问题
Suppose I have a JLayeredPane called mainPanel that is using a BorderLayout. I also have a JLabel called backgroundLabel that contains an image. How would I go about adding the backgroundLabel to the bottom layer of mainPanel?
mainPanel.add(backgroundLabel, new Integer(-1), new Integer(0));
The above line seems like the obvious answer, and would work if the mainPanel was using a null layout. The BorderLayout in mainPanel is not liking the command and gives the following stack trace.
Exception in thread "AWT-EventQueue-0" java.lang.IllegalArgumentException: cannot add to layout: constraint must be a string (or null)
at java.awt.BorderLayout.addLayoutComponent(Unknown Source)
at java.awt.Container.addImpl(Unknown Source)
How would I go about adding the backgroundLabel to the bottom layer of the mainPanel without conflicting with the BorderLayout?
回答1:
As noted in How to Use Layered Panes: Laying Out Components in a Layered Pane, "All of the layout managers provided by the Java platform arrange the components as if they were all on one layer." You have specified BorderLayout. Your call to add() invokes addImpl(java.awt.Component, java.lang.Object, int). Because BorderLayout implements LayoutManager2, your value for the constraints parameter must be a String constraint defined for BorderLayout, not an Integer having the value -1, e.g.
mainPanel.add(backgroundLabel, BorderLayout.SOUTH, 0);
My intent is for the
backgroundLabelto be added to it's own bottom layer behind all other components.
Setting a layout on the JLayeredPane makes "the components as if they were all on one layer." Instead, set a layout on the component occupying the deepest layer, and add the label to that component. In this example, a label and button are added to each layer's JPanel having the default FlowLayout.
来源：https://stackoverflow.com/questions/37376915/java-layeredpane-layoutmanager-add-method-conflict