Andrey Breslav
74 lines
2.2 KiB
Plaintext
74 lines
2.2 KiB
Plaintext
Sometimes we need to create an object of a slight modification of some class, without explicitly declaring a new subclass for it. *Java* handles this case with _anonymous inner classes_. [Kotlin] slightly generalizes this concept with *object* expressions and *object* declarations.
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h3. Object expressions
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To create an object of an anonymous class that inherits from some type (or types), one writes:
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{jet}
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window.addMouseListener(object : MouseAdapter() {
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override fun mouseClicked(e : MouseEvent) {
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// ...
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}
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override fun mouseEntered(e : MouseEvent) {
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// ...
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}
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})
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{jet}
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If a supertype has a constructor, appropriate constructor parameters must be passed to it. Many supertypes may be specified as a comma-separated list after the colon:
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{jet}
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open class A(x : Int) {
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public virtual val y : Int = x
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}
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open class B(s : String)
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val ab = object : A(1), B("abc") {
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override val y = 15
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}
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{jet}
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If, by any chance, we need "just an object", with no nontrivial supertypes, we can simply say:
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{jet}
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val adHoc = object {
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var x : Int = 0
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var y : Int = 0
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}
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print(adHoc.x + adHoc.y)
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{jet}
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h3. Object declarations
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[Singleton|http://en.wikipedia.org/wiki/Singleton_pattern] is a very useful pattern, and [Kotlin] (after *Scala*) makes it easy to declare singletons:
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{jet}
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object DataProviderManager {
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fun registerDataProvider(provider : DataProvider) {
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// ...
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}
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val allDataProviders : Collection<DataProvider>
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get() = // ...
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}
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{jet}
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This is called an _object declaration_. If there's a name following the *object* keyword, we are not talking about an _expression_ any more. We cannot assign such a thing to a variable, but we can refer to it by its name. Such objects can have supertypes:
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{jet}
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object DefaultListener : MouseAdapter() {
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override fun mouseClicked(e : MouseEvent) {
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// ...
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}
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override fun mouseEntered(e : MouseEvent) {
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// ...
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}
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}
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{jet}
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h3. Semantical difference between object expressions and declarations
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There is one important semantical difference between *object* expressions and *object* declarations:
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* *object* declarations are initialized lazily, when accessed for the first time
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* *object* expressions are executed (and initialized) immediately, where they are used
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h3. What's next
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* [Generics] |