unit 21 factory method

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Unit 21Factory Method

Summary prepared by Kirk Scott

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Design Patterns in JavaChapter 16

Factory Method

Summary prepared by Kirk Scott

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• Ordinary construction relies on the existence of constructors in a base class

• The Factory Method design pattern also relies on these ordinary constructors

• The idea is that client code may not know or care specifically which kind of object it is working with

• What it cares about is that the object implements a desired interface

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• Suppose the client works with several different base classes

• Each of these base classes, in turn, may rely on/create instances of different underlying classes, which the client needs an instance of

• Each of the base classes will implement a factory method which returns an object of a given interface

• The underlying classes all implement that interface• The actual type of the underlying object returned to the

client will depend on the logic of the particular base class that implements the factory method

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• Book definition:• The intent of the Factory Method is to let a

class developer define the interface for creating an object while retaining control of which class to instantiate.

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A Classic Example: Iterators

• Iterators provide a way to access the elements of a collection sequentially

• There is a Collection interface• Every class that implements the Collection

interface has to implement the method iterator()

• The iterator() method returns an instance of an iterator

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• In this example, the Collection interface is not “the” interface of the Factory Method design pattern

• The Collection interface is simply the mechanism that makes sure that every implementing class has an iterator() method

• The iterator() method is the factory method

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• The point is this:• You can call iterator() on any collection class• The iterator() method returns something that

is typed Iterator• However, Iterator is not a class• Iterator is an interface

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• The class of the object returned by a call to the iterator() method differs according to which kind of object it was called on

• The iterator() method could be called on an ArrayList and the type of the actual object returned would be an iterator for an ArrayList

• The iterator() method could be called on a HashMap and the type of the actual object returned would be an interator for a HashMap

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• The client code doesn’t care what the actual type of object is

• Formally, it only cares that it be typed Iterator• Functionally, this means that the client code

can call on any iterator any of the methods defined in the Iterator interface

• The client code does not need to be able to call any class specific methods, if they even exist

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• The code on the following overhead illustrates the creation and use of an iterator for a collection of type List

• The List itself is constructed by hard coding a simple array of strings and passing it in to the List object

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• import java.util.*;• public class ShowIterator • {• public static void main(String[] args) • {• List list = Arrays.asList(• new String[] { "fountain", "rocket", "sparkler" });• • Iterator iter = list.iterator();• • while (iter.hasNext()) • System.out.println(iter.next());• • // Uncomment the next line • // to see the iterator's actual class:• // System.out.println(iter.getClass().getName());• }• }

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• Challenge 16.1• What is the actual class of the Iterator object

in this code?

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• Solution 16.1• A good answer, perhaps, is that you do not need to

know what class of object an iterator() method returns.

• What is important is that you know the interface that the iterator supports, which lets you walk through the elements of a collection.

• However, if you must know the class, you can print out its name with a line like:

• System.out.println(iter.getClass().getName());

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• This statement prints out:• java.util.AbstractList$Itr• The class Itr is an inner class of AbstractList.

You should probably never see this class in your work with Java.

• [End of Solution 16.1.]

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Recognizing Factory Method

• There are many methods floating around object-oriented code which return references to newly created objects of one class or another

• Just because a method returns such a reference doesn’t mean that it implements the Factory Method design pattern.

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• Challenge 16.2• Name two commonly used methods in the

Java class libraries that return a new object.• [And don’t implement the Factory Method

design pattern.]

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• Solution 16.2• There are many possible answers, but

toString() is probably the most commonly used method that creates a new object.

• For example, the following code creates a new String object:

• String s = new Date().toString();

• The creation of strings often happens behind the scenes.

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• Consider:• System.out.println(new Date());• This code creates a String object from the Date

object, ultimately by calling the toString() method of the Date object.

• Another frequently used method that creates a new object is clone(), a method that usually returns a copy of the receiving object.

• [End of Solution 16.2.]

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• The point of the answers to the previous challenge is this:

• toString() and clone() don’t exhibit the Factory Method design pattern

• They don’t protect the client (the calling code) from knowing what kind of object is being constructed and returned

• There is no set of classes under the covers that implement a common interface which is the type returned by the calls to the methods

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• Challenge 16.3• The class javax.swing.BorderFactory sounds

like it ought to be an example of the Factory Method pattern.

• Explain how the intent of the Factory Method pattern is different from the intent of the BorderFactory class.

