Bug 124864 - Problems with nested generics
Problems with nested generics
Status: NEW
Product: java
Classification: Unclassified
Component: Compiler
All All
: P4 with 3 votes (vote)
: 6.x
Assigned To: Dusan Balek
Depends on:
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Reported: 2008-01-08 07:16 UTC by nicolo
Modified: 2009-10-30 19:38 UTC (History)
0 users

See Also:
Issue Type: DEFECT


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Description nicolo 2008-01-08 07:16:33 UTC
if I have a class Class1<B> and another class Class2<B, V, C>, and a class Class3 with a method
 public boolean meth(final Class2<? extends B, ?, ? extends Class1<? extends B>>); when I call the method meth from an
other class the editor shows me an error message: "incompatible types: found Class3.meth, required boolean"
The compiler works, but the editor shows me the error
Comment 1 Jan Lahoda 2008-01-15 06:30:33 UTC
Could you please try to find a small reproducible test case and attach it to this issue? Thanks.
Comment 2 nicolo 2008-01-15 08:09:51 UTC
File Base.java
public class Base {

File Model.java
public class Model<B extends Base> {

File Contribution.java
public abstract class Contribution <B extends Base, V, C extends Model<?>> extends Model<B> {

	public Contribution(){

File Manager.java
public class Manager {

	public boolean createContribution(final Contribution<? extends Base, ?, ? extends Model<? extends Base>> contribution) {
		return false;

File Main.java
public class Main {

	public static void main (String[] args){
		final Contribution <?, ?, ?> contr = new Contribution () {};
		Manager man = new Manager();
		if (man.createContribution(contr)){ //It shows the error here!!!!!!!!!!!!!!!!!

It shows an error: Incompatible types: required boolean, found createContribution.
But it compiles and run
Comment 3 matthies 2009-10-30 19:38:23 UTC
Still reproducable with build 200910261401, although with a different error message. But external javac from JDK 1.6 
also gives an error.

There is a simple workaround though, just declare createContribution as:

    public boolean createContribution(final Contribution<?, ?, ?> contribution)

It can be taken as a general coding rule that one shouldn't repeat type parameter bounds in wildcard arguments. For 
example it's redundant to write Model<? extends Base>, as the "extends Base" is already implicit in "Model<?>", because 
Model is defined as "Model<B extends Base>". Likewise, it's redundant to write "Contribution<? extends Base, ?, ? 
extends Model<? extends Base>>", because all these bounds are already implicit in "Contribution<?, ?, ?>".

Of course javac should still be fixed to correctly recognize those equivalencies.

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