image - Java KeyListener isn't detecting keyboard input -

i'm trying modify program draws image of castle , able scale image using , down arrow keys. can't manage keylistener work, program runs isn't responding key presses. appreciated, thanks.

import java.awt.*; import java.awt.geom.*; import java.util.*; import javax.swing.*; import java.net.*; import java.awt.event.*;  public class drawimage extends jframe implements keylistener { int scale = 1;     private image image;     enter code here     public static void main(string[] args) {         new drawimage();     }      public drawimage() {         super("drawimage");         setsize(600,600);         setvisible(true);         setdefaultcloseoperation(jframe.exit_on_close);         toolkit tk = toolkit.getdefaulttoolkit();         image = tk.getimage(geturl("castle.png"));         addkeylistener(this);     }      private url geturl(string filename) {         url url = null;         try {             url = this.getclass().getresource(filename);         }         catch (exception e) { }         return url;     }      public void paint(graphics g) {         graphics2d g2d = (graphics2d) g;         affinetransform trans = new affinetransform();         trans.scale(scale, scale);         system.out.println("scale: " + scale);         g2d.setcolor(color.black);         g2d.fillrect(0, 0, getsize().width, getsize().height);         g2d.settransform(trans);         g2d.drawimage(image, 0, 40, this);         addkeylistener(this);     }      public void keyreleased(keyevent e) { }     public void keytyped(keyevent e) { }     public void keypressed(keyevent e) {         int key = e.getkeycode();         if (key == keyevent.vk_up) {             scale++;         }         if (key == keyevent.vk_down) {             scale--;         }     } } 

i can't manage keylistener work, program runs isn't responding key presses.

yours common problem asked here frequently, , problem due lack of focus -- component being listened not have current focus, , focus essential keylistener work.

the short answer give component being listened focus.

the longer better answer not use keylisteners , instead use key bindings such project.

---

edit
other problems:

• in code above, you're adding keylistener jframe, , shouldn't done, if had reason use keylisteners.
• you're drawing directly in jframe in opinion bigger problem, since risk unwanted side effects.
• you're adding listener inside of paint method, bigger problem. paint method should not overridden (in general), , if did have override it, should never used program logic, adding or removing listeners, or doing non-painting activities. should painting , painting only.
• instead should draw directly in jpanel or jcomponent.
• instead should drawing in paintcomponent(graphics g) override in painting jpanel or jcomponent.
• just described above, method should painting , painting , should fast possible.
• you should call super.paintcomponent(g) inside of paintcomponent(graphics g) override.
• again drawing component should listen key strokes using key bindings (tutorial here). tutorial explain why distinction (keylistener vs. key bindings) important.

  ---

edit 2
never want ignore exceptions code shows here since programming equivalent of driving blind:

  try {      url = this.getclass().getresource(filename);   } catch (exception e) {   } 

hopefully not how production code looks, ignored exceptions in posted code sake of brevity, understand many of seeing hearing nails on chalk-board.

---

edit 3
more on keylisteners vs. key bindings: assuming got code work keylistener, assume added other focusable components gui , somehow got focus via user interaction, keybindings no longer work. if had done correctly key bindings, not issue.

---

edit 4
want scale field double, not int. , don't want increment , decrement it, rather want multiply , divide multiplier constant, 1.2 example. want call repaint() whenever change scale.

---

edit 5
please check out 2 sample programs, first, called drawimagepanelkeylistener.java, uses keylistener, while second, called drawimagepanelkeybindings, uses key bindings. both should compile, run, , function expected: when press or down arrow keys, displayed image shrinks , grows. difference in behavior though can seen when jbutton both have pressed. press button , see happens key response. when component has keylistener loses focus, keylistener stop working, same not true component uses key bindings.

a kludge around problem prevent other components getting focus, not practical or desired gui's.

