Layout Manager-mock exam

[Cristi Tudose]

this is taken from a mock exam:
import java.awt.*;
public class TestFrame extends Frame
{
Button bNorth = new Button("North");
Button bSouth = new Button("South");
Button bEast = new Button("East");
Button bWest = new Button("West");
Button bCenter = new Button("Center");
public TestFrame()
{
setLayout(new FlowLayout());
add(bNorth);
add(bSouth);
add(bWest);
add(bEast);
add(bCenter);
setLayout(new BorderLayout());
validate();
setSize(300,300);
setVisible(true);
}
public static void main(String args[])
{
TestFrame tf = new TestFrame();
}
}

Attemping to compile and run the above code

#Will cause a compilation error - a Layout cannot be set after a component has been added with a preset Layout Manager.
#Will cause a Runtime Exception - a Layout cannot be set after a component has been added with a preset Layout
Manager.
#Will compile cleanly and throw no runtime Exception. Only the button with label "Center" is visible and occupies the whole screen.
#Will compile cleanly an throw no runtime Exception. All the buttons are arranged in a single line. Any other component added in future will follow the rules of the BorderLayout Manager.
#Will compile and run cleanly, but no component is visible.
#Will compile cleanly and throw no runtime Exception. The buttons are arranged as listed below
Button Label Position
CenterCenter
Noerth North
SouthSouth
EastEast
WestWest
Could someone give the explanations for the correct answer provided(#Will compile and run cleanly, but no component is visible).
thanks in advance
Cristi

[atin sehgal]

Hi!
You see it is necessary that the layout manager be associated with the container before any component is added. If we associate the layout manager after the components are added and the container is already made visible, the components appear as if they have been added by the previous layout manager (if none was associated before, then the default). Only subsequent operations (such as resizing) on the container use the new layout manager. But if the container was not made visible before the new layout is added, the components are re-laid out by the new layout manager.
In this case since the componenst have been already laid down and layout has been given.Again implementing layout on the components wd lead to the present scenario..
I wd also suggest u to look at this site and have a look at the study notes of velmurugan(look at chaper 11 on layout Managers)

http://www.geocities.com/velmurugan_p/

Originally posted by Cristi Tudose:
this is taken from a mock exam:
import java.awt.*;
public class TestFrame extends Frame
{
Button bNorth = new Button("North");
Button bSouth = new Button("South");
Button bEast = new Button("East");
Button bWest = new Button("West");
Button bCenter = new Button("Center");
public TestFrame()
{
setLayout(new FlowLayout());
add(bNorth);
add(bSouth);
add(bWest);
add(bEast);
add(bCenter);
setLayout(new BorderLayout());
validate();
setSize(300,300);
setVisible(true);
}
public static void main(String args[])
{
TestFrame tf = new TestFrame();
}
}

Attemping to compile and run the above code

#Will cause a compilation error - a Layout cannot be set after a component has been added with a preset Layout Manager.
#Will cause a Runtime Exception - a Layout cannot be set after a component has been added with a preset Layout
Manager.
#Will compile cleanly and throw no runtime Exception. Only the button with label "Center" is visible and occupies the whole screen.
#Will compile cleanly an throw no runtime Exception. All the buttons are arranged in a single line. Any other component added in future will follow the rules of the BorderLayout Manager.
#Will compile and run cleanly, but no component is visible.
#Will compile cleanly and throw no runtime Exception. The buttons are arranged as listed below
Button Label Position
CenterCenter
Noerth North
SouthSouth
EastEast
WestWest
Could someone give the explanations for the correct answer provided(#Will compile and run cleanly, but no component is visible).
thanks in advance
Cristi

[Prashant Pundrik]

Dear Cristi,
I am a new entrant and this is my first attempt at replying to anything on this forum. As far as your question is concerned, well the trick i think lies in the setVisible part.The default visibility for all components is true except for Window , Frame and Dialog classes.They must be set true explicitly for these classes at the time of creation of the instance. Since you are not doing it at that time(it is being done after the validate method is called) there is no object(frame) to be seen and then even if you try to change the layout or anything else would not work.
I hope I am right.
Thanx
prashant

Originally posted by Cristi Tudose:
this is taken from a mock exam:
import java.awt.*;
public class TestFrame extends Frame
{
Button bNorth = new Button("North");
Button bSouth = new Button("South");
Button bEast = new Button("East");
Button bWest = new Button("West");
Button bCenter = new Button("Center");
public TestFrame()
{
setLayout(new FlowLayout());
add(bNorth);
add(bSouth);
add(bWest);
add(bEast);
add(bCenter);
setLayout(new BorderLayout());
validate();
setSize(300,300);
setVisible(true);
}
public static void main(String args[])
{
TestFrame tf = new TestFrame();
}
}

