Nested Classes

Nested Classes
An Introduction with Examples in Java

Computer Science Department

bernstdh@jmu.edu

Motivation

One Common Situation:
- One or more classes is tightly coupled to one other class and only used by that class (e.g., a Course class that has associated Comparator classes; a ClosedInterval class that has associated Iterator classes)
A Simple Improvement:
- Allow classes to be nested (i.e., for one class to be defined inside of another)

Motivation

Another Common Situation:
- A (often abstract) base class has derived classes that vary in similar and limited ways (e.g., an abstract Rectangle2D class that has two concrete derived classes, one that uses double values and one that uses float values)
A Simple Improvement:
- Though historically this situation was dealt with using nested classes it is usually better to use a parameterized class

Nested Classes in UML

Containment

Two Different Approaches

Static Nested Class:
- Is associated with the class (not an instance) so can't directly refer to instance attributes/methods in its enclosing class (can only do so through an object reference)
Inner Class:
- Is associated with an instance of its enclosing class (not the class itself) so has direct access to that object's attributes/methods

Static Nested Classes

Interaction With Instances of the Outer Class:
- Identical to "top-level" classes
Primary Advantage Over "Top-Level" Classes:
- Organizational convenience
Construction:
- Must use the syntax OuterClassName.StaticNestedClassName

Static Nested Classes (cont.)

An Example from Another Lecture:
- A Course class with "top-level" CreditComparator and IDComparator classes
Improving the Design:
- Since the two Comparator classes will only ever be used with the Course class (i.e., they can't stand alone), it makes sense to make them static nested classes

Static Nested Classes (cont.)

The Improved Implementation

javaexamples/nested/Course.java

Static Nested Classes (cont.)

Using the Static Nested Classes

javaexamples/nested/CourseDriver.java

Inner Classes

Context:
- Instances of an inner class exist within an instance of the outer class
Construction:
- An instance of the outer class must be created before the instance of the inner class can be
An Implication/Restriction:
- Inner classes can't declare any static members

Inner Classes (cont.)

Interaction With Instances of the Outer Class:
- Has access to all of the members of the containing/enclosing object
Primary Advantages Over "Top-Level" Classes:
- Organizational convenience
- Access to private members
Construction:
- The constructor is referred to using the syntax object.InnerClassName()
- A factory method is often used

Inner Classes (cont.)

An Example:
- A ClosedInterval class and associated IncreasingIterator class
Why Not A Static Nested Class?
- An IncreasingIterator object is associated with a particular ClosedInterval object

Inner Classes (cont.)

An Implementation without an Inner Class

javaexamples/iterator/ClosedInterval.java

Inner Classes (cont.)

An Implementation without an Inner Class (cont.)

javaexamples/iterator/IncreasingIterator.java

Inner Classes (cont.)

An Implementation with an Inner Class

javaexamples/nested/ClosedInterval.java

Inner Classes (cont.)

Using the Inner Class

javaexamples/nested/IntervalDriver.java

Beyond the Scope of this Lecture

Local Classes:
- Classes defined in a block (usually a method) and, hence, not "widely available"
Anonymous Classes:
- Enable the declaration and instantation of a class at the same time
- Useful in situations where a local class could be used, but only once

There's Always More to Learn