Java generics + static factory methods = [panic]

Question

I thought I would already understand Java generics. But now I'm helpless again.

I have a generic class where c-tor builds a properly typed instance, while the static factory method creates a type mismatch.

Take a look at the following code:

public class _GenericFactoryMethods { public final static class DemoClass<T1, T2> { private final T1 _1; private final T2 _2; // static factory method public static <T1, T2> DemoClass<T1, T2> create(T1 _1, T2 _2) { return new DemoClass<T1, T2>(_1, _2); } // usual c-tor public DemoClass(T1 _1, T2 _2) { this._1 = _1; this._2 = _2; } public T1 get1() { return _1; } public T2 get2() { return _2; } } public void doSomething() { String str = "test"; Class<? extends CharSequence> _1 = str.getClass(); CharSequence _2 = str; // works DemoClass<Class<? extends CharSequence>, CharSequence> dc1 = new DemoClass<Class<? extends CharSequence>, CharSequence>(_1, _2); // compile error DemoClass<Class<? extends CharSequence>, CharSequence> dc2 = DemoClass.create(_1, _2); } } 

Error:

 Uncompilable source code - incompatible types required: _GenericFactoryMethods.DemoClass<java.lang.Class<? extends java.lang.CharSequence>,java.lang.CharSequence> found: _GenericFactoryMethods.DemoClass<java.lang.Class<capture#794 of ? extends java.lang.CharSequence>,java.lang.CharSequence> 

Please help to understand and solve this.
(I really need static factory methods for some of these classes).

EDIT
Can someone else explain why explicit type parameters should be used

Answer 1

Try

 DemoClass.<Type1, Type2>create(_1, _2); 

As I recall from Josh Bloch's presentation, Effective Java 2nd Edition: "God kills a kitten every time you specify an explicit type parameter." Try to avoid such designs.

Answer 2

In this case, you need to explicitly set the generic type:

 dc2 = DemoClass.<Class<? extends CharSequence>, CharSequence>create(_1, _2);