rspec/rules/S1609/java/rule.adoc

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.


*Note* that this rule is automatically disabled when the project's ``++sonar.java.source++`` is lower than ``++8++``.


== Noncompliant Code Example

----
public interface Changeable<T> {
  public void change(T o);
}
----


== Compliant Solution

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