Functional Interfaces in java

What is functional interface?

• Functional interface must have exactly one abstract method
• Functional interface can have any number of default methods.
• @FunctionalInterface annotation is used to verify the if the interface follows all the rules for a functional interface

Some of the functional Interfaces :

• java.util.function.Predicate
• java.util.function.Consumer
• java.lang.Runnable
• java.util.Comparator

Lambda expression is used to represent the functional interface. If you add more than one abstract method for the functional interface, you will get an error in representing the functional interface. So, the thumb rule to be followed is,

Functional interface should have only one abstract method and that rule can be verifed by “@FunctionalInterface ” annotation.

@FunctionalInterface
public interface FunctionalInterfaceDemo {
	public String sayHello(String s);
	default String covertUpperCase(String s) {
		String result =s.toUpperCase();
		return result;
	}
}

If you try to add more than one abstract method, you will get the below error.