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How to Use Split Panes

A JSplitPane displays two components, either side by side or one on top of the other. By dragging the divider that appears between the components, the user can specify how much of the split pane's total area goes to each component. You can divide screen space among three or more components by putting split panes inside of split panes, as described in Nesting Split Panes.

Instead of adding the components of interest directly to a split pane, you often put each component into a scroll pane. You then put the scroll panes into the split pane. This allows the user to view any part of a component of interest, without requiring the component to take up a lot of screen space or adapt to displaying itself in varying amounts of screen space.

Here's a picture of an application that uses a split pane to display a list and an image side by side:

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Try this: 

1. Compile and run the application. The main source file is SplitPaneDemo.java. This example requires some additional files. See the examples index for links to all the files required by this example. See Getting Started with Swing if you need help compiling or running this application.
   This program gets the names of the images for the list from a properties resource bundle. For this demo you just need to know that the resource bundle contains a space-separated list of file names and the program parses the list into a Vector. The properties resource bundle is backed by imagenames.properties, which you need to put in the same directory as the application's class files. You can learn more about resource bundles in Isolating Locale-Specific Data.
2. Drag the dimpled line that divides the list and the image to the left or right. Try to drag the divider all the way to the window's edge.
3. Click the tiny arrows on the divider to hide/expand the left or right component.

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Below is the code from SplitPaneDemo that creates and sets up the split pane.

//Create a split pane with the two scroll panes in it.
splitPane = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT,
                           listScrollPane, pictureScrollPane);
splitPane.setOneTouchExpandable(true);
splitPane.setDividerLocation(150);

//Provide minimum sizes for the two components in the split pane
Dimension minimumSize = new Dimension(100, 50);
listScrollPane.setMinimumSize(minimumSize);
pictureScrollPane.setMinimumSize(minimumSize);

The constructor used by this example takes three arguments. The first indicates the split direction. The other arguments are the two components to put in the split pane. Refer to Setting the Components in a Split Pane for information about JSplitPane methods that set the components dynamically.

The split pane in this example is split horizontally--the two components appear side by side--as specified by the JSplitPane.HORIZONTAL_SPLIT argument to the constructor. Split pane provides one other option, specified with JSplitPane.VERTICAL_SPLIT, that places one component above the other. You can change the split direction after the split pane has been created with the setOrientation method.

Two small arrows appear at the top of the divider in the example's split pane. These arrows let the user collapse (and then expand) either of the components with a single click. The current look and feel determines whether these controls appear by default. In the Java Look & Feel, they are turned off by default. The example turned them on with a call to the setOneTouchExpandable.

The range of a split pane's divider is determined in part by the minimum sizes of the components within the split pane. See Positioning the Divider and Restricting its Range for details.

The rest of this section covers these topics:

• Setting the Components in a Split Pane
• Positioning the Divider and Restricting its Range
• Nesting Split Panes
• The Split Pane API
• Examples that Use Split Panes

A program can set a split pane's two components dynamically with these four methods:

• setLeftComponent
• setRightComponent
• setTopComponent
• setBottomComponent

You can use any of these methods at any time regardless of the split pane's current split direction. Calls to setLeftComponent and setTopComponent are equivalent and set the specified component in the top or left position, depending on the split pane's current split orientation. Similarly, calls to setRightComponent and setBottomComponent are equivalent. These methods replace whatever component is already in that position with the new one.

Like other containers, JSplitPane supports the add method. Split pane puts the first component added in the left or top position. The danger of using add is that you can inadvertantly call it too many times, in which case, the split pane's layout manager will throw a rather esoteric-looking exception. If you are using the add method and a split pane is already populated, you first need to remove the existing components with remove.

If you put only one component in a split pane, then the divider will be stuck at the right side or the bottom of the split pane, depending on its split direction.

Use the setDividerLocation method to position the divider programmatically. You can specify the new position by pixel or by percentage. To get the current divider location specified in pixels call getDividerLocation.

//Set divider at pixel 150
splitPane.setDividerLocation(150);
//25% of the space goes to left/top
splitPane.setDividerLocation(0.25);

Another handy method, resetToPreferredSizes, sets the divider location such that the two components are at their preferred sizes. This is the initial arrangement of a split pane, unless specified otherwise.

The user can set the divider location by dragging it. A split pane does not allow the user to make either of its components smaller than the component's minimum size by dragging the divider. So, you can affect the divider's range by setting the minimum sizes of the two components in the split pane. For example, to ensure that a minimum amount of each component in a split is visible at all times, set the minimum size of each component. To allow the user to drag the divider all the way to the edge of the split pane, use 0 for the minimum size.

