CODEWITHSUNDEEP
javagenericstypes
Brett Y
Brett Y

Reputation: 7678

Incompatible types when List of generic objects constructed outside of method call

The following code will not compile

import java.util.List;

class Scratch {

  public static void main(String[] args) {
    Foo<Object> foo = new Foo<>();
    Bar bar = new Bar();

    bar.bar(List.of(foo));  // Okay

    List<Foo<Object>> foos = List.of(foo);
    bar.bar(foos);  // Error
  }

  static class Foo<T> {}

  static class Bar {
    void bar(List<Foo<?>> foos) {}
  }
}

The line bar.bar(foos); casese this error:

incompatible types: java.util.List<Scratch.Foo<java.lang.Object>> cannot be converted to java.util.List<Scratch.Foo<?>>

Why are the types incompatible when the List<Foo<?>> is constructed outside bar() method call?

Upvotes: 1

Views: 145

Answers (2)

ToygunU
ToygunU

Reputation: 141

You might also use a generic type in the method signature other than wildcard, that also still allows any Object type i.e below:

<T> void bar(List<Foo<T>> foos)

instead of

void bar(List<Foo<?>> foos)

Upvotes: 0

Oleg Cherednik
Oleg Cherednik

Reputation: 18245

You should fix the <?>, because Foo<?> and Foo<Object> are not the same.

void bar(List<Foo<Object>> foos) {}

Upvotes: 1

Related Questions