CITS2210 Object-Oriented Programming Topic 10 Java: Nested and Inner Classes Summary: Nested and inner classes allow classes do be defined within other classes, and even within bodies of methods. They enable certain styles of programming that are awkward otherwise. 1 What are nested and inner classes? A nested class is a class defined inside another class. public class OuterClass { class NestedClass { } } Nested class declarations can appear in classes and interfaces along with the more standard variable declarations and method declarations. – Like other declarations, they can have modifiers like public, static, etc. – A non-static nested class is called an inner class. You can always write separate classes instead of using nested classes, but they are still useful for the following reasons. – They allow “helper” classes to be logically grouped with a main class. – They allow improved encapsulation since nested classes can access private variables, and can be private themselves. – They allow code to be better organized, with related code appearing in one place. 2 Static nested classes Static nested classes are “attached” to the outer class. – They can only access static variables and methods from the outerclass directly. – As such, static nested classes can usually be easily moved to become top-level classes – the nesting is only used for organization. – Static nested classes are can be accessed via the outer class, just like static methods and class variables (assuming that they are not private). public class Outer { public static class StaticNested { } } Outer.StaticNested nested = new Outer.StaticNested(); 3 Inner classes Inner classes are attached to a particular instance of the outer class. – So, each instance of the inner class is also attached to this instance of the outer class. – To create instances of an inner class you need an instance of the outer class so that you can use “new” with the inner class in that instance. – Instances of inner classes can directly access all the methods and instance variables of the enclosing class for the instance they are attached to. public class Outer { private int instanceVar; public class Inner { public int getOuter() { return instanceVar; } } } Outer outer = new outer(); Outer.Inner inner = outer.new Inner(); int i = inner.getOuter; 4 Extended example of an inner class public class DataStructure { private final static int SIZE = 15; private int[] arrayOfInts = new int[SIZE]; public DataStructure() { for (int i = 0; i < SIZE; i++) { arrayOfInts[i] = i; //fill the array } } public void printEven() { //print out values of even indices of the array } private class InnerEvenIterator { private int next = 0; // Begin at the start of the array. public boolean hasNext() { return (next <= SIZE - 1); } public int getNext() { int retValue = arrayOfInts[next]; next += 2; return retValue; } } public static void main(String s[]) { DataStructure ds = new DataStructure(); InnerEvenIterator iterator = ds.new InnerEvenIterator(); while (iterator.hasNext()) { System.out.println(iterator.getNext() + " "); } } } 5 Local and anonymous classes Sometimes an inner class is created specifically to be used in one particular place. – In these cases, it makes sense to define the class where it is used rather than separately as a part of the outer class. – A local class is an inner class that is defined a method body. – An anonymous class is a class that is defined at the exact point where it is used, i.e., where an instance of the class is created. – Local classes may be used many times within the the method where they are defined, while anonymous classes are only used once. interface IntRunnable { int run(); } public class AnonEx { public static int callRun(IntRunnable runnable) { runnable.run(); } public static void main(String argv[]) { callRun(new IntRunnable() { // Anonymous class public int run() { return 3 * 5; } }); // Close class then call to callRun } } 6 What use are anonymous classes? Anonymous classes turn out to be particularly useful when creating complex objects from a collection of simpler ones, each with their own methods. – It is also often useful for “glue code”. – These styles of programming largely borrow from functional programming, and are generally referred to as “higher order”. – Increasingly this style is influencing object-oriented programming. – C# in particular has been highly influenced. 7.