Selenium Java WebDriver 中的空指针异常

Question

我的代码：

public class Testlogin {
    WebDriver driver;
    public Testlogin(WebDriver driver) {
        this.driver=driver;
    }
    WebElement userName = driver.findElement(By.id("username"));
    WebElement Password = driver.findElement(By.id("password"));
    WebElement login = driver.findElement(By.xpath("//button"));
    public void loginpages(String user,String pass) {
        userName.sendKeys(user);
        Password.sendKeys(pass);
        login.click();
    }
}

---

public class Testclass {
    public static void main(String[] args) {
        WebDriver driver = new FirefoxDriver(); 
        driver.get("https://the-internet.herokuapp.com/login");
        Testlogin test = new Testlogin(driver);
        test.loginpages("tomsmith","SuperSecretPassword!");
    }
}

收到以下错误：

Exception in thread "main" java.lang.NullPointerException
    at Test.Testlogin.<init>(Testlogin.java:18)
    at Test.Testclass.main(Testclass.java:14)

Answer 1

必须首先设置驱动程序对象。 例如，将其移动到构造器内：

public Testlogin(WebDriver driver)
{
    this.driver=driver;
    WebElement userName = driver.findElement(By.id("username"));
    WebElement Password = driver.findElement(By.id("password"));
    WebElement login = driver.findElement(By.xpath("//button"));
}

Answer 2

如果尚未设置驱动程序，则使testlogin类如下所示，它将指向null，并且当您尝试运行driver.findElement(By.id("username"));并且驱动程序为null时，这将无法解决此问题，因为Aiden Grossman说这些将在设置驱动程序时初始化

public class Testlogin {
    WebDriver driver;
    public Testlogin(WebDriver driver) {
        this.driver=driver;
        WebElement userName = driver.findElement(By.id("username"));
        WebElement Password = driver.findElement(By.id("password"));
        WebElement login = driver.findElement(By.xpath("//button"));
    }
    public void loginpages(String user,String pass) {
        userName.sendKeys(user);
        Password.sendKeys(pass);
        login.click();
    }
}