Handling Images in a Java GUI Application
Handling images in an application is a common problem for many beginning Java programmers. The standard way to access images in a Java application is by using the
+getResource()+ method. This tutorial shows you how to use the IDE’s GUI Builder to generate the code to include images (and other resources) in your application. In addition, you will learn how to customize the way the IDE generates image handling code.
The application that results from this tutorial will be a simple JFrame that contains one JLabel that displays a single image.
In this section, you create the JFrame form and add a JLabel to the form.
To create the JFrame form:
In the Projects window, expand the
Right-click the Source Packages node and choose New > JFrame Form.
For Class Name, type
For Package Name, type
To add the JLabel:
In the Palette, select the Label component and drag it to the JFrame.
For now, the form should look something like the following image:
When you use images or other resources in an application, typically you create a separate Java package for the resources. On your local filesystem, a package corresponds with a folder.
To create a package for the image:
In the Projects window, right-click the
org.me.myimageappnode and choose New > Java Package.
In the Projects window, you should see a new package appear within the
Source Packages folder.
In this application, the image will be embedded within a JLabel component.
To add the image to the label:
In the GUI Designer, select the label that you have added to your form.
In the Properties window, click the Properties category and scroll to the Icon property.
Click the ellipsis (…) button. The icon property editor is displayed.
In the icon property dialog box, click Import to Project.
In the file chooser navigate to any image that is on your system that you want to use. Then click Next.
In the Select target folder page of the wizard, select the
newpackagefolder and click Finish.
Click OK to close the icon property dialog box.
After you click OK, the IDE does the following things:
Copies the image to your project. Therefore, when you build and distribute the application, the image is included in the distributable JAR file.
Generates code in the ImageDisplay class to access the image.
Displays your image on the label in the Design view of your form.
At this point, you can do some simple things to improve the appearance of the form, such as:
In the Properties window, select the
textproperty and delete
jLabel1. That value was generated by the GUI Builder as display text for the label. However, you are using the label to display an image rather than text, so that text is not needed.
jLabel1to the center of the form.
To view the generated code:
In the GUI Designer, click the Source button. (Choose View > Source Editor Toolbar from the main menu if the Source button is hidden.)
Scroll down to the line that says Generated Code.
Click the plus sign (+) to the left of the Generated Code line to display the code that the GUI Designer has generated.
The key line is the following:
jLabel1.setIcon(new javax.swing.ImageIcon(getClass().getResource("/org/me/myimageapp/newpackage/image.png"))); // NOI18N
Since you have used the property editor for
jLabel1’s `Icon property, the IDE has generated the
setIcon method. The parameter of that method contains a call to the
+getResource()+ method on an anonymous inner class of
ImageIcon. Notice that the generated path for the image corresponds with its location in the application’s package structure.
If you use the External Image option in the icon property editor, the IDE will generate an absolute path to the image instead of copying the image to your project. Therefore, the image would appear when you run the application on your system, but it would probably not appear when running the application on another system.
getResourcemethod is also useful for accessing other types of resources, such as text files that contain data that your application might need to use.
To register event handlers for mouse events on the Jlabel:
In the Design View, right-click the JLabel and choose Events > Mouse > mouseClicked/mousePressed/mouseReleased from the popup menu. An event handler is generated for the corresponding event.
You can get the mouse coordinates (for example, the location of a mouse click) in the event handler using the
Now that you have generated the code for accessing and displaying the image, you can build and run the application to ensure that the image is accessed.
First you need to set the project’s main class. When you set the main class, the IDE knows which class to run when you run the project. In addition, this ensures that the
Main-Class element in the application’s JAR file is generated when you build the application.
To set the project’s main class:
Right-click the ImageDisplayApp project’s node and choose Properties.
In the Project Properties dialog box, select the Run category.
Click the Browse button that is next to the Main Class field. Then select the
Click the Select Main Class button.
Click OK to close the Project Properties dialog box.
To build the project:
Choose Run > Clean & Build Project (project_name) from the main toolbar.
You can view the build products of the application in the Files window. The
build folder contains the compiled class. The
dist folder contains a runnable JAR file that contains the compiled class and the image.
To run the project:
Choose Run > Run Project (project_name) from the main toolbar.
In many applications, the image that is displayed is not determined statically like it is in this example. For example, the image to display might be determined by something that the user clicks.
If you need to be able to choose the image to display programmatically, you can write your own custom code to access and display resources. The IDE prevents you from writing code directly in the Source view’s "guarded blocks" that contain code generated by the GUI Builder. However, you can insert code in the guarded blocks through property editors that you can access through the Properties window. Using the property editors in this manner ensures that your custom code is not lost when you make design changes in the GUI Builder.
For example, to write custom code for a JLabel’s
Select the JLabel in the Design View or in the Navigator window.
In the Properties window, click the ellipsis (…) button that is next to the
From the dropdown list at the top of the dialog box, select the Custom Code option.
The Custom Code option in this property editor lets you fill in the parameter of the
setIcon method yourself. You can fill in this parameter with the necessary logic or with a call to a separate method that you have hand-coded elsewhere in the class.
This tutorial has shown you how to access images from an application that you create in the NetBeans IDE. Image handling is further discussed in the Java Tutorial.
*Note: *The example given in this tutorial is very similar to the first example in the How to Use Icons section of the Java Tutorial. One difference is that the code that is generated when you follow this tutorial uses
+JLabel+’s `+setIcon+ method to apply the icon to the label. In the Java Tutorial example, the icon is applied to the label by being passed through its constructor.