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• Comment mode on:• This challenge basically boils down to a red

herring• It’s like asking whether the so-called Adapter

classes in Java implement the Adapter design pattern

• The answer is no, it’s just the use of the same word to mean a different thing

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• Solution 16.3• The intent of the Factory Method pattern is to

let an object provider determine which class to instantiate when creating an object.

• By comparison, clients of BorderFactory know exactly what object types they’re getting.

• The pattern at play in BorderFactory is Flyweight, in that BorderFactory uses sharing to efficiently support large numbers of borders.

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• The BorderFactory class isolates clients from managing the reuse of objects, whereas Factory Method isolates clients from knowing which class to instantiate.

• [End of Solution 16.3.]

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Taking Control of Which Class to Instantiate

• The book now moves on from examples like iterator(), which already exist in Java

• It paints a scenario where client code needs to obtain a credit limit for a customer

• If a credit agency is online, then the credit limit is generated with an instance of a class named CreditCheckOnline

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• If the credit agency is offline, then the credit check is generated with an instance of a class named CreditCheckOffline

• The method that the client code is going to call is creditLimit()

• The client code doesn’t care exactly what kind of object is returned

• The UML diagram on the following overhead illustrates the situation so far

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• The key point of this example is that we’re one step lower down in the implementation details than we were in the iterator example

• In the iterator example, the iterator() method was simply a given for each class that implemented the Collection interface

• If you called iterator() on an object, it returned an instance of the right kind of iterator

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• In this example, the CreditCheckOnline and the CreditCheckOffline classes are roughly analogous to the different types of iterator

• When this example is fully worked out, there will be another class which contains the factory method

• When the factory method is called on an instance of that class, it will determine whether to construct an instance of CreditCheckOnline or CreditCheckOffline

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• The book outlines these elements of an application of the Factory Method design pattern to this situation:

• 1. Make a CreditCheck interface that includes a creditLimit() method

• 2. Have the classes CreditCheckOnline and CreditCheckOffline implement this interface

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• 3. Make a CreditCheckFactory class with a createCreditCheck() method that returns an object of type CreditCheck

• When you first look at these classes, it may be tempting to think that the creditLimit() method in the two base classes is analogous to the iterator() method in the foregoing example

• In other words, you might think that the creditLimit() method is the factory method

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• However, this is not the case• The creditLimit() method returns a reference,

to an instance of Dollars• This is practically the same as returning a

simple type• More importantly, both implementations of

creditLimit() return the same type of thing

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• There is no choice there of what to create• The creditLimit() method is analogous to the

hasNext() method in the Iterator interface• hasNext() can be called on any object that

implements the Iterator interface

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• In the iterator example the overarching question is, what kind of iterator is it, an ArrayList iterator, a HashMap iterator?

• In this example the question is, what kind of credit limit is it, an online or offline one?

• It is the method createCreditCheck() that returns a reference to one kind of object or another

• It is this method that is the factory method

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• Challenge 16.4• Draw a class diagram that establishes a way

for this new scheme to create a credit-checking object while retaining control of which class to instantiate.

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• Solution 16.4• Figure B.18 shows that the two credit check

classes implement the CreditCheck interface.• The factory class provides a method that

returns a CreditCheck object.• The client that calls createCreditCheck() does

not know the precise class of the object it receives.

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• Solution 16.4, continued• The createCreditCheck() method is a static

method, so clients need not instantiate the CreditCheckFactory class in order to get a CreditCheck object.

• You can make this class abstract or give it a private constructor if you want to actively prevent other developers from instantiating it.

• [End of Solution 16.4.]

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• Once again, this is the basic idea of the example:• Client code doesn’t know whether credit checking

is available online or offline• It simply wants a credit check generated• It does this by calling createCreditCheck()• The logic of the code for that method determines

which kind of actual credit check object is returned• However, whatever is returned, it will implement

the CreditCheck interface

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• Challenge 16.5• Assume that the CreditCheckFactory class has

an isAgencyUp() method that tells whether the credit agency is available, and write the code for createCreditCheck().