Attemping to compile and run the above code

#Will cause a compilation error - a Layout cannot be set after a component has been added with a preset Layout Manager.
#Will cause a Runtime Exception - a Layout cannot be set after a component has been added with a preset Layout
Manager.
#Will compile cleanly and throw no runtime Exception. Only the button with label "Center" is visible and occupies the whole screen.
#Will compile cleanly an throw no runtime Exception. All the buttons are arranged in a single line. Any other component added in future will follow the rules of the BorderLayout Manager.
#Will compile and run cleanly, but no component is visible.
#Will compile cleanly and throw no runtime Exception. The buttons are arranged as listed below
Button Label Position
CenterCenter
Noerth North
SouthSouth
EastEast
WestWest
Could someone give the explanations for the correct answer provided(#Will compile and run cleanly, but no component is visible).
thanks in advance
Cristi

[Cristi Tudose]

Atin&Prashant
I admited i'm not study complet this part(AWT) but... i'm a little confused about your arguments.
If you only switch the lines which set the LayoutManager could u tell me what's output.You may compile&run,but please give
me a reson
And if you know( or anybody else) some URLs where the AWT is properly treat give me a sign.
Rgds,Cristi

Originally posted by atin sehgal:

Hi!
You see it is necessary that the layout manager be associated with the container before any component is added. If we associate the layout manager after the components are added and the container is already made visible, the components appear as if they have been added by the previous layout manager (if none was associated before, then the default). Only subsequent operations (such as resizing) on the container use the new layout manager. But if the container was not made visible before the new layout is added, the components are re-laid out by the new layout manager.
In this case since the componenst have been already laid down and layout has been given.Again implementing layout on the components wd lead to the present scenario..
I wd also suggest u to look at this site and have a look at the study notes of velmurugan(look at chaper 11 on layout Managers)

http://www.geocities.com/velmurugan_p/

[Jane Griscti]

Hi Cristi,
Read this earlier post http://www.javaranch.com/ubb/Forum24/HTML/001736.html for further explanation.
Hope that helps.
------------------
Jane
The cure for boredom is curiosity.
There is no cure for curiosity.
-- Dorothy Parker

[Prashant Pundrik]

Cristi,
I tried and was right.If you put the two lines setSize and setVisible right where the constructor is being define i.e after the line add(bCenter) you will get a frame showing five buttons arranged in a row as per the layout manager which is set to Flow.
in the first line of the constructor.
Regards
Prashant

Originally posted by Cristi Tudose:
Atin&Prashant
I admited i'm not study complet this part(AWT) but... i'm a little confused about your arguments.
If you only switch the lines which set the LayoutManager could u tell me what's output.You may compile&run,but please give
me a reson
And if you know( or anybody else) some URLs where the AWT is properly treat give me a sign.
Rgds,Cristi

[Cristi Tudose]

Hi all,
I repet the important code:
�.
Button bNorth = new Button("North")
�
Button bCenter = new Button("Center");
setLayout(new FlowLayout());//0
add(bNorth);
add(bSouth);
add(bWest);
add(bEast);
add(bCenter);
//set a new layout manager
setLayout(new BorderLayout());//1
validate();
setSize(300,300);
setVisible(true);
any combinations of layouts(FlowLayout,BorderLayout,GridLayout) from line 0 to line 1 work fine(the components are re-laid out by the new layout manager, re-set in line 1),EXCEPT we set a BorderLayout-as we do in line 1.
as I read in a earlier post(thanks to Jane Griscti) the clue is: both FlowLayout and GridLayout implement the LayoutManager interface,while the BorderLayout implements the LayoutManager2 interface-that part is true.
And further the suposition:
BorderLayout calls the method 'invalidateLayout(Container target)' (from LayoutManager2 interface) when it is set to any container.
This method invalidates the layout, indicating that if the layout manager has cached information it should be discarded.
Could anyone know if it's true???
I checked out all my books and I didn't find the answer.
Please a solution...
Rgds,
Cristi

[Jane Griscti]

Hi Cristi,
I took a look at the API for both LayoutManager2 and BorderLayout. BorderLayout does implement LayoutManager2 and LayoutManager2 does have an invalidateLayout() method but there was no indication of it being called; so I took a look at the source code for BorderLayout.
Neither constructor calls invalidateLayout().

