在线程“main”中获取错误异常 java.lang.NullPointerException

Question

当我在 selenium 中运行以下类文件时，会启动浏览器并打开 URL，但之后出现错误：

exception in thread "main" java.lang.NullPointerException
    at guru99project.guru99project.printtitlepage(guru99project.java:25)
    at guru99project.Main_method.main(Main_method.java:10)

代码试验：

package guru99project;

import org.openqa.selenium.By;
import org.openqa.selenium.Dimension;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.chrome.ChromeDriver;

public class guru99project {

 public WebDriver driver;

    public void invokechrome() {
        System.setProperty("webdriver.chrome.driver","C:\\Users\\A\\eclipse-workspace\\libs\\chromedriver.exe");
        WebDriver driver = new ChromeDriver();
        Dimension dim = new Dimension(640,480);
        driver.manage().window().setSize(dim);
        driver.manage().window().maximize();
        driver.manage().deleteAllCookies();
        driver.get("http://demo.guru99.com/v4");            
    }

    public void printtitlepage() {

        System.out.println(driver.getTitle());      
    }

    public void login (String uname, String pwd) {
        WebElement usid = driver.findElement(By.name("uid"));       
        usid.sendKeys(uname);

     //   driver.findElement(By.name("uid")).sendKeys(uname);
        driver.findElement(By.name("password")).sendKeys(pwd);
        driver.findElement(By.name("btnLogin")).click();



    }

}

    package guru99project;

public class Main_method {

    public static void main(String[] args) {

        guru99project gp = new guru99project();

        gp.invokechrome();
        gp.printtitlepage();
        gp.login("mnr160177", "hynydej");

    }

}

Answer 1

您已经将驱动程序定义为全局（类级别）的WebDriver实例：

public WebDriver driver;

因此，您不需要在同一个Class内的任何methods()定义WebDriver 的任何其他新实例，并继续使用WebDriver的相同全局定义实例，即driver 。

您需要从行中删除WebDriver ：

WebDriver driver = new ChromeDriver();

所以行将是：

driver = new ChromeDriver();

Answer 2

您已被指定为public WebDriver driver;

所以你不需要再次指定为，

Webdriver driver = new ChromeDriver();//if you need This , Then Don't Specify public Webdriver driver at the Top. Initialize here it self and Import it. 

请将上面的行更改为，

driver = new ChromeDriver();