Overloading, Overriding, Runtime Types and Object Orientation - Inner Classes

• Inner classes are non-static classes defined within other classes (JLS§8.1.2)

    class Outer {
        class Inner {}      // class definition within the
                            // the body of class Outer
    }

• the compiled class files for the above are: Outer.class and Outer$Inner.class
• the Inner class type is: Outer.Inner
• instances of inner classes can be created in a number of ways

Create an Outer class object:
    Outer o1 = new Outer();

Then create an Inner class object:
    Outer.Inner i1 = o1.new Inner();    

Or, create the inner class directly:
    Outer.Inner i2 = new Outer().new Inner();
    
Or, create one from within the outer class constructor
    class Outer {
        Outer() {
            new Inner();
        }
    }    

• inner classes may have no declared access modifier, defaulting the class access to package
• or, inner classes may be declared public, protected, private, abstract, static or final

class Outer {
    public class PublicInner{}
    protected class ProtectedInner {}
    private class PrivateInner{}
    abstract class AbstractInner {}
    final class FinalInner {}
    static class StaticInner {}
}    

• each instance of a non-static inner class is associated with an instance of their outer class
• static inner classes are a special case. See Static Inner Classes

• inner classes may not declare static initializers or static members unless they are compile time constants ie static final var = value; (JLS§8.1.2)
• you cannot declare an interface as a member of an inner class; interfaces are never inner (JLS§8.1.2)
• inner classes may inherit static members (JLS§8.1.2)
• the inner class can access the variables and methods declared in the outer class
• to refer to a field or method in the outer class instance from within the inner class, use Outer.this.fldname

Example Code

• TestInner.java