== Why is this an issue?

Just as there is little justification for writing your own String class, there is no good reason to re-define one of the existing, standard functional interfaces.


Doing so may seem tempting, since it would allow you to specify a little extra context with the name. But in the long run, it will be a source of confusion, because maintenance programmers will wonder what is different between the custom functional interface and the standard one.


=== Noncompliant code example

[source,java]
----
@FunctionalInterface
public interface MyInterface { // Noncompliant
	double toDouble(int a);
}

@FunctionalInterface
public interface ExtendedBooleanSupplier { // Noncompliant
  boolean get();
  default boolean isFalse() {
    return !get();
  }
}

public class MyClass {
    private int a;
    public double myMethod(MyInterface instance){
	return instance.toDouble(a);
    } 
}
----


=== Compliant solution

[source,java]
----
@FunctionalInterface
public interface ExtendedBooleanSupplier extends BooleanSupplier { // Compliant, extends java.util.function.BooleanSupplier
  default boolean isFalse() {
    return !getAsBoolean();
  }
}

public class MyClass {
    private int a;
    public double myMethod(IntToDoubleFunction instance){
	return instance.applyAsDouble(a);
    }
}
----


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

* Drop this interface in favor of "XXX".
* Make this interface extend "XXX" and remove the functional method declaration.


'''
== Comments And Links
(visible only on this page)

=== on 14 Sep 2018, 07:02:32 Michal Domagala wrote:
Current implementation: 5.7.0.15470  sonar-java/java-checks/src/main/java/org/sonar/java/checks/StandardFunctionalInterfaceCheck.java

does not recognize two well known functional interfaces, Runnable and Callable


Link to master \https://github.com/SonarSource/sonar-java/blob/master/java-checks/src/main/java/org/sonar/java/checks/StandardFunctionalInterfaceCheck.java

endif::env-github,rspecator-view[]