If you don't set the minimum size of the two components in a split pane, then you might end up with a split pane whose divider won't move. By default, a split pane sizes its components at their preferred size, which for many components is equal to the minimum size. This means that both components are already displayed at their minimum sizes and the divider is stuck. Most programs that put standard components in a split pane need to set the minimum sizes of the components in the split pane to a smaller size.

Here's a picture of a program that achieves a three-way split by nesting one split pane inside of another:

If the top portion of the split pane looks familiar to you, it's because the program puts the split pane created by SplitPaneDemo inside a second split pane. A simple JLabel is the other component in the second split pane. This is not the most practical use of a nested split pane, but it gets the point across. Here's the code, which you can find in SplitPaneDemo2.java:

//Create an instance of SplitPaneDemo
SplitPaneDemo splitPaneDemo = new SplitPaneDemo();
JSplitPane top = splitPaneDemo.getSplitPane();

...
//Create a regular old label
label = new JLabel("Click on an image name in the list.",
		   JLabel.CENTER);

//Create a split pane and put "top" (a split pane)
//and JLabel instance in it.
JSplitPane splitPane = new JSplitPane(JSplitPane.VERTICAL_SPLIT,
				      top, label);

Refer to Solving Common Component Problems for information about fixing a border problem that can appear when nesting split panes.

The Split Pane API

The following tables list the commonly used JSplitPane constructors and methods. Other methods you are most likely to invoke on a JSplitPane object are those such as setPreferredSize that its superclasses provide. See The JComponent API for tables of commonly used inherited methods.

The API for using lists falls into these categories:

• Setting up the Split Pane
• Managing the Split Pane's Contents
• Positioning the Divider

Setting Up the Split Pane

Method	Purpose
JSplitPane() JSplitPane(int) JSplitPane(int, boolean) JSplitPane(int, Component, Component) JSplitPane(int, boolean, Component, Component)	Create a split pane. When present, the int parameter indicates the split pane's orientation, either HORIZONTAL_SPLIT (the default) or VERTICAL_SPLIT. The boolean parameter, when present, sets whether the components continually repaint as the user drags the split pane. If left unspecified, this option (called continuous layout) is turned off. The Component parameters set the initial left and right, or top and bottom components, respectively.
void setOrientation(int) int getOrientation()	Set or get the split pane's orientation. Use either HORIZONTAL_SPLIT or VERTICAL_SPLIT defined in JSplitPane. If left unspecified, the split pane will be horizontally split.
void setDividerSize(int) int getDividerSize()	Set or get the size of the divider in pixels.
void setContinuousLayout(boolean) boolean getContinuousLayout()	Set or get whether the split pane's components are continually layed out and painted while the user is dragging the divider. By default, continuous layout is turned off.
void setOneTouchExpandable(boolean) boolean getOneTouchExpandable()	Set or get whether the split pane displays a control on the divider to expand/collapse the divider. The default depends on the look and feel. In the Java Look & Feel, it's off by default.

Managing the Split Pane's Contents

Method	Purpose
void setTopComponent(Component) void setBottomComponent(Component) void setLeftComponent(Component) void setRightComponent(Component) Component getTopComponent() Component getBottomComponent() Component getLeftComponent() Component getRightComponent()	Set or get the indicated component. Each method works regardless of the split pane's orientation. Top and left are equivalent, and bottom and right are equivalent.
void remove(Component) void removeAll()	Remove the indicated component(s) from the split pane.
void add(Component)	Add the component to the split pane. You can add only two components to a split pane. The first component added is the top/left component. The second component added is the bottom/right component. Attempt to add more components result in an exception.

Positioning the Divider

Method	Purpose
void setDividerLocation(double) void setDividerLocation(int) int getDividerLocation()	Set or get the current divider location. When setting the divider location, you can specify the new location as a percentage (double) or a pixel location (int).
void resetToPreferredSizes()	Move the divider such that both components are at their preferred sizes. This is how a split pane divides itself at startup, unless specified otherwise.
void setLastDividerLocation(int) int getLastDividerLocation()	Set or get the previous position of the divider.
int getMaximumDividerLocation() int getMinimumDividerLocation()	Get the minimum and maximum locations for the divider. These are set implicitly by setting the minimum sizes of the split pane's two components.

Examples that Use Split Panes

This table shows the examples that use JSplitPane and where those examples are described.

Example	Where Described	Notes
SplitPaneDemo	This page and How to Use Lists.	Shows a split pane with a horizontal split.
SplitPaneDemo2	This page	Puts a split pane within a split pane to create a three-way split.
TreeDemo	How to Use Trees	Uses a split pane with a vertical split. Does not use the one-touch expandable feature.

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