Chapter 21 Template Method

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Chapter 21Template Method

Summary prepared by Kirk Scott

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One half of a bronze mold for casting a socketed spear head dated to the period 1400-1000 BC. There are no known parallels

for this mold.

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Stone mold of the Bronze Age used to produce spear tips

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Design Patterns in JavaChapter 21

Template Method

Summary prepared by Kirk Scott

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The Introduction Before the Introduction

• In this unit I will only cover the first example given by the book, sorting, taken from the Java API

• The book ends that discussion with an example with rockets, which will be covered

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• Next, the book develops some examples using rockets which apply the template pattern to an operation other than sorting

• Those examples will not be covered• You will only be responsible for the template

design pattern as illustrated by sorting

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• The Template design pattern, at heart, is not very complicated

• The idea can be summarized as follows:• You write the code for the outline of an

algorithm, leaving certain steps of the algorithm as calls to other methods

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• If the overall algorithm is complex, this is a divide and conquer approach to code writing

• Whether the algorithm is complex or not, the pattern has another potential benefit which is actually more important

• The overall algorithm may be designed generally so that it can apply to multiple kinds of objects

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• The changes needed for specific kinds of objects can be implemented in the subparts

• The subparts could also be altered so that they accomplish something different, or in a different way, without changing the overall outline of the algorithm

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Book Definition of Pattern

• Book definition:• The intent of the Template Method is to

implement an algorithm in a method, deferring the definition of some steps of the algorithm so that other classes can redefine them.

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A Classic Example: Sorting

• Sorting provides a good example where the template method can be applied

• There are many different sorting algorithms• These would be templates• Each algorithm depends on the ability to do

pairwise comparisons• How the pairwise comparison is done is the

subpart of the implementation of the algorithm overall

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Multiple Templates

• Here is one perspective on the design pattern• You might have multiple templates• You could implement a template for quick sort,

for example• You could also implement a template for merge

sort• Each template could rely on the same method

that supported the pairwise comparison of the values or objects to be sorted

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Multiple Subparts

• Here is another perspective on the design pattern

• Given one template, you might also be interested in varying implementations of the subpart

• For example, assume for a moment that you are just interested in merge sort

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• You might be interested in applying the sorting template to different kinds of objects

• Or you might be interested in applying it to the same kinds of objects, but sorting them on different attributes at different times

• This means having various different implementations of the subpart, the pairwise comparison operation that the template depends on

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Sorting in the Java API

• The book doesn’t actually explain sorting in the Java API in great detail

• It shows an example and moves on• I will try and explain it in detail• It is this information that you are responsible

for

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• There are two parallel tracks to sorting in Java• The first of the two tracks is based on what

the API refers to as “natural order”• In essence the second track is an extension

that makes it possible to compare objects on some order other than the so-called natural order

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• For each of the two tracks, pairwise comparison of objects will be addressed first

• Then using the comparison in a template method will be addressed

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Natural Order

• The phrase “natural order” sounds god-given• This is not the case• Natural order means the order that is defined by

a class that implements the Comparable interface

• The Comparable interface contains a compareTo() method

• This is known as the natural comparison method

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• The compareTo() method has this signature• compareTo(obj:Object):int• Given an implicit and an explicit parameter

that are objects, it will compare the two• It will return an integer value as the result of

the comparison

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• In a class that implements the Comparable interface, the compareTo() method contains the logic for comparison

• In general, the comparison is based on the value of the most important instance variable of the class

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compareTo()’s Return Values

• If the return value is -1, the implicit parameter is less than the explicit parameter

• If the return value is 0, the implicit parameter is equal to the explicit parameter

• If the return value is 1, the implicit parameter is greater than the explicit parameter

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The Arrays and Collections Classes

• The Java API contains classes named Arrays and Collections

• These classes contain sorting methods that make use of calls to compareTo()

• A little bit of background on the classes will be presented before getting to sorting

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• Do not to confuse the Arrays and Collections classes with the class Array or the interface Collection

• It is a little inconvenient that there are classes Array, Arrays, and Collections, and an interface Collection

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• Recall that the Array class is a concrete class• Instances of that class are arrays• Collection is an interface• This interface is at the root of the a hierarchy

of classes that implement it, like ArrayList, HashMap, etc.

