Progyadeep Moulik Progyadeep Moulik - 2 years ago 65
Java Question

Why doesn't the String object display that gets pressed key characters appended to it?

I wrote a Java applet code using Key Event handling to demonstrate Non buffered input in Java. My code is working fine and the output is alright but I'm having trouble accomplishing another objective in this program: in the overriden keyPressed() method, I write the line: showStatus(s); , where s is the global static StringBuffer object which the characters typed from the keyboard are appended to. But the showStatus() method displays text on the status bar of the applet viewer. Therefore, the present program is usable only in an applet viewer and not in a web browser. I tried putting the statement g.drawString(String.valueOf(s),10,90); (g=Graphics class object as argument of paint()) in the paint() method to display the text in the canvas. I expected this to work as s is global and static but it's not showing any output. I used the drawString() method in paint() both with and without overriding the boolean action() method but I still didn't get anything. I just want help in displaying the typed text in the canvas of the applet.

Below is my code. Please view it for reference and to help me out the best way possible. Thank you.

/* An applet that implements the concept of non buffered user input
using Event handling method.

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class NonBufferInput extends Applet implements KeyListener {
public static StringBuffer s;

public void init() {
s=new StringBuffer();

public void keyTyped(KeyEvent K) {}

public void keyPressed(KeyEvent K) {
char c=K.getKeyChar();
int i=K.getKeyCode();
if(i==32) //space bar
s.append(' ');
else if(i==10||i==16||i==20||i==32||i==17||i==524||i==18||i==525||i==27
||i==35||i==34||i==144||(i>=37&&i<=40)); //characters I don't want as input
else if(i==8) { //backspace
showStatus("Typed : "+s);

public void keyReleased(KeyEvent K) {}

public void paint(Graphics g) {
g.drawString("Text will be displayed on status bar as you type.",10,50);
g.drawString(String.valueOf(s),10,80); //PROBLEM

public boolean action(Event event, Object obj) {
return true;

Answer Source

The problem is: how does the EDT (Event Dispatch Thread) know that it should repaint the applet?

It works after changing your method keyPressed to

    public void keyPressed(KeyEvent K) {
       /* .. many lines omitted .. */
       showStatus("Typed : "+s);
       repaint(); // <<- this line is missing!

As an explanation: the EDT is the thread that runs your applet. It fetches events from the event queue and dispatches them to the various event listeners. One of its responsabilities is the repainting of your components (either on request, since someone / something has changed a component) or after your component has been obscured and is now shown again.

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