Java泛型+静态工厂方法= [恐慌]

Question

我想,我现在会理解Java泛型.但现在我又无助了.

我有一个泛型类,其中c-tor构造正确类型的实例,而静态工厂方法产生类型不匹配.

请看下面的代码:

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);
  }

}

错误是:

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>

请帮助理解并解决这个问题.
(对于某些类,我真的需要静态工厂方法).

编辑
有人也可以解释为什么必须使用显式类型参数？

Answer 1

尝试

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

我记得Josh Bloch关于他的Effective Java 2nd Edition的演讲:"每当你指定一个明确的类型参数时,上帝会杀死一只小猫".尽量避免像这样的结构.