运行Java FXML程序时出错

Question

I'm getting an error on netbeans when trying to run my java fxml application.

Here is the error I am getting:

Caused by: java.lang.NullPointerException at apnc7dsimulator.UIController.handleStart(UIController.java:4‌​2) at apnc7dsimulator.UIController.handleNew(UIController.java:37) at apnc7dsimulator.UIController.initialize(UIController.java:31‌​) at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2548) ... 17 more

It appears the error is originating from my controller. Here's my code:

public class UIController implements Initializable {

@FXML
private TextArea screen;

@FXML
private TextField commandLine; 

private Login login;

@Override
public void initialize(URL location, ResourceBundle resources) {
    handleNew();
}

public void handleNew(){
    screen.clear();
    commandLine.clear();
    handleStart();

}

public void handleStart(){
    login.loginSys(screen);

}}

The code the controller references is this:

public class Login implements PrintToScreen{

@Override
public void print(String text, TextArea screen) {
    screen.appendText(text);
}

@Override
public String read(TextArea screen) {
    Scanner scanner = new Scanner(System.in);
    String readString = scanner.nextLine();
    while(readString != null){

        print(readString,screen);

        if(scanner.hasNextLine()){
            readString = scanner.nextLine();
        }
        else{
            readString = null;
        }
    } 
    return readString;   
}

public void loginSys (TextArea screen)
{
    String user = "...";
    String pass = "...";

    print("Please Enter Username:",screen); 
    user = read(screen);

    print("Please Enter Password:",screen);
    pass = read(screen);

    print("Check Credentials... Please Wait.",screen);
}}

I'm sure it's a simple mistake I'm making or some sort of convention I'm not following correctly but this has had me stumped for a while now. Any help would be great.

Answer 1

Assuming you've posted all of your code, it appears that the login member variable is never initialized. By default it is initialized to null . The result is that when this line of code runs:

login.loginSys(screen);

Since login is null, the code is throwing a NullPointerException .

Somewhere in your code, before handleStart() is called, possibly in the UIController constructor, you need to initialize login , something like:

login = new Login();