Universidad Nacional de Colombia

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C
ertificación en
J
AVA

• Universidad Nacional de Colombia
• Facultad de Ingeniería
• Departamento de Sistemas

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9. LAYOUT MANAGERS

• Objectives
• Why Java uses Layout Managers
• Layout Manager Theory
• Layout Policies

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Objectives
Write code using component, container, and layout
manager classes of the java.awt package to
present a GUI with specified appearance and
resize behavior, and distinguish the
responsibilities of layout managers from those
of containers
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Introduction
The Abstract Windowing Toolkit (AWT) provides a
handful of layout managers, each of which
implements its own layout policy In Java, you
create a GUI by choosing one or more layout
managers and letting them take care of the
details.
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• When you started working with layout managers,
  you probably had two impressions
• You no longer bore the burden of specifying the
  exact position and dimensions of each component
• You no longer had the power to specify the exact
  position and dimensions of each component

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• Some people enjoy working with layout managers,
  and others resent them
• They are here to stay, so the job at hand is to
  master this feature of the language. Acquiring
  this competence requires three things
• An understanding of why Java uses layout managers
• An understanding of the layout policies of the
  more basic layout managers
• Some practice

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Why Java Uses Layout Managers
The theory lies in the position that precise
layout (that is, specification in pixels of each
component's size and position) is a repetitious
and often-performed task therefore, according to
the principles of object-oriented programming,
layout functionality ought to be encapsulated
into one or more classes to automate the task

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The practical reason for having layout managers
systems from Java's platform independence. Java
components borrow their behavior from the window
system of the underlying hardware on which the
Java Virtual Machine is running. Thus on a
Macintosh, an AWT button looks like any other Mac
button on a Motif platform, a Java button looks
like any other Motif button, and so on
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Problem buttons and other components have
different sizes when instantiated on different
platforms!!
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On a Windows 95 machine 32 pixels wide by 21
pixels high. On a Motif platform 32 pixels
wide, but 22 pixels high, even though it uses the
same font The difference seems small until you
consider the effect such a difference would have
on a column of many buttons.
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Layout Manager Theory
There are five layout manager classes in the AWT
toolkit java.awt.LayoutManager it is an
interface, not a class, because the layout
managers are so different from one another that
they have nothing in common except a handful of
method names java.awt.LayoutManager2
interface the GridBag, Border, and Card layout
managers implement it Layout managers work in
partnership with containers
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Containers and Components
Containers are Java components that can contain
other components There is a java.awt.Container
class which, like java.awt.Button and
java.awt.Choice, inherits from the
java.awt.Component superclass
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Inheritance of java.awt.Container
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The Container class was abstract, but now it
isn't its most commonly used concrete subclasses
are Applet, Frame, and Panel, (Note that Applet
is a subclass of Panel)
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Container
Panel
Window
ScrollPane
Applet
Frame
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If Java encouraged precise pixel-level sizing
and positioning, there would be a lot of Java
GUIs that looked exquisite on their platform of
origin, and terrible on other hosts There is no
guarantee that fonts with identical names will
truly be 100 percent identical from platform to
platform there could be minute differences.
Therefore, Java cannot even guarantee that two
strings drawn with the same text and font will
display at the same size across platforms
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Java GUIs reside in applets or in frames Simple
applets Put your components in your
applet Simple applications put your components
in your frame. In both cases, you might wonder
how the components end up where they do layout
managers are lurking in the background, taking
care of details)
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More complicated GUIs it is convenient to
divide the applet or frame into smaller regions
These regions might constitute, for example, a
toolbar or a matrix of radio buttons GUI
sub-regions are implemented most commonly with
the Panel container.
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Panels

• can contain other components
• buttons,
• canvases,
• check boxes,
• scroll bars,
• scroll panes,
• text areas,
• text fields, and
• other panels

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• Complicated GUIs sometimes have very complicated
  containment hierarchies of panels within panels
  within panels within panels, and so on, down
  through many layers of containment
• In Java, the term hierarchy is ambiguous.
• When discussing classes, hierarchy refers to the
  hierarchy of inheritance from superclass to
  subclass
• When discussing GUIs, hierarchy refers to the
  containment hierarchy of applets or frames, which
  contain panels containing panels containing panels

