How to initialize driver once in WebDriver class and then use it to start other classes?

Question

If I have 10 classes in testng.xml, it will open 10 browser sessions. How can I initialize the driver once and only open one browser for one test case then close and open a window again for the second test case and so on?

I have my driver setup code in a constructor, which is maybe a bad way to approach it? How can I initialize it in TNGDriver class and then use it in Testcase1 class?

I tried using @BeforeClass and have a setUp method but that did not work.

TNGDriver class

public abstract class TNGDriver {

public static WebDriver driver;
private static String chromeDriverPath = "C:\\Program Files (x86)\\Google\\Chrome\\Application\\chromedriver.exe";  

@SuppressWarnings("deprecation")
public TNGDriver() {        
    DesiredCapabilities capabilities = DesiredCapabilities.chrome();
    ChromeOptions options = new ChromeOptions();
    options.addArguments("--incognito");
    capabilities.setCapability(ChromeOptions.CAPABILITY, options);
    System.setProperty("webdriver.chrome.driver", chromeDriverPath);        
    driver = new ChromeDriver(capabilities);    
    driver.manage().window().maximize();            
}

public static WebDriver getDriver() {
    return driver;
}

public static void setDriver(WebDriver driver) {
    TNGDriver.driver = driver;
}

Testcase1 class

public class Testcase1 extends Registration {

    @Test(priority = 1) 
    public void step1_checkSomething() {        
        //do something
    }

    @Test(priority = 2)
    public void step2_clickOnSomething() {          
        //click on something            
    }

}

testng.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
<suite name="AutomationFramework" parallel="false" preserve-order="true">
    <test name="Registration">      
        <classes>
            <class name="regression.Testcase01" />
            <class name="regression.Testcase02" />              
        </classes>      
    </test>             
</suite>

Answer 1

Chrome Driver opens every time private window by default, You don't need this really.

//options.addArguments("--incognito");

You can make structure like below :

public class TNGDriver {

public static WebDriver driver;
private static String chromeDriverPath = "C:\\Program Files (x86)\\Google\\Chrome\\Application\\chromedriver.exe";  

public void DriverConfiguration() {        
    DesiredCapabilities capabilities = DesiredCapabilities.chrome();
    ChromeOptions options = new ChromeOptions();
    options.addArguments("--incognito");  
    capabilities.setCapability(ChromeOptions.CAPABILITY, options);
    System.setProperty("webdriver.chrome.driver", chromeDriverPath);        
    driver = new ChromeDriver(capabilities);    
    driver.manage().window().maximize();            
}

public void QuitDriver(){
    driver.quit();
}

}

Unit Test cases:

public class Testcase1 extends Registration {

TNGDriver objTND = new TNGDriver();

    @BeforeTest
    public void initializeDriver(){
    objTND.DriverConfiguration();  
    }

    @Test(priority = 1) 
    public void step1_checkSomething() {        
        //do something
    }

    @Test(priority = 2)
    public void step2_clickOnSomething() {          
        //click on something            
    }

    @AfterTest
    public void initializeDriver(){
    objTND.QuitDriver();  
    }
}

If you want to use browser(to open) before each @Test, You can use this same method with @BeforeMethod annotation.