drawimagepanelkeylistener.java

import java.awt.*; import javax.imageio.imageio; import javax.swing.*; import java.io.ioexception; import java.net.*; import java.awt.event.*;  @suppresswarnings("serial") public class drawimagepanelkeylistener extends jpanel implements keylistener {    public static final string image_path = "https://duke.kenai.com/"          + "nyanya/.midsize/nyanya.jpg.png";    private static final double multiplier = 1.2;    double scale = 1.0;    private image image;    private dimension initialsize = new dimension(0, 0);     public static void main(string[] args) {       swingutilities.invokelater(new runnable() {          public void run() {             drawimagepanelkeylistener drawimage = new drawimagepanelkeylistener();             drawimage.add(new jbutton("foo"));             jframe frame = new jframe("draw image");             frame.setdefaultcloseoperation(jframe.exit_on_close);             frame.add(drawimage);             frame.pack();             frame.setlocationrelativeto(null);             frame.setvisible(true);          }       });    }     public drawimagepanelkeylistener() {       setfocusable(true);       requestfocusinwindow();       url imgurl;       try {          imgurl = new url(image_path);          image = imageio.read(imgurl);          initialsize = new dimension(image.getwidth(this),                image.getheight(this));          addkeylistener(this);          setvisible(true);       } catch (malformedurlexception e) {          e.printstacktrace();       } catch (ioexception e) {          e.printstacktrace();       }    }     @override    public dimension getpreferredsize() {       return initialsize;    }     @override    protected void paintcomponent(graphics g) {       super.paintcomponent(g);       g.setcolor(color.black);       g.fillrect(0, 0, getsize().width, getsize().height);       graphics2d g2d = (graphics2d) g.create();       g2d.scale(scale, scale);       g2d.drawimage(image, 0, 0, this);    }     public void keyreleased(keyevent e) {    }     public void keytyped(keyevent e) {    }     public void keypressed(keyevent e) {       int key = e.getkeycode();       if (key == keyevent.vk_up) {          scale *= multiplier;       }       if (key == keyevent.vk_down) {          scale /= multiplier;       }       repaint();    } } 

drawimagepanelkeybindings.java

import java.awt.*; import javax.imageio.imageio; import javax.swing.*; import java.io.ioexception; import java.net.*; import java.awt.event.*;  @suppresswarnings("serial") public class drawimagepanelkeybindings extends jpanel {    public static final string image_path = "https://duke.kenai.com/"          + "nyanya/.midsize/nyanya.jpg.png";    private static final double multiplier = 1.2;    double scale = 1.0;    private image image;    private dimension initialsize = new dimension(0, 0);     public static void main(string[] args) {       swingutilities.invokelater(new runnable() {          public void run() {             drawimagepanelkeybindings drawimage = new drawimagepanelkeybindings();             drawimage.add(new jbutton("foo"));             jframe frame = new jframe("draw image");             frame.setdefaultcloseoperation(jframe.exit_on_close);             frame.add(drawimage);             frame.pack();             frame.setlocationrelativeto(null);             frame.setvisible(true);          }       });    }     public drawimagepanelkeybindings() {       setbindings();       url imgurl;       try {          imgurl = new url(image_path);          image = imageio.read(imgurl);          initialsize = new dimension(image.getwidth(this),                image.getheight(this));          setvisible(true);       } catch (malformedurlexception e) {          e.printstacktrace();       } catch (ioexception e) {          e.printstacktrace();       }    }     private void setbindings() {       int condition = when_in_focused_window;       inputmap inputmap = getinputmap(condition);       actionmap actionmap = getactionmap();        final keystroke[] keystrokes = {             keystroke.getkeystroke(keyevent.vk_up, 0),             keystroke.getkeystroke(keyevent.vk_down, 0)       };       (final keystroke keystroke : keystrokes) {          inputmap.put(keystroke, keystroke.tostring());          actionmap.put(keystroke.tostring(), new abstractaction() {             @override             public void actionperformed(actionevent evt) {                mykeypressed(keystroke.getkeycode());             }          });       }    }     @override    public dimension getpreferredsize() {       return initialsize;    }     @override    protected void paintcomponent(graphics g) {       super.paintcomponent(g);       g.setcolor(color.black);       g.fillrect(0, 0, getsize().width, getsize().height);       graphics2d g2d = (graphics2d) g.create();       g2d.scale(scale, scale);       g2d.drawimage(image, 0, 0, this);    }     public void mykeypressed(int key) {       if (key == keyevent.vk_up) {          scale *= multiplier;       }       if (key == keyevent.vk_down) {          scale /= multiplier;       }       repaint();    } }