selenium send text in alert window

How to insert text in text field of model window using selenium webdriver ?

package SeleniumLearn.Com;
import org.testng.annotations.Test;
import org.openqa.selenium.Alert;
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.testng.annotations.BeforeTest;

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