Thread: java top level container and EventLstenerList

Hello everyone

I have two questions about event handling (Button) and top level container

In the following sentences it says :

Each GUI component can be contained only once. If a component is already in a container and you try to add it to another container, the component will be removed from the first container and then added to the second

Using Top-Level Containers (The Java™ Tutorials > Creating a GUI With JFC/Swing > Using Swing Components)

I can't understand how we can have to add a component from one container to - another container - also how we can have two containers in a
Class (means we should have two frames possible)

2. When we want to add a button listener it sent ActionListenrt to a list of itself why we should use a list ?
A button only wants to add on one ActionListener or one source, but why it uses List

thanks

1 - Multiple containers are possible. JFrame, JPanel, and JDialog are all containers and can all be used in multiple numbers in a single application. You may have multiple containers displayed that all have a Cancel button that does the same thing, closes the dialog. To save time, you might think of using the same Cancel button in each of the containers. The component rule you've quoted says that using the same Cancel button in multiple containers AT THE SAME TIME is not possible.

2 - Components can have multiple ActionListeners.

Thanks
But there is a question about "Components can have multiple ActionListeners" suppose in a button which can only do one thing
What is the use of multiple ActionListener. Can you elaborate more.
Thanks again

a button which can only do one thing

That's a self-imposed, unnecessary limit, or incorrect assumption. A button, even with only one action listener assigned to it, could do multiple things.

Perhaps one action listener is common to multiple components, providing the ability to do the same thing multiple ways, resetting the program to a previous state, for example. While multiple buttons may perform this action when pressed, they may each have their own specific function, closing a dialog, or reading an input field, etc.

Knowing that one component can be assigned multiple action listeners is the important point. You may someday find that useful.