Creating a Custom Layout Manager
Before you start creating a custom layout manager, make sure that no existing layout manager meets your requirements. In particular, layout managers such as GridBagLayout
, SpringLayout
, and BoxLayout
are flexible enough to work in many cases. You can also find layout managers from other sources, such as from the Internet. Finally, you can simplify layout by grouping components into containers such as panels.
Note: This lesson covers writing layout code by hand, which can be challenging. If you are not interested in learning all the details of layout management, you might prefer to use the
GroupLayout
layout manager combined with a builder tool to lay out your GUI. One such builder tool is the
NetBeans IDE. Otherwise, if you want to code by hand and do not want to use
GroupLayout
, then
GridBagLayout
is recommended as the next most flexible and powerful layout manager.
If you are interested in using JavaFX to create your GUI, see Working With Layouts in JavaFX .
To create a custom layout manager, you must create a class that implements the LayoutManager
interface. You can either implement it directly, or implement its subinterface, LayoutManager2
.
Every layout manager must implement at least the following five methods, which are required by the LayoutManager
interface:
-
void addLayoutComponent(String, Component)
-
Called by the
Container
class'sadd
methods. Layout managers that do not associate strings with their components generally do nothing in this method. -
void removeLayoutComponent(Component)
-
Called by the
Container
methodsremove
andremoveAll
. Layout managers override this method to clear an internal state they may have associated with theComponent
. -
Dimension preferredLayoutSize(Container)
-
Called by the
Container
class'sgetPreferredSize
method, which is itself called under a variety of circumstances. This method should calculate and return the ideal size of the container, assuming that the components it contains will be at or above their preferred sizes. This method must take into account the container's internal borders, which are returned by thegetInsets
method. -
Dimension minimumLayoutSize(Container)
-
Called by the
Container
getMinimumSize
method, which is itself called under a variety of circumstances. This method should calculate and return the minimum size of the container, assuming that the components it contains will be at or above their minimum sizes. This method must take into account the container's internal borders, which are returned by thegetInsets
method. -
void layoutContainer(Container)
-
Called to position and size each of the components in the container. A layout manager's
layoutContainer
method does not actually draw components. It simply invokes one or more of each component'ssetSize
,setLocation
, andsetBounds
methods to set the component's size and position.This method must take into account the container's internal borders, which are returned by the
getInsets
method. If appropriate, it should also take the container's orientation (returned by thegetComponentOrientation
method) into account. You cannot assume that thepreferredLayoutSize
orminimumLayoutSize
methods will be called beforelayoutContainer
is called.
Besides implementing the preceding five methods, layout managers generally implement at least one public constructor and the toString
method.
If you wish to support component constraints, maximum sizes, or alignment, then your layout manager should implement the LayoutManager2
interface. In fact, for these reasons among many others, nearly all modern layout managers will need to implement LayoutManager2
. That interface adds five methods to those required by LayoutManager
:
addLayoutComponent(Component, Object)
getLayoutAlignmentX(Container)
getLayoutAlignmentY(Container)
invalidateLayout(Container)
maximumLayoutSize(Container)
Of these methods, the most important are addLayoutComponent(Component, Object)
and invalidateLayout(Container)
. The addLayoutComponent
method is used to add components to the layout, using the specified constraint object. The invalidateLayout
method is used to invalidate the layout, so that if the layout manager has cached information, this should be discarded. For more information about LayoutManager2
, see the LayoutManager2
API documentation.
Finally, whenever you create custom layout managers, you should be careful of keeping references to Component
instances that are no longer children of the Container
. Namely, layout managers should override removeLayoutComponent
to clear any cached state related to the Component
.
Example of a Custom Layout
The example CustomLayoutDemo
uses a custom layout manager called DiagonalLayout
. You can find the layout manager's source code in DiagonalLayout.java
. DialogLayout
lays out components diagonally, from left to right, with one component per row. Here is a picture of CustomLayoutDemo using DialogLayout
to lay out five buttons.
Click the Launch button to run CustomLayoutDemo
using Java™ Web Start (download JDK 7 or later ). Alternatively, to compile and run the example yourself, consult the example index.