Java 8 allows you to write lambda expressions in few varying syntaxes. Let us quickly see those here.

With no arguments

Runnable r = () -> System.out.println(“H”);

Only one argument: with or without parenthesis

ActionListener al1 = (event) -> System.out.println(“Button clicked”);

ActionListener al2 = event -> System.out.println(“Button clicked”);

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Lambda expressions implement an interface with only one single abstract methodl. Such interfaces are called as functional interfaces. Since the interface has only one single method (function), passing across that interface implementation gives the impression of passing across that function. Already existing interfaces in Java such as Runnable, Comparator, ActionListener are already functional interfaces, as they have only one abstract method. We will see how we can use lambda expressions with those existing interfaces.

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Before Java 8, interfaces could only have abstract methods and constants. Java 8 allows you to have default as well as static methods inside an interface. This was mainly done for interface unlocking: Now you can add methods to an interface without needing the implementing classes to change.

Default methods

You can add an instance method to an interface through the keyword default.

They are also public similar to the abstract methods. 

Default methods are inherited by subclasses and can be overriden. [node:read-more:link]

Method references or constructor references can be used to refer to an existing method or constructor by name. Classes containing these methods can be regular classes without the need to implement or extend anything. However, the target type needs to be a functional interface, as the signature of the method is infered from the functional interface's abstract method. [node:read-more:link]

Java 8 introduces few general purpose functional interfaces based on the input and output to the single abstract method.

Most common ones are:

1. Predicate<T> whose method

   1. takes in an argument of type T and

   2. rerturn a boolean.

2. Consumer<T> whose method

   1. takes in an argument of type T and

   2. rerturn nothing (void).

3. Function<T,R> whose method