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Containment Hierarchy Example
Red
Green
Blue
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45
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Containment hierarchy
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• A component inside a container receives certain
  properties from the container
• Font, foreground and background color same as
  the container
• Panels default layout manager Flow
• Applets default layout manager Flow
• Frames default layout manager Border

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• Each panel is built in four steps
• Construct the panel
• Give the panel a layout manager
• Populate the panel
• Add the panel to its own container

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Component Size and position
Components know where they are and how big they
are The java.awt.Component class has four
instance variables x,y position of the
components upper left corner width, and height
in pixels
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Position and Size
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A components size and position can be changed by
calling the components setBounds() method
C
setBounds(x, y, width, height)
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Line 7 has no effect, because after it executes,
the button is added to the applet. The applet
calls on its layout manager to enforce its layout
policy on the button
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A disappointing Button
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Layout Policies
Every Java component has a preferred size The
preferred size expresses how big the component
would like to be, barring conflict with a layout
manager Preferred size is generally the smallest
size necessary to render the component in a
visually meaningful way
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a buttons preferred size is the size of its
label text, plus a little border of empty space
around the text, plus the shadowed decorations
that mark the boundary of the button Thus a
button's preferred size is "just big
enough. Preferred size is platform dependent,
since component boundary decorations vary from
system to system
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The Flow Layout Manager
arranges components in horizontal rows Default
layout manager type for panels and applets
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Component Size and position
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A narrower applet
Applet ViewerNeatR...
Beowulf
Name
Ok
Applet started.
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An even narrower applet
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The Flow Layout Manager
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A right-justifying Flow layout manager
Applet Viewer FlowRight.class
Applet started.
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A narrower right-justifying Flow layout manager
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EJEMPLO
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The Grid Layout Manager
always ignores a component's preferred size The
Grid layout manager subdivides its territory into
a matrix of rows and columns The number of rows
and number of columns are specified as parameters
to the manager's constructor
GridLayout(int nRows, int ncolumns)
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Components know where they are and how big they
are
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EJEMPLO
Components know where they are and how big they
are
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The Border Layout Manager
The Flow layout manager always honors a
component's preferred size the Grid layout
manager never does. The Border layout manager
does something in between It is the default
manager for frames
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The Border layout manager divides its territory
into five regions North South East West Center Ea
ch region may contain a single component No
region is required to contain a component!!
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The Border Layout Manager
Components know where they are and how big they
are
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• The overloaded version of add() takes two
  parameters
• The component being added
• An Object
• Proper use of the Border Layout manager requires
  that the second parameter be a String that
  specifies the name of the region
• North
• South
• East
• West
• Center

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• Border Layout constants to use instead of the
  strings
• BorderLayout.NORTH
• BorderLayout.SOUTH
• BorderLayout.EAST
• BorderLayout .WEST
• BorderLayout.CENTER

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East and West
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East and West, with North
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East and West, with South
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East and West, with both North and South
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Center
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Center, no North
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Center, no South, East or West
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Other Layout Options
If you are in a situation where Flow, Grid, and
Border will not create the layout you need, your
choices are   To use a GridBag layout
manager To use a Card layout manager To use
no layout manager To create your own layout
manager
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GridBag Layout Manager
is by far the most complicated layout manager It
divides its container into an array of cells, but
(unlike the cells of a Grid layout manager)
different cell rows can have different heights,
and different cell columns can have different
widths A component can occupy a single cell or
it can span a number of cells. A GridBag layout
manager requires a lot of information to know
where to put a component
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GridBag Layout Manager ...
A helper class called GridBagConstraints is used
to hold all the layout position information.
When you add a component, you use the
add(Component, Object) version of the add()
method, passing an instance of GridBagConstraints
as the Object parameter.
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Card Layout Manager ...
Lays out its components in time rather than in
space At any time, a component using a Card
layout manager is displaying one or another of
its components all the other components are
unseen
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You always have the option of using no layout
manager at all
myContainer.setLayout(null)
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