The Function functional interface is a predefined interface introduced in Java 8 as part of the java.util.function package. It represents a function that accepts one input argument and produces one result. This interface is widely used in lambda expressions, Stream API operations, and functional programming.

A key feature of the Function interface is that it enables developers to define reusable transformation logic, where an input object is converted into another form. For example, converting a String into its length, a Student object into a grade, or an entity into a DTO.

Function is functional interface that takes one argument and produces a result ,

Syntax:

package com.test.rkdigitalschool.pred;
@FunctionalInterface
public class Function<T, R> {
	R apply(T t);
}

• T: Type of input
• R: Type of the results
• apply(T t ) : Abstract Method that perform the operation

package com.test.rkdigitalschool.pred;

import java.util.function.Function;
public class Example {
	public static void main(String[] args) {
		Function<String, Integer> strLength = str -> str.length();
        System.out.println(strLength.apply("ChatGPt Full course by Rkdigital school"));
	}
}

• Function can take 2 type parameters first one represent input argument type and second one represent return type .
• Function interface defines one abstract method call apply() .
• Function can return any type of value .

Program1 : Write a program to remove space present in the given string by using function .

package com.test.rkdigitalschool.pred;
import java.util.function.Function;
public class FuncTest {
	public static void main(String[] args) {
		String s="Rkdigital school Navi Mumbai";
		Function<String,String> f= s1 -> s1.replaceAll(" ", "");
		System.out.println(f.apply(s));
	}
}

Program2: Writ a program to find student grade by using function

Student.java

package com.test.rkdigitalschool.pred;

public class Student {

    String name;
    int marks;

    public Student(String name, int marks) {
        this.name = name;
        this.marks = marks;
    }
}

FunTest.java

package com.test.rkdigitalschool.pred;

import java.util.ArrayList;
import java.util.function.Function;
public class FunTest {

    public static void main(String[] args) {

        // Create student list
        ArrayList<Student> sl = new ArrayList<>();
        sl.add(new Student("Rohit", 85));
        sl.add(new Student("Amit", 65));
        sl.add(new Student("Sneha", 55));
        sl.add(new Student("Neha", 40));

        // Function to calculate grade
        Function<Student, String> f = s -> {
            int marks = s.marks;

            if (marks >= 80) {
                return "A [Distinction]";
            } else if (marks >= 60) {
                return "B [First Class]";
            } else if (marks >= 50) {
                return "C [Second Class]";
            } else {
                return "D [Failed]";
            }
        };
        Predicate<Student> p=s.marks >=60;
        // Apply function and print result
        for (Student s : sl) {
            if(p.test(s)){
            System.out.println(
                "Name: " + s.name +
                ", Marks: " + s.marks +
                ", Grade: " + f.apply(s)
            );
          }
        }
    }
}