Why doesn't the value type match?

Question

Java

Hello. I am writing a plugin for Jira, in the overridden overridemethod it is required that the method returns a type: List<IssueAction>

Listand IssueActionthese are interfaces.
I googled and found an example return value, it was like this:

return Lists.newArrayList(
    new GenericMessageAction("Запись 1"),
    new GenericMessageAction("Запись 2")
);

GenericMessageAction- This is a class that implements the interface, a IssueAction
newArrayListfunction from which com.google.common.collect.Listsreturns ArrayListthat implements the interface. List

That is, it turns out that by passing instances GenericMessageActionto the list, the method newArrayListreturns to me ArrayList<GenericMessageAction>

. Then I decided to return the list through a variable and return it, like this:

ArrayList<GenericMessageAction> list = Lists.newArrayList(
    new GenericMessageAction("Запись 1"),
    new GenericMessageAction("Запись 2")
);

return list;

So Java won't let me return a list because types do not match.
But I don’t understand why, because it is required to return interfaces List<IssueAction>, I return ArrayList<GenericMessageAction>where it ArrayListimplements Listand GenericMessageActionimplements IssueAction, and that’s it List<IssueAction>.



What am I doing wrong?

Answer 1

Funny, this is the first time I've seen this happen. This is called invariance when ArrayList{GenericMessageAction} is not a child of List{IssueAction}. Made this way because of overwriting generics in java, problems due to absolute compatibility. If you don’t understand what I mean, it’s better to read the articles, it is detailed there.
in your case the solution would be to declare list. You're directly violating the solid by announcing as it is now.