Leap Year detection issues

Questions about the leap year test:

Condition: 1. 4 Years a leap, 100 years do not leap, 400 years and leap.

2. Enter the year greater than 0.

3. Enter the number only.

Source:

public class Leapyear extends application {

public static void Main (string[] args) {
Leapyear.launch (args);
}

Private TextField TextField = new TextField ();

@Override
public void Start (Stage arg0) throws Exception {
Arg0.settitle ("Leapyear");

HBox HBox = new HBox (8);
Textfield.setprefcolumncount (25);
Hbox.setalignment (Pos.center_left);
Button btn = New button ();
Btn.settext ("OK");
Btn.setonaction (New Listener ());
Hbox.getchildren (). AddAll (New Label ("Please enter Year:"), TextField, BTN);

Arg0.setscene (New Scene (Hbox, 450, 80));
Arg0.show ();
}

public class Listener implements eventhandler<actionevent> {

@Override
public void handle (ActionEvent arg0) {
String str = Textfield.gettext ();
String inf = "";
if (isleapyear (Integer.parseint (str))) {
inf = "Leap year";
}
Else {
inf = "Non-leap Year";
}
Joptionpane.showmessagedialog (NULL, INF, "Information",
Joptionpane.information_message);
}
}

Private boolean isleapyear (int year) {
if (year% 4! = 0) {
return false;
}
Else if (year%!! = 0) {
return true;
}
Else if (year%!! = 0) {
return false;
}
Else {
return true;
}
}
}

Number	Test Cases	Results
1	1995	Non-leap year
2	2000	Leap
3	2012	Leap
4	0000	Leap
5	1900	Non-leap year

Test results:

1.

2.

3.

4.

5.

But there is a problem with the error when the input is empty. There are also errors when you enter a flight number.

Leap Year detection issues