如何使用滑块

A JSlider component is intended to let the user easily enter a numeric value bounded by a minimum and maximum value. If space is limited, a spinner is a possible alternative to a slider.

The following picture shows an application that uses a slider to control animation speed:

Below is the code from the SliderDemo.java file that creates the slider in the previous example.

static final int FPS_MIN = 0;
static final int FPS_MAX = 30;
static final int FPS_INIT = 15;    //initial frames per second
. . .
JSlider framesPerSecond = new JSlider(JSlider.HORIZONTAL,
                                      FPS_MIN, FPS_MAX, FPS_INIT);
framesPerSecond.addChangeListener(this);

//Turn on labels at major tick marks.
framesPerSecond.setMajorTickSpacing(10);
framesPerSecond.setMinorTickSpacing(1);
framesPerSecond.setPaintTicks(true);
framesPerSecond.setPaintLabels(true);

By default, spacing for major and minor tick marks is zero. To see tick marks, you must explicitly set the spacing for either major or minor tick marks (or both) to a non-zero value and call the setPaintTicks(true) method. However, you also need labels for your tick marks. To display standard, numeric labels at major tick mark locations, set the major tick spacing, then call the setPaintLabels(true) method. The example program provides labels for its slider in this way. But you are not constrained to using only these labels. Customizing Labels on a Slider shows you how to customize slider labels. In addition, a slider feature allows you to set a font for the JSlider component.

Font font = new Font("Serif", Font.ITALIC, 15);
framesPerSecond.setFont(font);

When you move the slider's knob, the stateChanged method of the slider's ChangeListener is called. For information about change listeners, refer to How to Write a Change Listener. Here is the change listener code that reacts to slider value changes:

public void stateChanged(ChangeEvent e) {
    JSlider source = (JSlider)e.getSource();
    if (!source.getValueIsAdjusting()) {
        int fps = (int)source.getValue();
        if (fps == 0) {
            if (!frozen) stopAnimation();
        } else {
            delay = 1000 / fps;
            timer.setDelay(delay);
            timer.setInitialDelay(delay * 10);
            if (frozen) startAnimation();
        }
    }
}

Notice that the stateChanged method changes the animation speed only if the getValueIsAdjusting method returns false. Many change events are fired as the user moves the slider knob. This program is interested only in the final result of the user's action.

Customizing Labels on a Slider

The demo below is a modified version of the SliderDemo that uses a slider with custom labels:

The source for this program can be found in SliderDemo2.java. Click the Launch button to run SliderDemo2 using Java™ Web Start (download JDK 7 or later). Alternatively, to compile and run the example yourself, consult the example index.

The following code creates the slider and customizes its labels:

//Create the slider
JSlider framesPerSecond = new JSlider(JSlider.VERTICAL,
                                      FPS_MIN, FPS_MAX, FPS_INIT);
framesPerSecond.addChangeListener(this);
framesPerSecond.setMajorTickSpacing(10);
framesPerSecond.setPaintTicks(true);

//Create the label table
Hashtable labelTable = new Hashtable();
labelTable.put( new Integer( 0 ), new JLabel("Stop") );
labelTable.put( new Integer( FPS_MAX/10 ), new JLabel("Slow") );
labelTable.put( new Integer( FPS_MAX ), new JLabel("Fast") );
framesPerSecond.setLabelTable( labelTable );

framesPerSecond.setPaintLabels(true);

Each key-value pair in the hashtable specified with the setLabelTable method gives the position and the value of one label. The hashtable key must be of an Integer type and must have a value within the slider's range at which to place the label. The hashtable value associated with each key must be a Component object. This demo uses JLabel instances with text only. An interesting modification would be to use JLabel instances with icons or buttons that move the knob to the label's position.

Use the createStandardLabels method of the JSlider class to create a set of numeric labels positioned at a specific interval. You can also modify the table returned by the createStandardLabels method in order to customize it.

The Slider API

The following tables list the commonly used JSlider constructors and methods. See The JComponent Class for tables of commonly used inherited methods.

The API for using sliders is divided into these categories:

• Creating the Slider
• Fine Tuning the Slider's Appearance
• Watching the Slider Operate
• Working Directly with the Data Model

Examples that Use Sliders

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

If you are programming in JavaFX, see Slider.