CODEWITHSUNDEEP
javagenerics
Duncan3142
Duncan3142

Reputation: 371

inference variable T has incompatible bounds

Given a method with the following signature.

public static <T extends Comparable<T>> int dummyMethod(List<T> list, T elem) {
  // Body
}

Of the following two methods only the first compiles successfully.

public void call() {
  MyClass v = new MyClass();
  List<MyClass> ls = new ArrayList<>();
  dummyMethod(ls, v);
}

public void brokenCall() {
  Comparable<MyClass> v = new MyClass();
  List<Comparable<MyClass>> ls = new ArrayList<>();
  dummyMethod(ls, v);    // Compilation error here.
}

The error returned by javac (JDK 8u60) reads as follows:

.\MyClass.java:23: error: method dummyMethod in class MyClass cannot be applied to given types;
    dummyMethod(ls, v);    // Compilation error here.
    ^
  required: List<T>,T
  found: List<Comparable<MyClass>>,Comparable<MyClass>
  reason: inference variable T has incompatible bounds
    equality constraints: MyClass,Comparable<MyClass>
    lower bounds: Comparable<MyClass>
  where T is a type-variable:
    T extends Comparable<T> declared in method <T>dummyMethod(List<T>,T)
1 error

I'm confused as to why the types of the variables provided as arguments to dummyMethod in brokenCall don't match it's signature.

Upvotes: 3

Views: 5846

Answers (1)

Andreas
Andreas

Reputation: 159215

In brokenCall, you're making T = Comparable<MyClass>, but Comparable<MyClass> extends Comparable<Comparable<MyClass>> is invalid.

Upvotes: 4

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