while execuiting javascript in selenium webdriver error occur

Question

 static WebDriver driver = new  FirefoxDriver();
    public static void main(String[] args) throws IOException, InterruptedException {
        // TODO code application logic here
        BufferedReader br= new BufferedReader(new InputStreamReader(System.in));
        System.out.println("Enter base url....!!!");
        String baseurl = br.readLine();
        connection c= new connection();
        c.makeConnection(driver,baseurl);
       JavascriptExecutor js;
      if (driver instanceof JavascriptExecutor) {
    js = (JavascriptExecutor)driver;
    js.executeScript("alert('hello world');");
  }

Following error occurs

Exception in thread "main" org.openqa.selenium.UnhandledAlertException: Modal dialog present: hello world
Build info: version: '2.39.0', revision: 'ff23eac', time: '2013-12-16 16:11:15'
System info: host: 'comp74', ip: '192.168.0.74', os.name: 'Windows 7', os.arch: 'x86', os.version: '6.1', java.version: '1.7.0_17'
Session ID: f6ba3a2b-87d4-4192-ad1a-94cf260d8f6e
Driver info: org.openqa.selenium.firefox.FirefoxDriver
Capabilities [{platform=XP, acceptSslCerts=true, javascriptEnabled=true, cssSelectorsEnabled=true, databaseEnabled=true, browserName=firefox, handlesAlerts=true, browserConnectionEnabled=true, webStorageEnabled=true, nativeEvents=true, rotatable=false, locationContextEnabled=true, applicationCacheEnabled=true, takesScreenshot=true, version=26.0}]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
    at org.openqa.selenium.remote.ErrorHandler.createThrowable(ErrorHandler.java:193)
    at org.openqa.selenium.remote.ErrorHandler.createUnhandledAlertException(ErrorHandler.java:174)
    at org.openqa.selenium.remote.ErrorHandler.throwIfResponseFailed(ErrorHandler.java:141)
    at org.openqa.selenium.remote.RemoteWebDriver.execute(RemoteWebDriver.java:554)
    at org.openqa.selenium.remote.RemoteWebDriver.findElements(RemoteWebDriver.java:332)
    at org.openqa.selenium.remote.RemoteWebDriver.findElementsByXPath(RemoteWebDriver.java:408)
    at org.openqa.selenium.By$ByXPath.findElements(By.java:358)
    at org.openqa.selenium.remote.RemoteWebDriver.findElements(RemoteWebDriver.java:295)
    at javaapplication4.count.linkCount(count.java:27)
    at javaapplication4.JavaApplication4.main(JavaApplication4.java:34)

Answer 1

Is per my understanding. After executing an alert if you try to do any other operation in the page when the alert already exists this errors comes. Are you providing the actual code?

Exception shows 
at javaapplication4.count.linkCount(count.java:27)
at javaapplication4.JavaApplication4.main(JavaApplication4.java:34)

as per above stacktrace you do something in linkCount. If you have anycode after .executeScript("alert('hello world' line. Please comment them and then check.