Java，JFrame:getWidth() 返回 0

setExtendedState(getExtendedState()|JFrame.MAXIMIZED_BOTH);
setResizable(false);
setUndecorated(true);
System.out.println("--------> "+getContentPane().getWidth()); //----> 0 why is this happening?

I'm trying to determine the size of a JFrame. I've searched on google and checked the documentation and I still don't know why it won't work It works fine on about any other control I try it on

EDIT: frame.getWidth() works when called outside of the class (that extends JFrame) still, if I replace

System.out.println("--------> "+getContentPane().getWidth());

with

System.out.println("--------> "+this.getWidth());

getWidth will still return 0

EDIT2: I need the frame's size before setting it visible and stuff. I need to add other controls to the frame and their coordinates and size depend on the frame's size.

解决方案

The reason why you got 0 is because you didn't call any of the pack(), setSize(int, int) or setSize(Dimension). This is only when calling one of these method that the layout of your frame will be computed.

JFrame frame = new JFrame("My Frame");
frame.setExtendedState(getExtendedState()|JFrame.MAXIMIZED_BOTH);
frame.setResizable(false);
frame.setUndecorated(true);
frame.pack(); // Important line!!! 
frame.setVisible(true);
System.out.println("--------> "+getContentPane().getWidth());