# Java Notes: Generics and New Data Structures

## What's new in Java 5

Java Collections have been extended in two directions.

- Generics -- Specifying the object type in a collection. See example below.
- Additional structures designed to make concurrency easier

## Additional data structures

Interfaces
  `Queue`, `BlockingQueue`, `ConcurrentMap`, `Delayed`.
Classes
  `PriorityQueue`, `ConcurrentLinkedQueue`, `ArrayBlockingQueue`, `LinkedBlockingQueue`, `PriorityBlockingQueue`, `DelayQueue`, `SynchronousQueue`, `ConcurrentHashMap`, and maybe others.

The concurrent collections can be executed by more than one thread without blocking! In other words, they don't have the performance cost of the synchronized classes. See John Zukowski's article at http://www-106.ibm.com/developerworks/java/library/j-tiger06164.html for an overview of these new structures.

## Example using Java 1.2-1.4 data structures

    // Typical Collections usage before Java 5
    List greetings = new ArrayList();
    greetings.add("We come in peace.");
    greetings.add("Take me to your leader.");
    greetings.add("Resistance is futile.");

    Iterator it = greetings.iterator();
    while (it.hasNext()) {
        String aGreeting = (String)it.next();
        attemptCommunication(aGreeting);
    }

## Same example rewritten in Java 5 using generics

Java 5 let's you specify the type of element in the collection. This has two nice consequences:

- Trying to add something of the wrong type gives a compile-timer error.
- Getting elements out doesn't require an explicit downcast. The example below doesn't show how nice that can be.

    // Same example using generics.
    List<String> greetings = new ArrayList<String>();
    greetings.add("We come in peace.");
    greetings.add("Take me to your leader.");
    greetings.add("Resistance is futile.");

    Iterator<String> it = greetings.iterator();
    while (it.hasNext()) {
        String aGreeting = it.next();     // No downcast.
        attemptCommunication(aGreeting);

## Links to resources on new Collections features

- There are no notes here yet, but if you already have some idea of what they are, this article (Talking Tiger, generically speaking) has some nice explanations of what's available.

- Sun also has a tutorial on generics at java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf. It's not a great article, but you might find something useful here.

- A discussion of generics in Java, C++, and C# is at www.artima.com/intv/generics.html