Interface in generic type Java

Question

Today I found strange code into jdk8 sources and couldn't find any explanation.

 static final Comparator<ChronoLocalDate> DATE_ORDER =
    (Comparator<ChronoLocalDate> & Serializable) (date1, date2) -> {
        return Long.compare(date1.toEpochDay(), date2.toEpochDay());
    };

Can anyone explain me why & Serializable out of <> ?
And it would be great to provide link on documentation.

Link to source: AbstractChronology

Answer 1

& in that context indicates an intersection of types. Say you have classes like these:

interface SomeInterface
{
  public boolean isOkay();
}

enum EnumOne implements SomeInterface { ... }

enum EnumTwo implements SomeInterface { ... }

You want to be able to use any enum that implements SomeInterface as a type parameter in a generic type. Of course, you want to be able to use methods on both Enum and SomeInterface, such as compareTo and isOkay, respectively. Here's how that could be done:

class SomeContainer<E extends Enum<E> & SomeInterface>
{
  SomeContainer(E e1, E e2)
  {
    boolean okay = e1.isOkay() && e2.isOkay();
    if (e1.compareTo(e2) != 0) { ... }
  }
}

See http://docs.oracle.com/javase/specs/jls/se5.0/html/typesValues.html#4.9

Answer 2

There are two parts to your question:

what is & Serializable?

It's a intersection of types - the type must be both Comparator<ChronoLocalDate> and Serializable

why is it not in angle brackets < >?

Because its a cast, not a generic parameter type