Input from jtextfield doesnt get read while in a for loop

My problem is that I have a for loop and while this loop is running I still want inputs from JTextField to get read and not interrupt the loop or timing when the line gets printed.

public class test {

    public void Play() {

        JTextField textField = new JTextField();
        textField.addKeyListener(new KeyChecker());
        JFrame jframe = new JFrame();
        jframe.add(textField);
        jframe.setSize(200, 30);
        jframe.setVisible(true);
    }
}

class KeyChecker extends KeyAdapter {

    @Override
    public void keyPressed(KeyEvent event)
    {
        for(int i = 0; i < 1; i = 0)
        {
            char Input = event.getKeyChar();
            if(Input == 'a')
            {
                System.out.println("working");
            } else {
                System.out.println("not working");
            }
            try
            {
                Thread.sleep(1000);
            }
            catch(InterruptedException ex)
            {
                Thread.currentThread().interrupt();
            }
        }
    }
}

In your code, the keyPressed method gets called by the Event Dispatch Thread (https://docs.oracle.com/javase/tutorial/uiswing/concurrency/dispatch.html) every time you press a key in the textfield.

In this context the KeyEvent event parameter represents a sing key press event, so it makes no sense to create a loop inside an event method, as maloomeister correctly pointed out.

Instead you should perform your logic using the state of your classes, either KeyChecker or test. I don't really understand why you should regularly check for a key being pressed (and the purpose of a textfield BTW), anyway the following is a possible solution using a separate thread. Of course you can also do the loop in the main thread (which is not the EDT), but in a non-trivial application you should never do that.

import java.awt.event.KeyEvent;

import javax.swing.JFrame;
import javax.swing.JTextField;

public class Test {
    public static void main(String[] args) {
        KeyChecker keyChecker = new KeyChecker();
        JTextField textField = new JTextField(40);
        textField.addKeyListener(keyChecker);
        JFrame jframe = new JFrame();
        jframe.add(textField);
        jframe.pack();
        jframe.setVisible(true);

        Thread t = new Thread(() -> {
            while(true) {
                char currentKey = keyChecker.getCurrentPressedKey();
                switch (currentKey) {
                    case 'a':
                        System.out.println("working");
                        break;
                    default:
                        System.out.println("not working");
                }
                    
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException ex) {
                    break;
                }
            }
        });
        
        t.start();
    }
}

class KeyChecker extends KeyAdapter {
    private char currentPressedKey = 0;

    @Override
    public void keyPressed(KeyEvent event) {
        currentPressedKey = event.getKeyChar();
    }

    @Override
    public void keyReleased(KeyEvent e) {
        currentPressedKey=0;
    }
    
    public char getCurrentPressedKey() {
        return currentPressedKey;
    }
}