Inheriting from a Base Class and a Mixin in Java

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Inheriting from a Base Class and a Mixin
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package javafxclasses; mixin class Mixin { var name = "Mixin name";
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public function doSomething() { println("mixin doSomething"); } } class Base { var name = "Base name"; public function doSomething() { println("Base doSomething"); } } class Mixee extends Mixin, Base { } var m = Mixee {}; println(mname); mdoSomething();
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// Compilation error
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Here, the class Mixee is derived from Base and incorporates the mixin class Mixin Both Base and Mixin declare a variable called name and a function called doSomething() The reference on line 23 to the name variable is resolved to the declaration in the Base class on line 12, in preference to the variable of the same name in the mixin However, the code on line 24 causes a compilation error the compiler does not choose to prefer the declaration of this function in the base class over that in the mixin
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Mixins and Triggers
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When a variable declaration in a mixin includes a trigger, that trigger applies even if the variable declaration is apparently overridden by one in another mixin, in the mixee, or in the mixee s base classThe same applies to the triggers in the mixee class and its base class, if anyThis is no different from the situation that exists with normal inheritance, as the example in Listing 11-14 shows
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Listing 11-14
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Using Triggers with Mixins
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package javafxclasses; mixin class Mixin { var name = "Mixin" on replace { println("Mixin trigger, value is {name}"); } }
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Mixins
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class Mixee extends Mixin { override var name = "Mixee" on replace { println("Mixee trigger, value is {name}"); } } var m = Mixee {};
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In this example, both the mixin and mixee class attach a trigger to the name variable (on lines 4 and 10)As you have already seen, the precedence rules dictate that the initial value assigned to this variable is the one set in the mixee class, but both triggers are attached and both fire, as the output from this example shows:
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Mixin trigger, value is Mixee Mixee trigger, value is Mixee
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Initialization and Mixins
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Like ordinary classes, mixins can have both init and postinit blocks, which are called during the initialization of any class of which the mixin is a partThe order of invocation of the init and postinit blocks in the mixee class is as follows: 1 The init block of the mixee s superclass is called 2 The init blocks of the mixin classes are called, in the order in which they appear in the extends clause of the mixee class declaration 3 The init block of the mixee class is called 4 The postinit block of the mixee s superclass is called 5 The postinit blocks of the mixin classes are called, in the order in which they appear in the extends clause of the mixee class declaration 6 The postinit block of the mixee class is called The code in Listing 11-15 illustrates the initialization order
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Listing 11-15
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Order of Initialization in Classes with Mixins
package javafxclasses; mixin class Mixin1 { init { println("Mixin1 init") } postinit { println("Mixin1 postinit") } } mixin class Mixin2 extends Mixin1 {
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init { println("Mixin2 init") } postinit { println("Mixin2 postinit") } } mixin class Mixin3 { init { println("Mixin3 init") } postinit { println("Mixin3 postinit") } } class Base { init { println("Base init") } postinit { println("Base postinit") } } class Mixee1 extends Mixin1, Mixin2, Mixin3, Base { init { println("Mixee init") } postinit { println("Mixee postinit") } } println("Create instance of Mixee1"); var m1 = Mixee1 {}; class Mixee2 extends Mixin3, Mixin1, Mixin2, Base { init { println("Mixee init") } postinit { println("Mixee postinit") } } println("\nCreate instance of Mixee2"); var m2 = Mixee2 {};
This code contains the following:
Three mixin classes called Mixin1, Mixin2, and Mixin3 Note that the class Mixin2 is derived from Mixin1 A class called Base Two mixee classes, Mixee1 and Mixee2, that are both derived from Base and incorporate all three of the mixinsThe only difference between these two classes is the order of mixins in their extends clauses
Each of these classes declares init and postinit blocks that write a message to indicate that they have been executedWhen an instance of Mixee1 is created on line 29 of the code, execution of these blocks results in the following output: