BaseTest.java

package tests;

import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
import org.testng.annotations.AfterMethod;
import org.testng.annotations.BeforeMethod;
import pages.LoginPage;
import pages.ProductsPage;

import java.time.Duration;

public class BaseTest {

    WebDriver driver;
    LoginPage loginPage;
    ProductsPage productsPage;

    @BeforeMethod
    public void setup() {
        driver = new ChromeDriver();
        driver.manage().window().maximize();
        driver.manage().timeouts().implicitlyWait(Duration.ofSeconds(20));
        loginPage = new LoginPage(driver);
        productsPage = new ProductsPage(driver);
    }

    @AfterMethod(alwaysRun = true)
    public void close() {
        driver.quit();
    }
}

CheckoutTest.java

package tests;

public class CheckoutTest extends BaseTest{
}

LoginPage.java

package pages;

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;

public class LoginPage {

    WebDriver driver;
    By usernameField = By.id("user-name");
    By passwordField = By.id("password");
    By loginButton = By.id("login-button");
    By errorMessage = By.cssSelector("[data-test=error]");

    public LoginPage(WebDriver driver) {
        this.driver = driver;
    }

    public void open() {
        driver.get("https://www.saucedemo.com/");
    }

    public void login(String user, String password) {
        driver.findElement(usernameField).sendKeys(user);
        driver.findElement(passwordField).sendKeys(password);
        driver.findElement(loginButton).click();
    }

    public String getErrorMessage() {
        return driver.findElement(errorMessage).getText();
    }
}

LoginTest.java

package tests;

import org.openqa.selenium.By;
import org.testng.Assert;
import org.testng.annotations.Test;

public class LoginTest extends BaseTest {

    @Test
    public void checkPositiveLogin() {
        loginPage.open();
        loginPage.login("standard_user", "secret_sauce");
        Assert.assertTrue(productsPage.isPageOpened());
    }

    @Test
    public void checkLoginWithEmptyUsername() {
        loginPage.open();
        loginPage.login("", "secret_sauce");
        Assert.assertEquals(loginPage.getErrorMessage(), "Epic sadface: Username is required");
    }

    @Test
    public void checkLoginWithEmptyPassword() {
        loginPage.open();
        loginPage.login("standard_user", "");
        Assert.assertEquals(loginPage.getErrorMessage(), "Epic sadface: Password is required");
    }
}

ProductsPage.java

package pages;

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;

public class ProductsPage {

    WebDriver driver;
    By title = By.cssSelector("[data-test=title]");

    public ProductsPage(WebDriver driver) {
        this.driver = driver;
    }

    public boolean isPageOpened() {
        return driver.findElement(title).isDisplayed();
    }
}