Binding to the value of an expression in Java

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Binding to the value of an expression
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Here, the bound value is a string with a nested expression that depends on the value of the rawText variable of the TextBoxWhenever that value changes, the nested expression is reevaluated and substituted into the string, and the result is assigned to the text variable of the Label You can embed more than one expression in the string, and the bound variable will be updated when the value of either expression changesThe following code, which is taken from the file javafxbinding/BindLabelToExpression2fx in the JavaFX Book GUI project, causes the label to display the length of the string that you type as well as the string itself, as you can see in Figure 9-3:
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text: bind "You typed: {textBoxrawText}" "\n({textBoxrawTextlength()} characters)"
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Figure 9-3
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Binding with two embedded expressions
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Binding also works with arithmetic operations:
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var a = 23; var b = bind a + 10;
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This code initializes b with the value 33 and ensures that it is always 10 greater than the value assigned to a
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Binding to Variables and Expressions
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Binding and the def Statement
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As you saw in 5, Variables and Data Types, the def statement creates a variable that is initialized when declared and which cannot subsequently be changed More correctly, the variable cannot be changed by assignment, but its value can change if it is bound to an expression whose value can change For example
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var value = 10; def twiceTheValue = bind 2 * value;
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The variable twiceTheValue is initially assigned the value 20, as a result of the binding Even though its value cannot be directly changed, the binding ensures that it tracks the variable value so that the following code prints 30:
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value = 15; println("{twiceTheValue}");
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Binding to an Instance Variable
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You have already seen an example that bound the value of the text variable of a Label to that of the rawText variable of a TextBox in an object initializerYou can also bind a script variable to the value of an instance variable, as the following example shows:
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1 2 3 4 5 6 7 8 9 10 11 12 class ValueHolder { var value:Integer; } var holder1 = ValueHolder { value: 1234 }; var holder2 = ValueHolder { value: 3456 }; var holder = holder1; var v = bind holdervalue; println("Value #1: {v}"); holder1value = 9876; println("Value #2: {v}");
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The code on lines 1 through 3 are a repeat of the definition of a JavaFX class called that we used in an earlier example in this chapter Lines 5 through 7 create two instances of this class and then assign a reference to one of them to the script variable holder On line 9, we create a binding between the variable v and the value variable of the ValueHolder referred to by holder, which currently refers to the instance created on line 5 It should be no surprise that the result of the println() call on line 10 is the following:
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ValueHolder Value #1: 1234
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ValueHolder
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This is the expected result because 1234 is the value of the value variable of the first object It should also be clear that because line 11 assigns 9876 to the value
9 Binding
variable of the first ValueHolder and v is bound to this variable, the output from line 12 will be the following:
Value #2: 9876
Now suppose that we do this:
holder = holder2; println("Value #3: {v}");
This code makes the script variable holder refer to the other ValueHolder instance Given that v is bound to holdervalue, what effect should this assignment have The answer is that after this assignment, v will have the value 3456:
Value #3: 3456
This is quite logical, because we bound v to the value variable of whatever the variable holder refers to, not to the value variable of the ValueHolder that holder initially referred toAs a result, changing the value of either the variable holder or the value variable of the ValueHolder to which it refers will cause v to be updatedAnother way of thinking about this is that if a variable is bound to abcd, it is effectively also bound to all of abc, ab, and a