Introduction

This tutorial covers:

• Some details of implementation of the Variable class for programming project 1.
• The flyweight pattern to share objects from a "pool" of objects.
• Factories to manufacture objects instead of calling the objects' constructors directly.

Class Variable

Below is an UML diagram for the design of class Variable.

• Class Variable is both a subclass of ABooleanExp and an implementation of ISNForm.  This is the Java specific way of doing multiple-inheritance.  In Java, a class can have only one superclass, but can have as many super interfaces as desired.  Use StructureBuilder to discover the syntax for mutliple inheritance in Java.  As a concrete subclass of ABooleanExp, Variable must have code for the accept method.  As a concrete "implementation" of ISNForm, Variable must have code for the acceptSNFV method.  Write the code for these methods.
  

• Class Variable maintains ("has-a") static reference to a QFList,   _symbolTable, and uses it to hold NameVarPair objects which serve no other purpose than pairing a String name with its corresponding Variable object.  The static method  Variable makeVariable (String name) asks _symbolTable to use LookUp, a singleton visitor to QFList, to find and return a matching Variable object for a given String name.  If the result is null, then a new NamePair (name, new Variable (name)) is inserted into _symbolTable and returned.  This way, the next time when the same name is looked up again, the same Variable (name) object will be returned.  The static method  Variable makeVariable (String name)is called a factory method, and class Variable is said to be a factory that manufactures its own instances.   Since the constructor Variable (String name) is private, no external clients can instantiate a Variable object directly.  The only way to obtain a Variable object with a particular String name is to call Variable.makeVariable(name).   Write the code for Variable makeVariable (String name).
  

• LookUp is an IAlgo singleton.  It knows its QFList host contains NameVarPair objects. It is given as input a String name, and its task is to find and return the matching Variable (name) object in the host QFList.   If there is no match, it returns null (for non-existence).  Write the code for LookUp.
  

• QFList and IAlgo constitute the quasi-functional list framework studied in class.  QFList serves as a "symbol table" where the pairs (name, Variable(name)) are stored.  Re-use the framework.
  

• Class NameVarPair serves no other purpose than pairing a name with its corresponding Variable object..  As such, its fields are public and it has no methods.  Write the code for NameVarPair.

The Flyweight Pattern

The flyweight pattern is a design pattern used to save space and in some cases time.   To save space, objects of certain type are stored in one place called a storage "pool".  These objects are called flyweights.  The flyweight pattern is used when objects need to be shared because there is expensive to have duplicate copies and/or because the construction of these objects are expensive.  For example, in a word processing application, there can be hundreds of thousands of "character" objects.  It is thus necessary to define a Character class with a minimal set of attributes and behaviors and share the Character objects.

A client cannot directly instantiate a flyweight but has to ask a flyweight "factory" to "vend" it one by calling an appropriate factory method.   A factory class is a class that is capable of manufacturing instances of objects of certain type.  As such, it has access to the constructors of the objects it can create.  When a flyweight factory is asked to vend a particular flyweight, it looks it up in the storage pool.  The flyweight object is returned if found, if not, the factory instantiates a new flyweight (it has access to the constructor!), adds it to the pool, and returns it to the calling client.  In general the factory maintains a reference to the storage pool to have quick access to it.  In some cases, the factory serves as the storage pool itself.  The high level specification for flyweights and its factory is "a flyweight factory has (access to) many flyweight objects".   Below is the general UML diagram describing the flyweight pattern.