Type parameter declaration overrides another type parameter already declared


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;


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