Java Iterable<Iterable<T>> to ArrayList<ArrayList<T>>

Question

Maybe I am missing something but I thought that If I declare my class as such:

public class Something<T> implements Iterable<Iterable<T>> {
  public Something(Iterable<Iterable<T>> input) {
  ...

I should be able to instantiate it as such:

ArrayList<ArrayList<String>> l = new ArrayList<ArrayList<String>>();

Something<String> s = Something<String>(l);

Unfortunately this gimes me an error. I thought ArrayLists are Iterable so that should map exactly to my constructor definition.

Jeremy · Accepted Answer · 2011-02-17 15:47:49Z

7

You need to modify your constructor to accept anything that extends Iterable<T>

public Something(Iterable<? extends Iterable<T>> list){}

The reason for this is because generic types cannot literally extend other generic types. A List<String> cannot be assigned to a List<CharSequence>. The keyword extends allows this type of assignment to occur.

answered Feb 17, 2011 at 15:47

Jeremy

22.5k4 gold badges71 silver badges82 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Pass Over a year ago

Thanks, but now I am getting warnings here:

public class Something<T> implements Iterable<Iterable<T>> { 	 	private Iterable<Iterable<T>> input; 	 	public Something(Iterable<? extends Iterable<T>> input) { 		this.input = (Iterable<Iterable<T>>)input; 	}

Jeremy Over a year ago

I suggest find a copy of Effective Java by Josh Bloch and read up on "PECS". It will help you really understand what is going on. In any case, you need to have Iterable<? extends Iterable<T> everywhere you expect to assign ArrayList<ArrayList<String>>

newacct Over a year ago

or perhaps even Iterable<? extends Iterable<? extends T>>

mblinn · Accepted Answer · 2011-02-17 15:49:41Z

1

Subtyping with generics is very non-intuitive. For what you want to do, you need wildcards, and it would be pretty ugly.

There's a great explanation here in Section 3 - http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf

answered Feb 17, 2011 at 15:49

mblinn

3,27419 silver badges15 bronze badges

Collectives™ on Stack Overflow

Java Iterable<Iterable<T>> to ArrayList<ArrayList<T>>

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related