collections in tandem with another API that requires you to pass in legacy collections. To make the two APIs interoperate smoothly, you have to transform modern collections into old collections. Again, the Java Collections Framework makes this easy.
Suppose the new API returns a Collection
, and the old API requires an array of Object
. As you're probably aware, the Collection
interface contains a toArray
method designed expressly for this situation.
Collection c = newMethod(); oldMethod(c.toArray());
What if the old API requires an array of String
(or another type) instead of an array of Object
? You just use the other form of toArray
the one that takes an array on input.
Collection c = newMethod(); oldMethod((String[]) c.toArray(new String[0]));
If the old API requires a Vector
, the standard collection constructor comes in handy.
Collection c = newMethod(); oldMethod(new Vector(c));
The case where the old API requires a Hashtable
is handled analogously.
Map m = newMethod(); oldMethod(new Hashtable(m));
Finally, what do you do if the old API requires an Enumeration
? This case isn't common, but it does happen from time to time, and the
Collections.enumeration
method was provided to handle it. This is a static factory method that takes a Collection
and returns an Enumeration
over the elements of the Collection
.
Collection c = newMethod(); oldMethod(Collections.enumeration(c));