4- Java Comparator Interface

 Java Comparator Interface

Java Comparator interface is used to order the objects of a user-defined class.

It provides multiple sorting sequences, i.e., you can sort the elements on the basis of any data member, for example, rollno, name, age or anything else.

Image 1 -  Methods of Java Comparator Interface

Create a package that have java classes  to create a workable code based on concepts of comparator and lamda expression that sort books on the basis of names

Book.java (Model class for code)

package com.java.lamda.example;

public class Book {

    private int id;

    private String name;

    private int pages;

    /*

     * Getter , Setters,toString , AllArgsConstructor , NoArgsConstructor in SpringBoot framework

     * handled by Lombok Library with annotations like @Data,@Getter,@Setter,etc

     * Refer to SpringBoot Blog

     */

    public int getId() {

        return id;

    }

    public void setId(int id) {

        this.id = id;

    }

    public String getName() {

        return name;

    }

    public void setName(String name) {

        this.name = name;

    }

    public int getPages() {

        return pages;

    }

    public void setPages(int pages) {

        this.pages = pages;

    }

    @Override

    public String toString() {

        return "Book [id=" + id + ", name=" + name + ", pages=" + pages + "]";

    }

    @Override

    public int hashCode() {

        final int prime = 31;

        int result = 1;

        result = prime * result + id;

        result = prime * result + ((name == null) ? 0 : name.hashCode());

        result = prime * result + pages;

        return result;

    }

    @Override

    public boolean equals(Object obj) {

        if (this == obj)

            return true;

        if (obj == null)

            return false;

        if (getClass() != obj.getClass())

            return false;

        Book other = (Book) obj;

        if (id != other.id)

            return false;

        if (name == null) {

            if (other.name != null)

                return false;

        } else if (!name.equals(other.name))

            return false;

        if (pages != other.pages)

            return false;

        return true;

    }

    public Book() {

        super();

    }

    public Book(int id, String name, int pages) {

        super();

        this.id = id;

        this.name = name;

        this.pages = pages;

    }

}

BookDAO.java(Data Layer with an array list or Data from database)

package com.java.lamda.example;

import java.util.ArrayList;

import java.util.List;

public class BookDAO {

   

    public List<Book> getBooks() {

        List<Book> books = new ArrayList<>();

        books.add(new Book(100, "Core Java", 500));

        books.add(new Book(101, "ZCore Java", 400));

        books.add(new Book(363, "Hibernate", 180));

        books.add(new Book(275, "Spring", 200));

        books.add(new Book(893, "WebService", 300));

        return books;

    }

}

BookService.java(Service Layer i.e performs functional code)

package com.java.lamda.example;

import java.util.Collections;

import java.util.List;

public class BookService {

    public List<Book> getBooksInSortByName() {

        List<Book> books = new BookDAO().getBooks();

       

            //Sort to organize in ascending order of name(Format for dictionary)

        Collections.sort(books, (o1, o2) -> o1.getName().compareTo(o2.getName()));

        return books;

    }

    public static void main(String[] args) {

        System.out.println(new BookService().getBooksInSortByName());

    }

}

CONSOLE

[Book [id=100, name=Core Java, pages=500], Book [id=363, name=Hibernate, pages=180], Book [id=275, name=Spring, pages=200], Book [id=893, name=WebService, pages=300], Book [id=101, name=ZCore Java, pages=400]]