Java 8 - Lambda & Functional Interfaces

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Java 8 supposted to be a major release after Java 5 and it brought a lot of new changes and ease for java developers i.e. language,compiler,libraries,tools and runtime (JVM)

Lambdas and Functional Interfaces
Lambdas allow us to treat functionality as a method argument (passing functions around), or treat a code as data. So, this is must be good news for functional developers and from now on no need for anonymous classes in order to do functional programming with java.

Examples (-> lambda):
  1. Arrays.asList(“a”,”b”,”c”,”d”).forEach( e -> System.out.println( e ) );
  2. Arrays.asList(“a”,”b”,”c”,”d”).forEach( ( String str ) -> System.out.println( str ) );
  3. Arrays.asList(“a”,”b”,”c”,”d”).forEach( ( String str ) -> {
        System.out.println( str ) ;
        System.out.println( str + ”,”) ;
    } );
Obviously lambda expression returns a value to but, it must not need to be declared, if not required.
  1. Arrays.asList(“a”,”b”,”c”,”d”).sort( ( e1, e2 ) -> e1.compareTo( e2 ) );
OR
  1. Arrays.asList(“a”,”b”,”c”,”d”).sort( ( e1, e2 ) ->
    {
        int result = e1.compareTo( e2 );
        return result;
    });

Interface’s Default and Static Methods

Two new concepts of methods in interface; default and static methods.
Default method, make interfaces somewhat similar to traits but serve a bit different goal. They allow adding new methods to existing interfaces without breaking the binary compatibility with the code written for older versions of those interfaces.
The difference between default methods and abstract methods is that abstract methods are required to be implemented.

Example:
private interface Defaultable{
    // Interfaces now allow default methods, the implementer may or
    // may not implement (override) them.
    default String notRequired(){
        return “Default Implementation”;
    }
}
private static class DefaultableImpl implements Defaultable{
}
private static class OverridableImpl implements Defaultable{
   @Override
    public static String notRequired(){
         return “New Implementation”;
    }
}


In java 8 interface can declare and implement static methods too:

private interface DefaultableFactory{
    //a new static method’s
    static Defaultable create(Supplier<Defaultable> supplier){
         return supplier.get();
    }
}
The small code snippet below glues together the default methods and static methods from the examples above.

public static void main(String[] args){
       Defaultable defaultable = DefaultableFactory.create(DefaulatableImpl::new);
       System.out.print(defaultable.notRequired());
       defaultable = DefaultableFactory.create( OverridableImpl::new);
       System.out.print(defaultable.notRequired());
}


Default methods implementation on JVM is very efficient and is supported by the byte code instructions for method invocation. Default methods allowed existing Java interfaces to evolve without breaking the compilation process. The good examples are
java.util.Collection interface:stream(),parallelStream(),forEach() and etc.

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