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• Solution 16.5• If you take a leap of faith that the static method

isAgencyUp() accurately reflects reality, the code for createCreditCheck() is simple:

• public static CreditCheck createCreditCheck()• {• if(isAgencyUp())• return new CreditCheckOnline();• return new CreditCheckOffline();• }

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Factory Method in Parallel Hierarchies

• Given a hierarchy of classes, you may decide to move a subset of behavior out of the classes and implement it in separate classes

• The result is a parallel hierarchy of classes• The Factory Method design pattern can arise

in such a situation

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• The book illustrates this with machines and machine managers in a fireworks factory

• The example is not exactly the same, but you may notice similarities with the material presented in the chapter on the Bridge pattern

• The UML diagram on the next overhead gives the starting point for the example

• There are various concrete types of machine that extend the abstract Machine class

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• The scenario is that you would like to have a getAvailable() method for machines for planning purposes

• However, the logic for implementing getAvailable() is relatively complex

• A parallel hierarchy arises when you decide to factor out the management functionality

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• More background information includes the following:

• getAvailable() is supposed to forecast when a machine will finish its current work and become available

• Most machine types will have different logic, requiring different kinds of planner (classes)

• Mixers and fusers are simple and quick enough that the are essentially always available and they can share the same kind of planner (class)

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• Challenge 16.6• Fill in the diagram of a

Machine/MachinePlanner parallel hierarchy in Figure 16.3.

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• Solution 16.6• Figure B.19 shows a reasonable diagram for

the Machine/MachinePlanner parallel hierarchy.

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• Solution 16.6 continued• This diagram indicates that subclasses of MachinePlanner

must implement the getAvailable() method.• The diagram also indicates that classes in the

MachinePlanner hierarchy accept a Machine object in their constructors.

• This allows the planner to interrogate the object it is planning for, regarding such criteria as the machine’s location and the amount of material it is currently processing.

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• Notice that the discussion so far hasn’t quite made it clear how the Factory Method design pattern fits into the picture

• The book now addresses that question by means of a challenge

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• Challenge 16.7• Write a createPlanner() method for the

Machine class to return a BasicPlanner object, and write a createPlanner() method for the StarPress class

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• Comment mode on:• Don’t be confused by the reference arrows in

the UML diagram• When an object of a machine class creates a

planner, it passes the planner a reference to itself

• However, construction of planners happens on the machine side of the hierarchy

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• Each kind of machine knows what kind of planner to create for itself

• The Factory Method pattern results from the fact that the actual machines extend the abstract Machine class

• Each concrete machine class implements the createPlanner() method

• This is the factory method

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• Client code makes calls of the form someMachine.createPlanner()

• All the client code cares about is that the return value is of the MachinePlanner interface type

• The client is not concerned with which actual type of planner object is returned

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• Solution 16.7• A createPlanner() method for the Machine

class might look like:• public MachinePlanner createPlanner()• {• return new BasicPlanner(this);• }

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• The Fuser and Mixer classes can rely on inheriting this method, whereas the ShellAssembler and StarPress class will need to override it.

• For the StarPress class, the createPlanner() method might be:

• public MachinePlanner createPlanner()• {• return new StarPressPlanner(this);• }

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• These methods show the Factory Method pattern at work.• When we need a planner object, we call the

createPlanner() method of the machine we want a planner for.

• The specific planner that we receive depends on the machine.

• It’s analogous to the iterator example• We ask for an iterator on a collection class object and

don’t care exactly what kind of iterator it is as long as it implements the Iterator interface

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Summary

• The Factory Method design pattern allows client code not to know exactly what kind of object is going to be created

• On the factory side, each different kind of object has to implement the same interface

• Then the client is only concerned that the returned object be of the type of the interface

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• The book illustrated the idea by noting how the iterator() method works in the Java API

• It illustrated a case where the client code would not know which kind of object to construct with the credit example

• It also brought out that the design pattern can be useful with parallel hierarchies where the client only needs to know the one hierarchy and the interface for the other

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• Comment mode on for the last time in this chapter:

• It seems that in the given parallel hierarchies example, it was the slight mismatch between the hierarchies that motivated the use of the pattern

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• If the hierarchies matched exactly, the client, in theory at least, would know:

• If I want to construct a planner for and instance of MachineX

• Then I can expect back from construction, whether ordinary or through a factory method, an instance of PlannerX

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• However, the same observation is true of the iterator example

• If every different collection class has its own iterator type

• Then in theory, client code would know what kind of iterator it was getting

• But the point is that the client code shouldn’t have to worry about this

• And it doesn’t have to when the Factory Method design pattern is applied

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The End