Can Java nest generics? The following is giving me an error in Eclipse:
ArrayList<ArrayList<Integer>> numSetSet = ArrayList<ArrayList<Integer>>();
The error is:
Syntax error on token "(", Expression expected after this token
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2Well now, there's an example of a terrible compiler error message!Cornelius Dol– Cornelius Dol2009-11-04 04:14:45 +00:00Commented Nov 4, 2009 at 4:14
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2Basic syntax FAIL. Back to driving school, Sonny!Bombe– Bombe2009-11-04 06:54:52 +00:00Commented Nov 4, 2009 at 6:54
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2Better do: List<List<Integer>> numSetSet = new ArrayList<List<Integer>>(); And if it's a set, why are you using lists?starblue– starblue2009-11-04 07:36:35 +00:00Commented Nov 4, 2009 at 7:36
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5 Answers
You forgot the word new.
Comments
That should be:
ArrayList<ArrayList<Integer>> numSetSet = new ArrayList<ArrayList<Integer>>();
Or even better:
List<List<Integer>> numListList = new ArrayList<List<Integer>>();
4 Comments
Nick Heiner
Actually, you can't have nested Abstract types, apparently:
List<ArrayList<Integer>> numSetSet = new ArrayList<ArrayList<Integer>>();
Asaph
@Rosarch:
List<List<Integer>> numListList = new ArrayList<List<Integer>>(); compiles just fine on my machine.
fastcodejava
The second solution may not be better. In the second solution any kind of list can be put in the main list which may not what you want. In fact you could put both ArrayList and LinkedList into the main list. It may not be a bad thing, but may not be what you want.
Asaph
@fastcodejava: That's exactly the point! Always code to an interface, not an implementation. That way you can swap out different implementations of the
List interface easily without changing lots of code.For those who come into this question via google, Yes Generics can be nested. And the other answers are good examples of doing so.
Comments
And here are some slightly tricky technic about Java template programming, I doubt how many people have used this in Java before.
This is a way to avoid casting.
public static <T> T doSomething(String... args)
This is a way to limit your argument type using wildcard.
public void draw(List<? extends Shape> shape) {
// rest of the code is the same
}
you can get more samples in SUN's web site:
http://java.sun.com/developer/technicalArticles/J2SE/generics/
Comments
You forgot 'new' keyword as in below code:
ArrayList<ArrayList<Integer>> numSetSet = new ArrayList<ArrayList<Integer>>();
You can also use Maps along with Lists for nested Generics as shown in Java 5 (J2SE 5.0/JDK 1.5) New Features with Examples
