调整JFrame大小后会发生什么？

Question

I am curious as to what happens exactly in Java when a JFrame is resized.

I am asking this because of an issue I am having with adding a component to my bigCOntainer (of type JPanel) inside the JFrame. I want to add something when the user performs a certain action but the component doesn't become visibly added until the JFrame is resized somehow.

I discovered this when I was doing some experimenting trying to get the new component to show up; I called myJFrame.pack(). As a secondary question I would like to know what pack() does, both generally and specific to resizing. The Java Documentation makes it sound like it will resize every component to its preferred size. My solution, if I decide to use pack() in my implementation to display new components is to always set a preferred size. Can anyone attest to this being an accepted programming practice? It sure seems like a good idea.

I also tried repaint() to get the new component to show up but that did not work.

Answer 1

You need to call revalidate() and then repaint() on the container that gets components added or removed during run-time.

revalidate() tells the container's layout managers to re-layout the components it contains, and this is what is missing from your code. Without this request, the layouts don't change, and you don't see the changes until you re-size the GUI, which pretty much does the same thing.

Note that if you're adding components to the GUI on start-up, then the solution is to call pack() and setVisible(true) to the top-level container only after all components have been added.

For specifics about say what pack() does, do what we all do -- read the source code.