java - Compile error when providing interface as an arraylist type

Question

I have an interface defined as

interface ListItem {
    public String toString();
    public String getUUID();
}

And a class (BrowseItem) implementing that interface. When I try:

ArrayList<ListItem> = (method returning ArrayList of type BrowseItem)

I get an incompatible type error (found ArrayList<BrowseItem>, require ...<ListItem>)

Am I approaching this wrong?

Answer 1

Java generics are not covariant.

See (among many other questions on SO):

• java generics covariance
• Java covariance
• Java collections covariance problem
• Using generic collections in arguments

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Solutions:

• Change the return type of the problematic method. That is, change

  List<listItem> = (method returning List of type browseItem)
  // to
  List<listItem> = (method returning List of type listItem)
  

• Use wildcard covariance (I think that's what this is called):

  List<? extends listItem> = (method returning List of type browseItem)
  

  Be aware that you cannot add items to the list if you take this route.

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_{N.B. it is generally good practice to declare list types as List<T> and not ArrayList<T>. The pseudocode above reflects this.}