Question

I’m taking a beginning Java class and an assignment requires that I write a class to represent a JPanel with buttons to increment and decrement a value and a label to display the value. Then, I have to create a separate class which instantiates the panel and adds it to a frame. I’m trying to have the frame resize to fit the size of the panel by running the pack method. I try to call the frame’s pack method by using:

SwingUtilities.getAncestorOfClass(JFrame.class, this).pack()

I get a “cannot find symbol – method pack()” error. The getAncestorOfClass is definitely returning a JFrame, and it is the correct JFrame. When I run the pack method from inside the driver class where the JFrame is created, there are no problems. Any ideas why it can’t find the pack method? Is it because I’m trying to run this from a separate class file? I also can’t access some other JFrame methods such as getContentPane, but I am able to access some others such as add. Huh?

Answer 1

The method SwingUtilities.getAncestorOfClass returns a Container. Now, you know that Container is really gonna be a JFrame, but the compiler doesn’t. And Java is a static language, not a dynamic one that’ll just try to call the method regardless of whether the class declares it or not.

Since Container doesn’t have pack method, the compiler’s gonna complain. You’ll need to cast to JFrame to make it work:

((JFrame)SwingUtilities.getAncestorOfClass(JFrame.class, this)).pack();

Careful, though… The method can return null if no suitable ancestor was found. You might want to check that first.