Variable in Java

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Table 22-14 Variable
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item selectedIndex
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Variables of the ChoiceBox Class Type
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Object[] Integer
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Access
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RW R
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Default
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Description
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The items to be displayed The index of the selected item, or -1 if there is no selection The selected item, or null if there is none
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true if the full list of items is visible, false if not
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selectedItem
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Object
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showing
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Boolean
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You can see the result of running this example in Figure 22-28 As you can see, ChoiceBox is more compact than ListView, and this is the usual reason for choosing one over the otherWhen the user clicks the control, a pop-up showing all of the values from the items variable appears, as shown in Figure 22-29
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The ScrollBar Control
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Figure 22-28
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A ChoiceBox control
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Figure 22-29
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A ChoiceBox control with selections showing
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As you can see, the item that is currently selected is indicated with a tick markThe user can use the mouse or the arrow keys on the keyboard to move the selection highlight and either click with the mouse or press the Enter key to change the selectionApplication code can track the selection by using the selectedItem and selectedIndex variables in the same way as with ListView In JavaFX 13, the items in the ChoiceBox are rendered by calling their toString() functionAt the time of this writing, there is no cell factory and therefore no way to change the rendering
The ScrollBar Control
The ScrollBar control allows the user to select a value from a bounded range of numbers given by its max and min variablesThe current value of the scrollbar is represented by its value variable Graphically, a scrollbar is typically rendered as a horizontal or vertical track with a moveable thumbThe ends of the track represent the minimum and maximum values of the scrollbar, while the position of the thumb gives the current value, as shown in Figure 22-30
22 Cross-Platform Controls
Unit increment button Thumb Track
Vertical scroll bar
Horizontal scroll bar
Figure 22-30
Horizontal and vertical scrollbars
A scrollbar is most commonly used when displaying a node or control that is too large to be seen all at once, such as the list of items in a list view control (as shown in Figure 22-27) or the large image shown in Figure 22-30 In this example, the user would drag the scrollbar to bring a different part of the image into view The ScrollView class, which we discuss in the next section, is a convenient control that makes internal use of two ScrollBars to provide just this functionality If you don t plan to make direct use of a ScrollBar, you can skip the rest of this section and review the coverage of ScrollView that follows it The variables of the ScrollBar class are listed in Table 22-15
Table 22-15 Variable
vertical
Variables of the ScrollBar Class Type
Boolean
Access
Default
false
Description
true for a vertical scrollbar, false for
horizontal
min max value visible Amount clickTo Position Number Number Number Number
RW RW RW RW
0 100 0
The minimum value for the scrollbar The maximum value for the scrollbar The current value of the scrollbar The extent of the scrollbar s range that is actually visible
Boolean
false
Whether clicking in the track set the value that corresponds to the track position
The ScrollBar Control
Table 22-15 Variable
unit Increment
Variables of the ScrollBar Class (Continued) Type
Number
Access
Default
Description
The amount by which the value changes when one of the buttons at its ends is clicked The amount by which the value changes when the mouse is clicked on the track if clickToPosition is
false
block Increment
Number
ScrollBar Value and Range
When the thumb is at the far left of a horizontal scrollbar (or at the top of a vertical scrollbar), its value variable has the minimum value; when it is at the far right (or bottom for a vertical scrollbar) the value variable has the maximum value In between, the value is proportional to the thumb bar s location For example, for a horizontal scrollbar with a minimum value of 0 and a maximum value of 100 (which are the defaults), when the thumb is at the midpoint of the track, the value variable will be 50 Moving the thumb further to right produces a value that is greater than 50, while moving it to the left gives a value that is less than 50 The value of the min variable must always be no greater than that of the max variable, and the value variable will always have some value equal to or between these extremes If you attempt to set any of these variables to values that would violate this constraint, the other variables are adjusted accordingly For example, if min is 0, value is 50, and max is 100, and you attempt to set min to 150, the result will be that all three variables are set to 100, because min is first clamped to the value of max, resulting in both having the value 100, value is also set to 100 because it must be no greater than max and no less than min