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.
@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); } }
@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); } }