Introduction to Function Functional Interface

Interface Name	Description	Abstract Method
Function<T, R>	Accepts one argument and produces a result.	R apply(T t)
DoubleFunction<R>	Accepts a double-valued argument and produces a result.	R apply(double d)
IntFunction<R>	Accepts a int-valued argument and produces a result.	R apply(int i)
LongFunction<R>	Accepts a long-valued argument and produces a result.	R apply(long l)
DoubleToIntFunction	Accepts a double-valued argument and produces an int-valued result.	int applyAsInt(double d)
DoubleToLongFunction	Accepts a double-valued argument and produces a long-valued result.	long applyAsLong(double d)
IntToDoubleFunction	Accepts a int-valued argument and produces an double-valued result.	double applyAsDouble(int i)
IntToLongFunction	Accepts a int-valued argument and produces an long-valued result.	long applyAsLong(int i)
LongToDoubleFunction	Accepts a long-valued argument and produces an double-valued result.	double applyAsDouble(long l)
LongToIntFunction	Accepts a long-valued argument and produces an int-valued result.	int applyAsInt(long l)
ToDoubleFunction<T>	Accepts a reference type and produces a double-valued result.	double applyAsDouble(T t)
ToIntFunction<T>	Accepts a reference type and produces a int-valued result.	int applyAsInt(T t)
ToLongFunction<T>	Accepts a reference type and produces a long-valued result.	long applyAsLong(T t)
BiFunction<T,U,R>	Accepts two arguments and produces a result.	R apply(T t, U u)
ToDoubleBiFunction<T, U>	Accepts two arguments and produces a double-valued result.	double applyAsDouble(T t, U u)
ToIntBiFunction<T, U>	Accepts two arguments and produces a int-valued result.	int applyAsInt(T t, U u)
ToLongBiFunction<T, U>	Accepts two arguments and produces a long-valued result.	long applyAsLong(T t, U u)

Function functional interface has following methods:

Applicant Example using Function functional Interface

public class Applicant {
    private String name;
    private String email;
    private int yearOfExperience;
    private double qualificationScore;

    public Applicant(String name, String email, int yearOfExperience, double qualificationScore) {
        this.name = name;
        this.email = email;
        this.yearOfExperience = yearOfExperience;
        this.qualificationScore = qualificationScore;
    }

    public int getYearOfExperience() {
        return yearOfExperience;
    }

    public double getQualificationScore() {
        return qualificationScore;
    }
}

public class Main {
    public static void main(String[] args) {
        Applicant newApplicant = new Applicant("Alice", "alice@gmail.com", 4, 6.7);
        Function<Applicant, Double> suitabilityCalculator = (Applicant applicant) ->{
            double experienceFactor = 0.2;
            double qualificationFactor = 0.8;
            return (experienceFactor * applicant.getYearOfExperience()) + (qualificationFactor*applicant.getQualificationScore());
        };

        Double suitabilityScore = suitabilityCalculator.apply(newApplicant);
        System.out.println("New Applicant's suitability score is:  " + suitabilityScore);
    }
}