Using the init and postinit Blocks in Java

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Using the init and postinit Blocks
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The init block is executed while its owning object is being initialized and after all the initial values for the variables in that class (and subclasses) have been setTypical tasks for this block include the following:
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Checking that the initial values of the instance variables are valid, if this has not already been done by their associated triggers Checking that the initial values are consistent with each other Setting the values of any variables that cannot be initialized from an object initializer and which depend on values that could be assigned by the object initializer
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As an example of the last bullet point, suppose you want to create a JavaFX class with an embedded HashMap and you want to allow application code to configure the initial
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capacity of the HashMapYou can do this by allocating the map in the init block of your JavaFX class6:
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class MapBasedClass { public-init var capacity = 32; var map:Map; init { map = new HashMap(capacity); } }
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The init block should not set the value of a variable that could be set from an object initializer, unless it is known that a value has not already been setThe reason for this restriction is that object initializer values are installed before the init block is run, so any value set from this block will overwrite the value set by application code If you need to assign a value to a variable that could be set from an object initializer, either make it the default value in the variable declaration, which allows application code to override it by using the object initialize, or use the isInitialized() function to check whether a value has already been supplied:
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1 2 3 4 5 6 7 8 9 class InitializeTest { public-init var value; init { if (not isInitialized(value)) { value = 32; } } }
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In the preceding code, no default value is assigned to the variable value on line 2 In the init block, a test is made to determine whether the variable has been initialized and, if not, the value 32 is assigned In a more realistic example, the value assigned would probably be computed from other state in the same object or in related objects The isInitialized() function returns true in the following circumstances:
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If a value has been assigned by an object initializer If this class is subclassed and the variable is overridden to assign a default value
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Instantiating the class without assigning a value causes the value 32 to be assigned:
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var i = InitializeTest {}; println(ivalue); // Prints 32
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Thanks to Brian Goetz for this example, which appeared as part of a discussion in the JavaFX compiler development forums
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However, a value set in an object initializer is not changed by the init block:
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i = InitializeTest { value: 64 }; println(ivalue); // Prints 64
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The postinit block can be used to perform actions that can take place only once the object is fully initialized, one example of which is publishing the object externally If your object needs to attach itself as a listener to some state in another class, for example, it should do this in the postinit block rather than in init if there is the possibility that an event might be delivered to it before it has fully initialized
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A JavaFX source file can contain any number of class definitions, some or all of which may be public classes None of the classnames need to match that of the source fileThis is different to Java, where only one class in a source file can be public and the name of that class must match that of the source file The code in Listing 11-4 shows two classes, a script function and a script variable, all defined in the same script file
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Listing 11-4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26