Getting Null pointer exception while running my code - selenium

Question

I am getting the null pointer error in the below code. If you run the code it will execute all the code just before the code where i mentioned 'captureScreenshot()' function after this its not executing.
package rough;
import java.io.File;
import java.io.IOException;
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.concurrent.TimeUnit;
import org.apache.commons.io.FileUtils;
import org.openqa.selenium.By;
import org.openqa.selenium.OutputType;
import org.openqa.selenium.TakesScreenshot;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
public class TestEmailAndScreenshot1 {
public static WebDriver dr;
public static String mailscreenshotpath;
public static void captureScreenshot(){
Calendar cal = new GregorianCalendar();
int month = cal.get(Calendar.MONTH);
int year = cal.get(Calendar.YEAR);
int sec = cal.get(Calendar.SECOND);
int min = cal.get(Calendar.MINUTE);
int date = cal.get(Calendar.DATE);
int day = cal.get(Calendar.HOUR_OF_DAY);
File scrFile = ((TakesScreenshot)dr).getScreenshotAs(OutputType.FILE);
try {
mailscreenshotpath = System.getProperty("user.dir")+"\\screenshot\\"+year+"_"+date+"_"+(month+1)+"_"+day+"_"+min+"_" +sec+".jpeg";
FileUtils.copyFile(scrFile, new File(mailscreenshotpath));
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public static void main(String[] args) {
WebDriver dr=new FirefoxDriver();
dr.manage().window().maximize();
dr.manage().timeouts().implicitlyWait(10L, TimeUnit.SECONDS);
dr.get("http://gmail.com");
dr.findElement(By.id("Email")).sendKeys("vijenderchhoker92");
captureScreenshot();
System.out.println(" Screenshot taken successfully....");
}
}

Answer 1

When you do
WebDriver dr = new FirefoxDriver();
In main you hide the public static WebDriver dr; variable, so in captureScreenshot() method it's still null and you get the error trying to use it. Change to
public static void main(String[] args) {
dr = new FirefoxDriver();
dr.manage().window().maximize();
//...
}