Question

Either I’m too stupid to use google, or nobody else encountered this problem so far.

I’m trying to compile the following code:

public interface MyClass {
  public class Util {
    private static MyClass _this;
    public static <T extends MyClass> T getInstance(Class<T> clazz) {
      if(_this == null) {
        try {
          _this = clazz.newInstance();
        } catch(Exception e) {
          e.printStackTrace();
        }
      }
      return _this;
    }
  }
}

Howerer, in the line “return _this;” I get the error “Type mismatch: cannot convert from MyClass to T”
Why is this? T extends MyClass, so where is the problem? If I change the line to “return (T)_this;”, i just get a warning about the unchecked cast, but I don’t like warnings 😉 Is there a way to achieve what i want without an error or warning?

Answer 1

Imagine you have two implementations of MyClass, Foo and Bar. As a field of type MyClass, _this could be a Foo or a Bar.

Now, since your getInstance method returns <T extends MyClass>, it’s legal to call it any of these ways:

MyClass myClass = Util.getInstance(MyClass.class);

This doesn’t work if it’s the first call, because MyClass is an interface and can’t be instantiated with newInstance().

Foo foo = Util.getInstance(Foo.class);
Bar bar = Util.getInstance(Bar.class);

Now, what would happen if _this was an instance of Foo and you called Util.getInstance(Bar.class)? That’s why you aren’t allowed to do this.