Java - what can and cannot be done with generics? Add your favourite trick here

[dribeas]

I once was an experienced Java programmer, but when Java 5 came out I stopped that path and went into C++. With time I have learned what can be done with C++ templates, and they are powerful.

With time I have never gone back to Java, but each so often I wonder what the real power inside Java generics is. I once believed that it was a nice utility to add type safety from containers, but I wonder what else can be done.

Anyone has experience with generics? What can be done with them? Any code snippets and/or short descriptions, links... will be appreciated.

David

[tinny]

Wildcards are worth knowing.



List<?> strs = new ArrayList<String>();
//TODO add some objects
for(Object obj : strs){
//TODO
}

List<? extends JComponent> comp = new ArrayList<JPanel>();
//TODO add any JPanel object

List<? super JPanel> comp2 = new ArrayList<JComponent>();
//TODO add any JComponent object


Wildcards are useful when you want to take advantage of polymorphism in generic collections



public void method(List<? extends JComponent> list) {
//deal with a collection of JComponents
}


The above method is only concerned with dealing with JComponents and doesn't care about the specific type.

[slavik]

this is something that C++ templates do as well.

[dribeas]

this is something that C++ templates do as well.

Well, I strongly believe that Java/.NET generics are far behind the power of C++ templates. What I want to know is how much can be done with Java.

The thing is that I used to believe that C++ templates were all about defining containers, and I know now how far from the truth I was. I believe that Java generics are about removing unneeded casts from Object, how wrong am I?

Note that Java has super and extends keywords for generic type definitions:



public static <T extends Comparable<? super T>> void sort(List<T> list) {...}


This means that sort will receive a list of T where some ancestor of T must implement Comparable. Not as complete as C++0x concepts but still it is more expressive than current templates in C++. Now this is solved with docs and compiler errors, where the latter are not always user friendly.

NOTE: My intention is learning the possibilities, not comparing languages. Please, no more language flamewars :)

[tinny]

I believe that Java generics are about removing unneeded casts from Object, how wrong am I?


More or less.

Its about type safety.
http://www.angelikalanger.com/GenericsFAQ/FAQSections/Fundamentals.html#FAQ004