TypeParameterUnusedInFormals
Declaring a type parameter that is only used in the return type is a misuse of generics: operations on the type parameter are unchecked, it hides unsafe casts at invocations of the method, and it interacts badly with method overload resolution.

Category
Severity
WARNING
Maturity

The problem

A method’s type parameters should always be referenced in the declaration of one or more formal parameters. Type parameters that are only used in the return type are a source of type-unsafety. First, operations on the type will be unchecked after the type parameter is erased. For example:

static <T> T doCast(Object o) {
  return (T) o; // this will always succeed, since T is erased
}

The ‘doCast’ method would be better implemented as:

static <T> T doCast(Class<T> clazz, Object o) {
  return clazz.cast(o); // has the expected behaviour
}

Second, this pattern causes unsafe casts to occur at invocations of the method. Consider the following snippet, which uses the first (incorrect) implementation of ‘doCast’:

this.<String>doCast(42); // succeeds
String s = doCast(42); // fails at runtime

Finally, relying on the type parameter to be inferred can have surprising results, and interacts badly with overloaded methods. Consider:

<T> T getThing()
void assertThat(int a, int b)
void assertThat(Object a, Object b)

This invocation will be ambiguous:

// both method assertThat(int,int) and method assertThat(Object,Object) match
assertThat(42, getThing());

Suppression

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