This is a follow up question from :: (double colon) operator in Java 8 in which Java allows you to refer to methods using :: operator.
Is it possible to provide some custom Functional Interface that I create and use it with :: operator? And how to do it?
“Is it possible to provide some custom Functional Interface that I create and use it with :: operator? And how to do it?”
It is possible and it’s as easy as you could imagine: just create an interface with exactly one method. You don’t even need the @FunctionalInterface annotation; this annotation just documents your intention and helps detecting errors at compile time similar to @Override.
So maybe you already have created such interfaces in your pre-Java 8 projects…
class Foo {
// nothing new:
public interface FooFactory {
Foo createFoo();
}
// new in Java 8:
public static final FooFactory DEFAULT_FACTORY = Foo::new;
}
How to provide custom functional interface implementation to use :: operator
public class TestingLambda {
public static void main(String[] args) {
int value1 = method(TestingLambda::customMethod1);
int value2 = method(TestingLambda::customMethod2);
System.out.println("Value from customMethod1: " + value1);
System.out.println("Value from customMethod2: " + value2);
}
public static int customMethod1(int arg){
return arg + 1;
}
public static int customMethod2(int arg){
return arg + 2;
}
public static int method(MyCustomInterface ob){
return ob.apply(1);
}
@FunctionalInterface
interface MyCustomInterface{
int apply(int arg);
}
}
I have created my own FunctionalInterface named MyCustomInterface and in Java 8 you have to declare the interface to be functional interface using @FunctionalInterface annotation. Now it has a single method taking int as param and returning int.
Created two methods customMethod1 and customMethod2 which confirm to the signature of that custom interface.
MyCustomInterface) as argumentmethod takes MyCustomInterface in the argument.
And you are ready to go.
In the main I have used the method and passed it the implementation of my custom methods.
method(TestingLambda::customMethod1);
Here you go
import java.util.Arrays;
class Sort {
public int compareByLength(String s1, String s2) {
return (s1.length() - s2.length());
}
}
public class LambdaReferenceExample1 {
public static void main(String[] javalatteLambda) {
String[] str = {"one", "two", "3", "four", "five", "sixsix",
"sevennnnn", "eight"};
Sort sort = new Sort();
Arrays.sort(str, sort::compareByLength);
for (String s : str) {
System.out.println(s);
}
}
}
