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 (Browse

Answer 1

ArrayList<listItem> is not equal to ArrayList<browseItem>

They are strictly type safe

you can make use of ?

Answer 2

your example doesnt work, but you can use

ArrayList<? extends ListItem> list = (method returning ArrayList of type browseItem)

this should work.

Answer 3

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.}