Using generic as method parameter

Question

I have a problem when using generic types. IDE show me following error:

Cannot select from a type variable

I think here problem with type erasure, but I think here exist some workaround...

My code:

class MyFactory {
  public Object getByClass(Class<?> clazz) {
     ..... 
  }
}


class<T> MyClass {
  private Object myObj = MyFactory.getByClass(T.class);  // HERE ERROR, `Cannot select from a type variable` 
  ...
}

How to solve this problem?

Because of type erasure, it's impossible to tell at runtime what T.class is supposed to refer to. — millimoose
– millimoose, Commented Feb 4, 2013 at 9:40

Tom Hawtin - tackline · Accepted Answer · 2013-02-04 09:38:43Z

3

Pass the Class object into the constructor.

private final Class<T> clazz;
private Object myObj
public MyClass(Class<T> clazz, MyFactory myFactory) {
    this.clazz = clazz;
    this.myObj = myFactory.getByClass(clazz);
}

answered Feb 4, 2013 at 9:38

Tom Hawtin - tackline

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4 Comments

JeroenWarmerdam Over a year ago

Shouldn't that read: this.myObj = myFactory.getByClass(clazz); Also, I don't think it's necessary to pass on the myFactory instance.

Tom Hawtin - tackline Over a year ago

@JeroenWarmerdam Yup, fixed.

Tom Hawtin - tackline Over a year ago

@JeroenWarmerdam (I mean I fixed the T.class to clazz. In the question getByClass is declared as an instance method. If it wasn't an instance method, then that would tend to suggest global state (a bad thing).)

JeroenWarmerdam Over a year ago

I was reading the lower class in which the method is invoked statically. I see now that the upper class defines an instance method. I'll remove my example.

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