BoxLayout for a JFrame

问题

Could you help me understand what is going on here. I consulted Javadoc: JFrame has setLayout method. So, what sharing error springs out is a mystery to me.

public class View extends JFrame {
    public View(){

        // LayoutManager for the whole frame.
        this.setLayout(new BoxLayout(this, BoxLayout.X_AXIS));
    }
}

Result

Exception in thread "main" java.awt.AWTError: BoxLayout can't be shared
    at javax.swing.BoxLayout.checkContainer(BoxLayout.java:465)
    at javax.swing.BoxLayout.invalidateLayout(BoxLayout.java:249)
    at java.awt.Container.invalidate(Container.java:1583)
    at java.awt.Component.invalidateIfValid(Component.java:2957)
    at java.awt.Container.setLayout(Container.java:1484)
    at javax.swing.JFrame.setLayout(JFrame.java:605)
    at View.<init>(View.java:16)
    at Init.main(Init.java:6)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

回答1:

Try this one on JFrame#getContentPane()

this.setLayout(new BoxLayout(this.getContentPane(), BoxLayout.X_AXIS));

Read more How to Use BoxLayout

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All the components are added in JFrame's content pane.

Read more Adding Components to the Content Pane

Here is the pictorial representation how JFrame looks like

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EDIT

From comments:

Well, not clear anyway. I analyze it like this: BoxLayout class needs to know it target. JFrame has setLayoutt method and needs to know its layout.

this.setLayout(manager) internally calls getContentPane().setLayout(manager);

The below line

this.setLayout(new BoxLayout(this, BoxLayout.X_AXIS));

is converted to below line that is not correct.

this.getContentPane().setLayout(new BoxLayout(this, BoxLayout.X_AXIS));

For more detail have a look at Source code

来源：https://stackoverflow.com/questions/24840860/boxlayout-for-a-jframe