In addition, B itself can be hidden from the outside world. By hiding class B within class A, A's members can be declared private and B can access them. It increases encapsulation: Consider two top-level classes, A and B, where B needs access to members of A that would otherwise be declared private. Nesting such "helper classes" makes their package more streamlined. It is a way of logically grouping classes that are only used in one place: If a class is useful to only one other class, then it is logical to embed it in that class and keep the two together. (Recall that outer classes can only be declared public or package private.) Why Use Nested Classes?Ĭompelling reasons for using nested classes include the following: As a member of the OuterClass, a nested class can be declared private, public, protected, or package private. Static nested classes do not have access to other members of the enclosing class. Non-static nested classes (inner classes) have access to other members of the enclosing class, even if they are declared private. Further, no matter whether the function has or not, we can call it through the Companion object in Java.A nested class is a member of its enclosing class. the pickOneRandomly() function without test passes if we give it a run.Īs we can see in the Java code above, due to the annotation, we can invoke the greaterThan() function as a standard Java static method.
calling it through the Companion objectĪssertEquals(Arrays.asList(THREE, FOUR, FIVE, SIX), (2)) Next, let’s create a test in Java and see how to call the two functions: // the greaterThan() function with FOUR, FIVE, SIX), eaterThan(2)) However, we can add the annotation to functions within the companion object body so that Kotlin will generate additional static methods to delegate the corresponding Companion.functions in the compiled bytecode. Then, we can call them as normal static methods that we’re familiar with.įor the sake of demonstration, we’ll add the only to one function: enum class MagicNumber(val value: Int) The functions in Kotlin enum’s companion object can be called in Java through (), for example, ().
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