Selenium TestNG - java.lang.NullPointerException in automation testing

Question

I am trying to test the login function to my android app. But it displays an exception error java.lang.NullPointerException.

Code as follows:

package tests;

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import io.appium.java_client.AppiumDriver;
import io.appium.java_client.MobileElement;
import io.appium.java_client.android.AndroidDriver;

import org.openqa.selenium.By;
import org.openqa.selenium.remote.DesiredCapabilities;

import org.testng.annotations.BeforeTest;

import org.testng.annotations.Test;

public class Amazon

{

    public static AppiumDriver<MobileElement> driver;

    @BeforeTest

    public void launch() throws IOException

    {

        // Set the Desired Capabilities
        DesiredCapabilities caps = new DesiredCapabilities();
        caps.setCapability("deviceName", "Bright Roots (Galaxy S4)");
        caps.setCapability("udid", "4d00b778d48a421b"); // Give Device ID of your mobile phone
        caps.setCapability("platformName", "Android");
        caps.setCapability("platformVersion", "5.0.1");
        caps.setCapability("appPackage", "com.example.babilok");
        caps.setCapability("appActivity", "com.example.babilok.MainActivity");
        caps.setCapability("noReset", "true");

        try {

            AppiumDriver<MobileElement> Driver = new AndroidDriver<MobileElement>(new URL("http://0.0.0.0:4723/wd/hub"),
                    caps);


        } catch (MalformedURLException e) {
            System.out.println(e.getMessage());
        }

    }

    @Test

    public void print() throws Exception

    {

        // locate the Text on the calculator by using By.index()
        MobileElement driver = driver.find_element_by_xpath("//android.widget.Button[@text='7']")
        driver.click();

    }

//  @AfterTest
//
//  public void close() throws IOException
//
//  {
//      driver.quit();
//
//  }

}

Error:

[RemoteTestNG] detected TestNG version 7.0.0 Oct 19, 2019 3:48:46 PM io.appium.java_client.remote.AppiumCommandExecutor$1 lambda$0 INFO: Detected dialect: W3C FAILED: print java.lang.NullPointerException at tests.Amazon.print(Amazon.java:58) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) at java.lang.reflect.Method.invoke(Unknown Source) at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:133) at org.testng.internal.TestInvoker.invokeMethod(TestInvoker.java:584) at org.testng.internal.TestInvoker.invokeTestMethod(TestInvoker.java:172) at org.testng.internal.MethodRunner.runInSequence(MethodRunner.java:46) at org.testng.internal.TestInvoker$MethodInvocationAgent.invoke(TestInvoker.java:804) at org.testng.internal.TestInvoker.invokeTestMethods(TestInvoker.java:145) at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:146) at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:128) at java.util.ArrayList.forEach(Unknown Source) at org.testng.TestRunner.privateRun(TestRunner.java:770) at org.testng.TestRunner.run(TestRunner.java:591) at org.testng.SuiteRunner.runTest(SuiteRunner.java:402) at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:396) at org.testng.SuiteRunner.privateRun(SuiteRunner.java:355) at org.testng.SuiteRunner.run(SuiteRunner.java:304) at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:53) at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:96) at org.testng.TestNG.runSuitesSequentially(TestNG.java:1180) at org.testng.TestNG.runSuitesLocally(TestNG.java:1102) at org.testng.TestNG.runSuites(TestNG.java:1032) at org.testng.TestNG.run(TestNG.java:1000) at org.testng.remote.AbstractRemoteTestNG.run(AbstractRemoteTestNG.java:115) at org.testng.remote.RemoteTestNG.initAndRun(RemoteTestNG.java:251) at org.testng.remote.RemoteTestNG.main(RemoteTestNG.java:77)

Answer 1

In the test you use a property driver which has not been instantiated, because you create the driver into local a variable called Driver. See the case-difference? The class property driver has a default value of NULL and that is why you get a nullpointer exception when you try to use it. You cannot use objects that are NULL.

This is a foundational Java programming language issue, I suggest you first learn the programming language before you start automating tests in it. This book helped me a lot to understand Java and how to use its classes and standard libraries better. I really suggest you learn how to use Java and its basic OO-concepts first before you continue with your trail and error.

Code change I think might make your example work. This code:

       AppiumDriver<MobileElement> Driver = new AndroidDriver<MobileElement>(new URL("http://0.0.0.0:4723/wd/hub"),
                caps);

Should be:

       driver = new AndroidDriver<MobileElement>(new URL("http://0.0.0.0:4723/wd/hub"),
                caps);

Because you want to use the class variable driver and not create a new local variable Driver.

This code:

    MobileElement driver = driver.find_element_by_xpath("//android.widget.Button[@text='7']")
    driver.click();

Should be:

    MobileElement element = driver.find_element_by_xpath("//android.widget.Button[@text='7']")
    element.click();

You cannot re-assing the driver, you want a new variable to interact with elements.