在用户输入完所有值之前，循环不会在输入负数时结束。

Question

I want the program to end as soon as user enters a value which is less than zero but this program keep asking all three value then exits.

For instance, if I enter -3, 2, 1, The program doesn't display error after entering -3 for the first height, instead it takes all three values then display the "invalid height" message.

How do I make it program display error message of invalid height as soon as user enters a negative value?

//Program Asking user to input three different heights
HeightOne = Integer.parseInt(JOptionPane.showInputDialog("Enter Height of First    Tower"));

HeightTwo = Integer.parseInt(JOptionPane.showInputDialog("Enter Height of Second 
Tower"));

HeightThree = Integer.parseInt(JOptionPane.showInputDialog("Enter Height of Third               Tower"));

If (centimeterHeightOne < 0 || centimeterHeightTwo < 0 || centimeterThree < 0) 
{
    JOptionPane.showMessageDialog(null, "Invalid height";
}
else 
{
   conditions...
}

Answer 1

Loop doesn't end upon entering negative number until user finish entering all values.

Because that's exactly what you're doing. If you want to terminate after one negative value, put if right after you ask for each input:

heightOne = Integer.parseInt(JOptionPane.showInputDialog("Enter Height of First  Tower"));
if(heightOne < 0) {
    displayError();
    //return; ?
}
heightTwo = ...
if(heightTwo < 0) {
    displayError();
}
...

Please follow Java Naming Conventions and change HeightOne to heightOne .

Answer 2

You need to include the check after every value, something like

heightOne = Integer.parseInt(JOptionPane.showInputDialog("Enter Height of First Tower"));
if (heightOne < 0) {
    JOptionPane.showMessageDialog(null, "Invalid height");
    return;
}

The return statement aborts the current method and returns, if the current method is main , the program stops. If you are not in the main method, but you want to forcefully exit the program nevertheless, you can do this by calling System.exit() .