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Java Forum / GUI / March 2007GridBagLayout and changing constraints objects.
Say I have the following code: JPanel myPanel = new JPanel(new GridBagLayout()); GridBagConstraints constaints = getConstraints(); myPanel.add(new JLabel("Hello world"), constaints); And then later (as a result of an Event), I do something like constraints.gridx = 3; myPanel.revalidate(); Will this have the expected effect, or do I need to remove the old label, and re-add it with the new constraints? > Say I have the following code: > [quoted text clipped - 8 lines] > Will this have the expected effect, or do I need to remove the old > label, and re-add it with the new constraints? The latter. The GridBagLayout class makes a copy of the layout constraints when the device is added. In fact, it's fairly common to create just one GridBagConstraints object, and use it to add a large number of components by just changing the few properties that differ each time.  SignatureChris Smith > Say I have the following code: > > JPanel myPanel = new JPanel(new GridBagLayout()); > GridBagConstraints constaints = getConstraints(); What class does the method getConstraints() belong to? > myPanel.add(new JLabel("Hello world"), constaints); > [quoted text clipped - 4 lines] > Will this have the expected effect, or do I need to remove the old > label, and re-add it with the new constraints? No. You will have to re-add it. SignatureKnute Johnson email s/nospam/knute/ > Say I have the following code: > [quoted text clipped - 8 lines] > Will this have the expected effect, or do I need to remove the old > label, and re-add it with the new constraints? It wont have any affect. The constraints object is cloned. Indeed it is normal to use one constraints object to add many components. To change the constraints for a component, just call LayoutManager[2].addLayoutComponent. Tom Hawtin > > Say I have the following code: > [quoted text clipped - 16 lines] > > Tom Hawtin Thanks to everyone who replied. I should have RTFM. Actually, its RTFA, isn't it? GridBagLayout.setConstraints is what I need to use. :-) Cheers, Daniel. On Mar 26, 7:07 am, "Daniel Pitts" <googlegrou...@coloraura.com> wrote: > Say I have the following code: > [quoted text clipped - 8 lines] > Will this have the expected effect, or do I need to remove the old > label, and re-add it with the new constraints? You could try it yourself. > On Mar 26, 7:07 am, "Daniel Pitts" <googlegrou...@coloraura.com> > wrote: [quoted text clipped - 13 lines] > > You could try it yourself. I could, but that might have only proved that sometimes it works. I was asking if it would always work... As I discovered, it won't ever work, but there is a different way to do it that will always work. On Mar 26, 7:07 am, "Daniel Pitts" <googlegrou...@coloraura.com> wrote: > Say I have the following code: > [quoted text clipped - 8 lines] > Will this have the expected effect, or do I need to remove the old > label, and re-add it with the new constraints? Why don't you try it yourself? > Say I have the following code: > [quoted text clipped - 8 lines] > Will this have the expected effect, or do I need to remove the old > label, and re-add it with the new constraints? You will have to remove it and re-add it. How is Java to know which Object is supposed to have the new constraint if you don't re-add the Object with that constraint? It could assume to apply that new constraint property to the only Object in a JPanel if there is only one in it but that wouldn't work for more than 1 Object in container and it isn't a good way to do it anyway. It is bad design to need to relocate/resize an Object after an event occurs. You should rethink how you are presenting the interface to the user because the user should not normally see new things appear when they do something; all the Objects in the GUI should be visible, their sizes unchanging, and the unused ones disabled until they are needed (with the enabling trigger being your event). As it is now, the line: myPanel.add(new JLabel("Hello world"), constaints); isn't going to do much because you aren't defining what your constraints are although some of the properties do have default values. The default for gridx is 'relative' which may not be what you want initially. > > Say I have the following code: > [quoted text clipped - 21 lines] > ones disabled until they are needed (with the enabling trigger being > your event). Actually, I have four objects which would only trade places, and they are similar enough that the change shouldn't bother users. Have you ever played a game on Yahoo games? notice when you sit at a table, your side becomes the southern most side. I'm basically emulating that effect. It is easier to move the components around than to change which model each component is looking at. Either way, the effect would be the same. > As it is now, the line: > myPanel.add(new JLabel("Hello world"), constaints); > > isn't going to do much because you aren't defining what your constraints > are although some of the properties do have default values. The default > for gridx is 'relative' which may not be what you want initially. Hmm, I don't see how you think that my constraints object is unset. I simply didn't copy the code that implemented getConstraints(); > Say I have the following code: OK, I shall ... > JPanel myPanel = new JPanel(new GridBagLayout()); > GridBagConstraints constaints = getConstraints(); [quoted text clipped - 6 lines] > Will this have the expected effect, or do I need to remove the old > label, and re-add it with the new constraints? Others have given the answer you were seeking. However I can't help noticing that the code you quoted can never have the expected effect since constaints != constraints :-) > > Say I have the following code: > [quoted text clipped - 14 lines] > noticing that the code you quoted can never have the expected effect > since constaints != constraints :-) Did I surround that code with an sscce tag? I don't think so. :-) > Say I have the following code: > [quoted text clipped - 8 lines] > Will this have the expected effect, or do I need to remove the old > label, and re-add it with the new constraints? It will have no effect. Normally you change the (single) constraints object between adding components. There is no need to create multiple constraints objects. To change after creation, I assume you can use GridBagLayout.setConstraints(). You could use GridBagLayout.getConstraints(), make the change, then use setConstraints() to avoid storing the constraints object. Oops, the OP was to cljp and cljg with followups to cljg, so I didn't see the answers already here. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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