    /**
     * Constructs a new border layout with
     * no gaps between components.
     */
    public BorderLayout() {
this(0, 0);
    }
    /**
     * Constructs a border layout with the specified gaps
     * between components.
     * The horizontal gap is specified by <code>hgap</code>
     * and the vertical gap is specified by <code>vgap</code>.
     * @param   hgap   the horizontal gap.
     * @param   vgap   the vertical gap.
     */
    public BorderLayout(int hgap, int vgap) {
this.hgap = hgap;
this.vgap = vgap;
    }

The API does say that LayoutManager2 is used by classes that know how to layout containers based on Constraints and the addLayoutComponent() method requires a Constraints object.
LayoutManager.addLayoutComponent() does not require a Constraints object.
So, what I think happens is when you add components using a Layout that implements LayoutManager, no Constraints exist. When you then switch to a Layout which implements LayoutManager2, as the existing components have no associated constraints, the default constructor sets everything to 0,0 ... which means none of the components are visible. Calling validate() doesn't help as BorderLayout doesn't know where to put the existing components.
Not sure if I've got this 100% right ... if anyone knows better please jump in.
Hope that helps.
Jane


[This message has been edited by Jane Griscti (edited January 07, 2001).]

[Cristi Tudose]

Hi Jane,
Thanks for trying to give an plausible explanation.
But,I'm a little confused about addLayoutComponent()
What is the connection with add() and setLayout() ?Could you be more exactly?
All I found in API about addLayoutComponent() in BorderLayout is:
"Adds the specified component to the layout, using the specified constraint object. For border layouts, the constraint must be one of the following constants: NORTH, SOUTH, EAST, WEST, or CENTER.
Most applications do not call this method directly. This method is called when a component is added to a container using the Container.add method with the same argument types."
Best rgds Jane,
Cristi

[Jane Griscti]

Hi Cristi,
Well I was poking around in another book last night (Java Algorithms by Scott Robert Ladd) I think I found the right answer
Basically, Containers keep track of the Components they contain but it is the LayoutManager that actually handles the positioning of the Components. Each Container can be associated with only one LayoutManager or have a null LayoutManager which means you have to supply the code to handle positioning and resizing separately.
LayoutManagers contain a number of methods: addLayoutComponent(), layoutContainer(), etc. However, in most situations you don't call these directly. Instead, you use the Container methods which in turn call the LayoutManager methods ie Container.add() calls LayoutManager addLayoutComponent().
The key to the difference between LayoutManagers which require constraints such as BorderLayout, and those which don't have constraints such as FlowLayout is this:

Classes implementing LayoutManager2 ie BorderLayout, CardLayout and GridBagLayout maintain an internal list of their components.
Classes implementing LayoutManager ie FlowLayout, GridLayout query the target container for a list of Components

This is why we see the behaviour shown in the original example. When components are added to a Container, the Container itself keeps a list of the objects. If, for example, the Layout has been set to FlowLayout, and the Container is resized, the FlowLayout manager will get a list of the component objects associated with the Container and then perform the size and position functions.
If the Layout is then changed to, for example, BorderLayout, and the Container is resized, no request is made to the Container for a list of the associated Components. The BorderLayout contract states it will keep it's own list of Components; as far as it's concerned, it has no components and nothing needs to be positioned.
This is why the Components need to be re-added to the Container.
Hope thats clearer.
Jane

------------------
Jane
The cure for boredom is curiosity.
There is no cure for curiosity.
-- Dorothy Parker

[Cristi Tudose]

Hi Jane,
Your explanations lighten me.Now I'm sure this is the correct answer.
Please,could u tell me if in SCJP exam ,might apper questions and invloved answers related to deprecated method from Thread class ??(ie,stop(),suspend()/resume())
best rgds,
cristi

[Jane Griscti]

Hi Cristi,
Apologies for the late response (ISP problems the last few days).
I don't remember encountering any questions on deprecated methods.
Ajith, Bill, Angela, Marcus ... have you seen anything related to deprecated methods on the exam?
Jane

[Ajith Kallambella]

Nope. Most likely questions on deprecated feature(s) will not be included in the exam.
Ajith

[natarajan meghanathan]

Wow!. Thats a beauty..
Thanks a lot Jane!!. For clearing this long-time doubt for me about changing layout managers. I am now reading all the threads in this forum and i believe this is worth than buying 4 or 5 certification books. I am bringing this thread again (i think this is a very catchy concept) to other students who haven't visited this forum before today.

[Kalpesh Soni]

hi jane!
wonderful explanation
really cool
thanks for sharing such things
------------------
KS