Java 8 : Frequently Asked Questions for Interviews - 5

What are Functional Interfaces

A functional interface is an interface with a single abstract method. That abstract method is used as the type of a lambda expression.

public interface ActionListner extends EventListener {
      public void actionPerformed( ActionEvent event);
}

In the above example ActionListener has only one method actionPerformed and we use it to represent the action that takes one argument and returns no result. => It is a functional Interfaces.

• It doesn't matter what the single method ( e.g. actionPerformed ) is called. 
• The method will match up to lambda expression as long as it has compatible method signature
• One can see the type of the parameter in the functional interface => improves readability and understanding in code.
• Functional interfaces can use generics also.
• Some important functional interfaces in Java :

Interface Name	Arguments	Returns	Example
Predicate<T>sasdadas	Tdfasdfasdfafsadfs	booleanasfsdfsdadfsd	Has this book been published yet?aasdfsdfasdfasdfasdfasdfasdfasfsdfsfsdfsdadfsd
Consumer<T>	T	void	Printing out a value. Getting an action/task done.
Function<T,R>	T	R	Get a property/instance var from an object. Eg: get the author from book object
Supplier<T>	None	T	Any Factory Method. Get Constant Values defined in certain context.
UnaryOperator<T>	T	T	Logical not (!)
BinaryOperator<T> (T,T)	(T,T)	T	Multiplying two numbers (*)

• There are many interfaces in Java that encapsulates blocks of code such as "Runnable" or "Comparator"
• Lambdas are backwards compatible with these interfaces.

When to supply Lambda Expression?

• When an object of an interfaces with a single method is expected. Eg. ActionListerner is the functional interfaces in  the following example: The Left Hand Side Code changes to written in Lambda expression in the right hand side.

button.addActionListener( new ActionListener() {     public void actionPerformed(ActionEvent event){         System.out.println("Click on Button");         } })

button.addActionListerner( event ->              System.out.println("Click on Button"); )

• The boiler plate code is minimized, readability enhanced and meaningfulness accomplished.

Why a functional interface must have a single abstract method. Aren’t all methods in an interface abstract?

• In Java 8, interfaces can declare nonabstract methods.
• Some interfaces in the Java API redeclare Objectmethods (such as toString or clone) to attach javadoc comments. But these declarations do not make the methods abstract.
• Check out the Comparator API for an example.