Loading image in Java J2ME

问题

I have a problem with loading image with java 2ME. I have a image file "picture.png" in location drive "C:". After that I wrote my like this to show image from this location.

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.io.*;

public class ImageMidlet extends MIDlet implements CommandListener{
    private Display display;
    private Command exitCommand;
    private Command backCommand;
    private Command okCommand;
    private Form form;

    private ImageItem imageItem;
    private Image image;

    public ImageMidlet(){
        display = Display.getDisplay(this);
        form=new Form("");
        exitCommand = new Command("Exit", Command.EXIT, 1);
        backCommand = new Command("Back", Command.BACK, 2);
        okCommand = new Command("OK", Command.OK, 3);

        try {
            image=Image.createImage("/picture.png");
            imageItem=new ImageItem(null,image,ImageItem.LAYOUT_NEWLINE_BEFORE,"");
        }
        catch(IOException ex){

        }
        form.append(imageItem);
        form.addCommand(okCommand);
        form.addCommand(exitCommand);
        form.addCommand(backCommand);
        form.setCommandListener(this);

        display.setCurrent(form);

    }

    public void commandAction(Command c,Displayable d){

    }

    public void startApp() {
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }
}

It shows me this error:

Unable to create MIDlet Test.ImageMidlet
java.lang.NullPointerException
     at javax.microedition.lcdui.Form.append(Form.java:638)
     at Test.ImageMidlet.<init>(ImageMidlet.java:39)
     at java.lang.Class.runCustomCode(+0)
     at com.sun.midp.midlet.MIDletState.createMIDlet(+34)
     at com.sun.midp.midlet.Selector.run(Selector.java:151)

I am starting learn to develop, so please guide to do this.

回答1:

As msell said - You can't access images from Your computer. Make sure that You have included the given image in midlet jar file. If You try to access it using '/picture.png', then it should be located a the root directory in jar.

回答2:

Image.createImage(String name) loads the given image as a resource. Resources are loaded with Class.getResourceAsStream(name), which looks up the resources from classpath, not from your file system root.

You should put the image file in your classpath, which is usually the final application .jar file. Usually a folder called resources or res is created under the project, where the images are placed. The contents of this folder are then copied to the .jar file. In development phase you should be able to append your resource folder to the classpath with a command-line parameter (java -cp resources in Java SE) or with a similar IDE setting.

If you are really interested in loading the images from actual file system, you can use optional FileConnection API (http://developers.sun.com/mobility/apis/articles/fileconnection/). The handset support for this API is limited though. For static images the classpath is the way to go.

回答3:

First of all place your image in default package. I have placed "My Network Places.png" in default package. Then create MIDlet named "ImageItemExample" then copy below code in that MIDlet file.

import java.io.*;
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class ImageItemExample extends MIDlet implements CommandListener{
private Display display;
private Command exit;
private Form form;
private ImageItem logo;

public ImageItemExample(){
form = new Form("Image Item");
exit = new Command("Exit", Command.EXIT, 0);
try{
  logo = new ImageItem(null, Image.createImage("/My Network Places.png"),
   ImageItem.LAYOUT_CENTER | ImageItem.LAYOUT_NEWLINE_BEFORE |
    ImageItem.LAYOUT_NEWLINE_AFTER, "Roseindia");
  form.append(logo);
}catch(IOException e){
  form.append(new StringItem(null, "Roseindia: Image not available: "+ e));
}
}

public void startApp(){
display = Display.getDisplay(this);
form.addCommand(exit);
form.setCommandListener(this);
display.setCurrent(form);
}

public void pauseApp(){}

public void destroyApp(boolean unconditional){
notifyDestroyed();
}

public void commandAction(Command c, Displayable d){
String label = c.getLabel();
if(label.equals("Exit")){
  destroyApp(true);
}
}
}

回答4:

My guess is that

image=Image.createImage("/picture.png");

throws an exception which prevents the creation of a new object of type ImageItem which leaves your imageItem variable as null. This gives you the null pointer exception.

Isn't your file Picture.png and not Pictur.png?