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A Single Abstract Method (SAM) interface is a Java interface containing only one method. The Java API is full of SAM interfaces, such as java.lang.Runnable, java.awt.event.ActionListener, java.util.Comparator and java.util.concurrent.Callable. SAM interfaces have a special place in Java 8 because they can be implemented using Lambda expressions or Method references.

Using @FunctionalInterface forces a compile break when an additional, non-overriding abstract method is added to a SAM, which would break the use of Lambda implementations.

Noncompliant Code Example
  public interface Changeable<T> {
    public void change(T o);
  }


Compliant Solution
  @FunctionalInterface
  public interface Changeable<T> {
    public void change(T o);
  }