Java GUI：关于getContentPane（）方法和内容

Question

In this piece of code:

JLabel emptyLabel = new JLabel("");
emptyLabel.setPreferredSize(new Dimension(175, 100));
frame.getContentPane().add(emptyLabel, BorderLayout.CENTER);

I can see it makes a new label and adds it to the JFrame object frame . But I want to understand what does getContentPane() do, and why do I need it?

I read this API but I still didn't understand.

Answer 1

Every Swing top level container (and JInternalFrame) has what's called a JRootPane . This is responsible for actually managing the overall layout of the window.

The root pane has a number of layers, one of which is the content pane. When you add something to a frame (since Java 5 I think), it is automatically added to the content pane for you, before this, you had to call getContentPane().add(...) yourself

Take a look at How to use RootPanes

Answer 2

Every JPanel is a container, so either add it to a panel then add it to the container or directly use add(component) or use the getContentPane().add method. Both add the component to the container in Java 7 ( I don't know if version 6 has a problem with this or not ).

Answer 3

A container has several layers in it. You can think of a layer as a transparent film that overlays the container . In Java Swing, the layer that is used to hold objects is called the content pane . Objects are added to the content pane layer of the container .

The getContentPane() method retrieves the content pane layer so that you can add an object to it. The content pane is an object created by the Java run time environment . You do not have to know the name of the content pane to use it. When you use getContentPane() , the content pane object then is substituted there so that you can apply a method to it.

Answer 4

A JFrame is the headcomponent which is put together with other subcomponents. With getContentPane() gets the component that represents the contents of a graphical user interface. A JMenuBar for example is placed in another area next to the contentPane of a frame.