Functional interfaces: Interfaces which define exactly one abstract method. They can be used by Lambdas to instantiate an object of that type with the abstract method implemented.
For e.g.
Runnable r1 = () -> {System.out.println("Hello world 1");}
process(r1); //prints Hello world 1
Runnable is an interface with exactly one abstract method named "run";
OR
process(() -> System.out.println("This is awesome!!"));
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