DisableSubmit

import java.awt.GridLayout;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextArea;


public class TextField extends JFrame {

    JTextArea inputField;
    JButton    submit;

    public TextField()
    {
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        inputField = new JTextArea("hi");
        submit = new JButton("Submit");
        inputField.addKeyListener(new KeyListener() {

            @Override
            public void keyPressed(KeyEvent arg0) {
                //func(arg0);
            }

            @Override
            public void keyReleased(KeyEvent arg0) {
                func(arg0);
            }

            @Override
            public void keyTyped(KeyEvent arg0) {
                //func(arg0);
            }
        });
        this.setLayout(new GridLayout(0,1));
        this.add(inputField);
        this.add(submit);
        this.pack();
        this.setVisible(true);
    }

    void func(KeyEvent arg0)
    {
        System.out.println(inputField.getText() + "length: " + inputField.getText().trim().length()); // test input
        if(inputField.getText().trim().length() == 0)
            submit.setEnabled(false);

        if(arg0.getKeyCode() == KeyEvent.VK_ENTER) {
            if(inputField.getText().trim().length() == 0)
                System.out.println("error");
            else
                System.out.println("success");
            return;
        }
        if(inputField.getText().trim().length() > 0) {
            submit.setEnabled(true);
        }
    }

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