Author.java

public class Author {

    private String name;
    private String email;
    private char gender;

    public Author(String name, String email, char gender) {
        this.name = name;
        this.email = email;
        this.gender = gender;
    }

    public String getName() {
        return this.name;
    }

    public String getEmail() {
        return this.email;
    }

    public void setEmail(String newEmail) {
        this.email = newEmail;
    }

    public char getGender() {
        return this.gender;
    }

    @Override
    public String toString() {
        return "Author[name="+this.name+",email="+this.email+",gender="+this.gender+"]";
    }
}

AuthorTest.java

import org.junit.Before;
import org.junit.Test;

import static org.junit.Assert.*;

public class AuthorTest {

    Author author;

    @Before
    public void setUp() throws Exception {
        author = new Author("Daniiar", "daniyarflash.m01@gmail.com", 'm');
    }

    @Test
    public void TestGetName(){
        assertEquals(author.getName(), "Daniiar");
    }

    @Test
    public void TestGetEmail(){
        assertEquals(author.getEmail(), "daniyarflash.m01@gmail.com");
    }

    @Test
    public void TestSetEmail(){
        String newEmail = "gmail@gmail.com";
        author.setEmail(newEmail);
        assertEquals(author.getEmail(), newEmail);
    }

    @Test
    public void TestGetGender(){
        assertEquals(author.getGender(), 'm');
    }


    @Test
    public void TestToString(){
        assertEquals(author.toString(), "Author[name=Daniiar,email=daniyarflash.m01@gmail.com,gender=m]");
    }
}

Book.java

public class Book {

    private String name;
    private Author[] authors;
    private double price;
    private int qty;

    public Book(String name, Author[] authors, double price) {
        this.name = name;
        this.authors = authors;
        this.price = price;
        this.qty = 0;
    }

    public Book(String name, Author[] authors, double price, int qty){
        this.name = name;
        this.authors = authors;
        this.price = price;
        this.qty = qty;
    }

    public String getName() {
        return this.name;
    }

    public Author[] getAuthors() {
        return this.authors;
    }

    public double getPrice() {
        return this.price;
    }


    public void setPrice(double newPrice) {
        this.price = newPrice;
    }

    public int getQty() {
        return this.qty;
    }

    public void setQty(int newQty) {
        this.qty = newQty;
    }

    @Override
    public String toString() {
        String result = "Book[name=" + this.name + "," + "authors={";
        StringBuilder authors_str = new StringBuilder();
        for (Author i: this.authors) {
            authors_str.append(i.toString()).append(",");
        }

        return  result + authors_str.substring(0, authors_str.length() - 1) + "},price=" + this.price + ",qty=" + this.qty + "]";
    }

    public String getAuthorNames() {
        StringBuilder authors_str = new StringBuilder();
        for (Author i: this.authors) {
            authors_str.append(i.toString()).append(",");
        }

        return authors_str.substring(0, authors_str.length() - 1);
    }
}

BookTest.java

import org.junit.Before;
import org.junit.Test;

import static org.junit.Assert.*;

public class BookTest {

    Author author;
    Author author2;
    Book book;
    Author[] authors;

    @Before
    public void setUp() throws Exception {
        author = new Author("Daniiar", "daniyarflash.m01@gmail.com", 'm');
        author2 = new Author("Dastan", "dastan@gmail.com", 'm');

        authors = new Author[2];
        authors[0] = author;
        authors[1] = author2;

        book = new Book("How to Python", authors, 1200.0);
    }

    @Test
    public void TestGetName(){
        assertEquals(book.getName(), "How to Python");
    }

    @Test
    public void TestGetAuthors(){
        assertArrayEquals(book.getAuthors(), authors);
    }

    @Test
    public void TestGetPrice(){
        assertEquals(book.getPrice(), 1200.0, 0);
    }

    @Test
    public void TestSetPrice(){
        double newPrice = 1000;
        book.setPrice(newPrice);
        assertEquals(book.getPrice(), newPrice, 0);
    }

    @Test
    public void TestGetQty(){
        assertEquals(book.getQty(), 0);
    }

    @Test
    public void TestSetQty(){
        int newQty = 1000;
        book.setQty(newQty);
        assertEquals(book.getQty(), newQty);
    }

    @Test
    public void TestToString(){
        assertEquals(book.toString(),
                "Book[name=How to Python,authors={Author[name=Daniiar,email=daniyarflash.m01@gmail.com,gender=m],Author[name=Dastan,email=dastan@gmail.com,gender=m]},price=1200.0,qty=0]");
    }

    @Test
    public void TestGetAuthorNames(){
        assertEquals(book.getAuthorNames(), "Author[name=Daniiar,email=daniyarflash.m01@gmail.com,gender=m],Author[name=Dastan,email=dastan@gmail.com,gender=m]");
    }
}