CODEWITHSUNDEEP
javauser-interfacenumbers
wong Mr
wong Mr

Reputation: 1

How to limit user to input only numbers?

I am currently creating this java GUI that will ask the user to input 10 entries, then use the values to execte the next action.

I want only numbers or decimal point to be inputted inside such that it can only be a float value.

If it is not number or decimal point, it should prompt the user to input that specific entry again before the next action is executed.

How should I do it?

Upvotes: 0

Views: 3040

Answers (2)

timo_kramer
timo_kramer

Reputation: 21

Wong,

not sure whether you are using Swing or not...

Ages ago I had the same problem and I solved it with creating a class RestrictedTextField extending JTextField. In the constructor I added a key listener (addKeyListener(new RestrictedKeyAdapter());)

private class RestrictedKeyAdapter extends KeyAdapter {

    @Override
    public void keyReleased(KeyEvent e) {

        if (getText().equals("")) {
            oldString = "";
            return;
        } else {
            // if you cannot parse the string as an int, or float, 
            // then change the text to the text before (means: ignore
            // the user input)
            try {
                if (type.equals("int")) {
                    int i = Integer.parseInt(getText());
                    oldString = getText();
                } else if (type.equals("float")) {
                    float f = Float.parseFloat(getText());
                    oldString = getText();
                } else {
                    // do nothing
                }
            } catch (NumberFormatException el) {
                setText(oldString);
            }

            // if the text is identical to the initial text of this
            // textfield paint it yellow. If the text was changed
            // paint it red.
            if (initialString.equals(getText())) {
                setForeground(Color.YELLOW);
            } else {
                setForeground(Color.RED);
            }
        }
    }
}

The idea is, that every time the user presses a key in the textfield (and releases it then), the text in the textfield is parsed. If the component should accept only floats for example then the component tries to parse it as an float (Float.parseFloat(..)). If this parsing is successful everything is fine. If the parsing fails (an NumberFormatException is thrown) then the old text is written back into the textfield (literally ignoring the user input).

I think you can add the KeyAdapter directly to the JTextField without creating a dedicated class for that, but with this solution you can remember the initial string and the old string.

you can play around with the code.. you can change the colour of the textfield if the input is valid or not (or like in my code snippet if the text is identical to the initial string).

one additional comment: I set the 'type' of the textfield in a variable with the name 'type', which is simply a String with the values "int", "float", etc.... a better solution would be here for example an enum of course...

I hope this is helpful...

timo

Upvotes: 2

npinti
npinti

Reputation: 52185

There are various options for what you would like to do. You can check here for one example of doing so. Another example could be to use Formatted TextFields, as shown here.

On the other hand, upon submission, you can try to parse the value to a float or double. If you get any exceptions, then, the value is not a number.

Lastly, you can use Regular Expressions. An expression such as ^\\d+(\\.\\d+)?$ should match any integer or floating point number.

Upvotes: 1

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