Implementing multiple interfaces

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Sometimes you may want to have a lambda expression implementing more than one interface. This is mostly useful with marker interfaces (such as since they don’t add abstract methods.

For example, you want to create a TreeSet with a custom Comparator and then serialize it and send it over the network. The trivial approach:

TreeSet<Long> ts = new TreeSet<>((x, y) ->, x));

doesn’t work since the lambda for the comparator does not implement Serializable. You can fix this by using intersection types and explicitly specifying that this lambda needs to be serializable:

TreeSet<Long> ts = new TreeSet<>(
    (Comparator<Long> & Serializable) (x, y) ->, x));

If you’re frequently using intersection types (for example, if you’re using a framework such as Apache Spark where almost everything has to be serializable), you can create empty interfaces and use them in your code instead:

public interface SerializableComparator extends Comparator<Long>, Serializable {}

public class CustomTreeSet {
  public CustomTreeSet(SerializableComparator comparator) {}

This way you’re guaranteed that the passed comparator will be serializable.

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