Question

Just come across with this problem:

List<DataNode> a1 = new ArrayList<DataNode>();
List<Tree> b1 = a1;  // compile error: incompatible type

Where the type DataNode is a subtype of Tree.

public class DataNode implements Tree

To my surprise, this works for array:

DataNode[] a2 = new DataNode[0];
Tree[] b2 = a2;   // this is okay

This likes a bit strange. Can anyone give an explanation on this?

Answer 1

What you’re seeing in the second case is array covariance. It’s a bad thing IMO, which makes assignments within the array unsafe – they can fail at execution time, despite being fine at compile time.

In the first case, imagine that the code did compile, and was followed by:

b1.add(new SomeOtherTree());
DataNode node = a1.get(0);

What would you expect to happen?

You can do this:

List<DataNode> a1 = new ArrayList<DataNode>();
List<? extends Tree> b1 = a1;

… because then you can only fetch things from b1, and they’re guaranteed to be compatible with Tree. You can’t call b1.add(...) precisely because the compiler won’t know whether it’s safe or not.

Have a look at this section of Angelika Langer’s Java Generics FAQ for more information.