TypeParameterShadowing
Type parameter declaration overrides another type parameter already declared

Category
JDK
Severity
WARNING
Maturity

The problem

When declaring type parameters, it’s possible to declare a type parameter with the same name as another type parameter in scope causing unintended incompatibilities when trying to use them together.

public class Foo<T> {
    void instanceMethod(T t) {}
    <T> void genericMethod(T t) {
      instanceMethod(t); // FAIL: T declared in this method doesn't correspond to Foo<T>'s T
    }
}

In some cases, the type variable being declared has no relation to the type variable being shadowed. It may be appropriate to rename the shadowing type variable:

class Logger<T> {
  void log(T t) {}
  <T> logOther(T t) { ... } // Really should be <O> void logOther(O o), since this T is unrelated.
}

Depending on the nature of the surrounding code, you might be able to remove the generic declaration on a method, or convert the generic method into a static method that doesn’t inherit the surrounding type parameter:

class Holder<T> {
  T held;

  public Foo<T> fooIt() {
    return fooify(held);
  }

  private <T> Foo<T> fooify(T t) { ... } // Could be static, or non-generic
}

If an inner class declaration shadows a type variable, you may be able to remove the type variable, make it a static inner class, or rename the type variable:

class BoxingBox<T> {
  // Works if you make the class static.
  // If you remove the type parameter, you'll need to update stuff to List<Container>
  class Container<T> {
    T held;
  }
  List<Container<T>> stuff = new ArrayList<>();

  T get(int index) {
    return stuff.get(index).held;
  }
}

Suppression

Suppress false positives by adding an @SuppressWarnings("TypeParameterShadowing") annotation to the enclosing element.