Any interface with a single abstract method is a functional interface and its implementation may be treated as lambda expression. It can have only a single functionality. However a functional interface can have unlimited default methods. Note, that default methods have implementation and abstract methods – not. The latter can be only one in functional interface.
When defining your own functional interfaces it is recommended to put an annotation @FunctionalInterface. It helps compiler to identify any inconsistencies and show an error during compilation.
There is a number of out of the box functional interfaces defined in Java8. You can check them listed here.
Here is a sample with custom created functional interface and lambda expression used with it:
<pre>public class FunctionalInterfaceSample {
public static void main(String[] args) {
printResult(5,x -> x * x);
printResult(5,x -> x * x * x);
printResult(6,x -> x * x);
}
public static void printResult(Integer s, MyFunction<Integer, Integer> f) {
System.out.println(f.getResult(s));
}
public interface MyFunction<T, K> {
public K getResult(T a);
}
}
For more information about Java functional interface please check documentation.