Java Create String While Typing

Question

I have a JTextField and an JLabel; if an user types a special character, such as /, I want the JLabel to setVisible(true).

I want this to be done without the user pressing any button, but in "real-time" as the typing is done, for each new character that is inserted, a check will be done. How do I do something like this?

Yes, use a DocumentListener. It is the API designed for usage with Swing. Read the Section from the Swing tutorial on How to Write a DocumentListener for more information and examples. — camickr
– camickr, Commented Jan 2, 2016 at 16:16
@Stultuske As a general rule of thumb, KeyListener is a bad idea, but especially for text components — MadProgrammer
– MadProgrammer, Commented Jan 2, 2016 at 21:07
@MadProgrammer: I never stated it to be ideal, but still better than "I don't know". If someone can't find a single way to do it, unfortunately, the "bad" ways are easier to explain than the "best design" practices — Stultuske
– Stultuske, Commented Jan 3, 2016 at 11:58

Whateverest · Accepted Answer · 2016-01-02 17:23:23Z

Simple solution using DocumentListener:

import java.awt.FlowLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;

public class DemoFrame extends JFrame {

    JTextField tf;
    JLabel lbl;

    public DemoFrame() {
        tf = new JTextField(10);
        lbl = new JLabel("Test");
        tf.getDocument().addDocumentListener(new DemoDocumentListener());
        lbl.setVisible(false);

        this.setLayout(new FlowLayout(FlowLayout.LEFT));
        this.add(tf);
        this.add(lbl);

        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(500, 100);
        this.setVisible(true);
    }

    public void checkForSpecialString() {
        if (tf.getText().contains("/")) {
            lbl.setVisible(true);
        } else {
            lbl.setVisible(false);
        }
    }

    class DemoDocumentListener implements DocumentListener {
        @Override
        public void insertUpdate(DocumentEvent e) {
            checkForSpecialString();
        }
        @Override
        public void removeUpdate(DocumentEvent e) {
            checkForSpecialString();
        }
        @Override
        public void changedUpdate(DocumentEvent e) {
            checkForSpecialString();
        }
    }

    public static void main(String[] args) {
        new DemoFrame();
    }
}

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