Introduction

• Function interface is a functional interface in Java 8 that represents a function which takes one argument and produces a result.
• It is part of the java.util.function package.

• Uses of Function Interface:
  • Used to transform data from one type to another.
  • Commonly used in map() operations with streams.
  • Helps write clean, functional-style code using lambda expressions.
  • Can be chained with andThen() and compose() methods for multiple transformations.

• Syntax :

  public interface Function
  {
      R apply(T t);
      //some default methods are also present
  }

Programs:

• 1. Simple program without lambda expression:

  import java.util.function.Function;
  
  public class MainApp
  {
      public static void main(String[] args)
      {
          // Function to find square of a number using anonymous class
          Function<Integer, Integer> square = new Function<Integer, Integer>() {
              @Override
              public Integer apply(Integer x) {
                  return x * x;
              }
          };
  
          // Test numbers
          System.out.println("Square of 5 : " + square.apply(5));
          System.out.println("Square of 9 : " + square.apply(9));
      }
  }

  Output:

  Square of 5 : 25
  Square of 9 : 81

• 2. Program with lambda expression:

  import java.util.function.Function;
  
  public class MainApp
  {
      public static void main(String[] args)
      {
          // Function to find square of a number using lambda expression
          Function<Integer, Integer> square = x -> x * x;
  
          // Test numbers
          System.out.println("Square of 5 : " + square.apply(5));
          System.out.println("Square of 9 : " + square.apply(9));
      }
  }

  Output:

  Square of 5 : 25
  Square of 9 : 81