Java Functional Interface Definition

In Java, an interface is a special kind of abstract class that contains only constants, method declarations, and default method implementations; it has no implementation code. An interface defines a set of rules specifying which methods and constants classes implementing the interface must provide.
Interfaces are used to define a set of related operations that can be shared by multiple classes. Classes that implement an interface can exhibit different behaviors, but they must provide all methods defined in the interface. Interfaces can be used to achieve polymorphism and are also important building blocks for implementing design patterns.
Interfaces in Java are defined using the interface keyword. Here is a simple example of an interface definition:

public interface MyInterface {
    void doSomething();
    int doSomethingElse();
}

In this example, MyInterface is an interface that defines two methods: doSomething() and doSomethingElse(). These methods have no implementation; they simply declare their signatures.
A class can implement one or more interfaces to provide the methods defined in those interfaces.

Examples

Here is an example of a class implementing an interface:

public class MyClass implements MyInterface {
    public void doSomething() {
        // Implement the method defined in the interface
    }
    public int doSomethingElse() {
        // Implement the method defined in the interface
        return 0;
    }
}

In this example, the MyClass implements the MyInterface, so it must provide implementations for the doSomething() and doSomethingElse() methods.

Functional Interfaces

A functional interface is an interface that contains exactly one abstract method. It is a new feature introduced in Java 8 to support functional programming.
In Java, a functional interface can be identified using the @FunctionalInterface annotation. This annotation is used to check if an interface conforms to the standard of a functional interface, meaning it contains only one abstract method. If an interface contains more than one abstract method, the compiler will report an error.
Functional interfaces can be used as the target type for lambda expressions or method references, making code more concise, readable, and maintainable.
Java 8 includes several commonly used functional interfaces, such as Function, Consumer, Supplier, and Predicate. They represent functions, consumers, suppliers, and predicates, respectively, and can be used in various scenarios like collection operations, thread pools, event handling, etc. Additionally, developers can define custom functional interfaces to meet specific requirements.

@FunctionalInterface
public interface Comparator<T>{
}