Why do I get a type mismatch during compile time

Question

I'm declaring a graph (using JUNG's Graph interface) as a class variable like this:

private Graph<Knoten, Kante> _graph;

I try to initialize it like this:

_graph = new DirectedSparseGraph<AttrKnoten, GewKante>();

AttrKnoten extends Knoten and GewKante extends Kante (they are just marker interfaces at the moment). I get the following error message during compile time:

"Type mismatch: cannot convert from DirectedSpareGraph<AttrKnoten, GewKante> to Graph<Knoten, Kante>"

Why is that? Is there any other way to handle this, but leaving out the parameters during declaration?

Answer 1

You can't do that with generics.

A simpler example:

List<CharSequence> list = new ArrayList<String>();

This doesn't work, even though String implements CharSequence.

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The easiest solution is to just do:

_graph = new DirectedSparseGraph<Knoten, Kante>();

You can still add AttrKnoten and GewKante objects to _graph.

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Alternatively, if you only want AttrKonten and GewKante objects, just declare it as:

private Graph<AttrKnoten, GewKante> _graph;