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The Arrays and Collections Classes Contain Static Methods

• So what are the classes Arrays and Collections?• They are classes that contain many static

methods that can be used to work on instances of arrays or collections

• We will be interested in just those methods that sort the contents of an array or collection based on pairwise comparison of the elements

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• This is the first paragraph of the Java API for the Arrays class:

• “This class contains various methods for manipulating arrays (such as sorting and searching).

• This class also contains a static factory that allows arrays to be viewed as lists.”

• (I only see static methods in the class.)

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• This is the first paragraph of the Java API for the Collections class:

• “This class consists exclusively of static methods that operate on or return collections.

• It contains polymorphic algorithms that operate on collections, "wrappers", which return a new collection backed by a specified collection, and a few other odds and ends.”

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• This is a simple analogy:• The Math class contains static methods for

performing mathematical operations on simple types

• The Arrays and Collections classes contain static methods for performing operations on instances of Array or Collection classes

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The sort() Methods in the Arrays Class

• The Arrays class has 18 different sort methods in it

• The methods sort an array that is passed in as a parameter

• Most of these methods differ according to the type of element in the array that is sorted

• For simple types, the sort order is simply based on numeric comparison

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Arrays with Numeric Elements

• There is a version of sort() which takes an array of integers as the explicit parameter

• The pairwise comparison that it’s based on is the numerical < operator

• For what it’s worth, this version of sort() implements a version of quick sort

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Sorting Arrays with Elements that are References

• How will an array be sorted if it contains references to objects?

• Remember that you typically declare the type of the object in the array

• You can count on each of an array’s elements being an instance of the same class

• The sort() method in Arrays will sort the elements in “natural” order

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Natural Order Sorting is Based on Implementing the Comparable Interface

• Here is part of the Java API documentation for the sort() method for objects in the Arrays class:

• “public static void sort(Object[] a)• Sorts the specified array of objects into

ascending order, according to the natural ordering of its elements.

• All elements in the array must implement the Comparable interface.”

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• The sort() method will successfully work on an array if the elements of the array are of a class that implements the Comparable interface

• The results of the sort will depend on the pairwise comparison of the array’s elements using the compareTo() method specified by Comparable and implemented in the class

• For what it’s worth, this version of sort() implements a version of merge sort

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Flexibility of the Template Pattern—Re-using the Comparison Operation

• Notice how sorting in Java illustrates one aspect of the template pattern

• You can have more than one sorting template, one for quick sort and the other for merge sort, for example

• They both can make use of the same pairwise comparison operation

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The sort() Methods in the Collections Class

• Sorting with the Collections class is similar to the Arrays class, but simpler

• Collections does not contain a large number of sort() methods because a collection can only contain object references, not simple types

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• The Collections class has two sort() methods• The first sort() method is analogous to the

sort() method for the Arrays class• It will be discussed next• The second sort() method is based on

something call a Comparator• That will be discussed after finishing with the

first sort() method

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Natural Order Sorting in the Collections Class

• Here is part of the Java API documentation for the first sort() method in the Collections class:

• “public static <T extends Comparable<? super T>> void sort(List<T> list)”

• Sorts the specified list into ascending order, according to the natural ordering of its elements.

• All elements in the list must implement the Comparable interface.”

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Differences between Sorting in Arrays and Collections

• The signature of the first sort() method in the Collections class differs from the signature of the sort() method in the Arrays class

• An Array contains elements of one kind• It is possible for a collection to contain

references to different kinds of objects

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• For a collection, angle bracket notation is used to specify a single object type that a collection contains

• The angle bracket notation shown is more complex than any we’ve seen before

• You don’t have to master this syntax in order to make use of the method

• All it apparently means is that the elements of the collection either have to be comparable themselves or be subclasses of classes that are comparable

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Similarities between Sorting in Arrays and Collections

• Aside from those two differences, the sort() methods in Arrays and Collections discussed so far are similar

• They both rely on the natural order defined on the objects by their implementation of the Comparable interface

• The sort order results from the pairwise application of compareTo() to the elements of the collection

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Kinds of Objects in Collections

• The elements of a collection may literally be instances of different classes

• Things will work if the objects are instances of subclasses of the type declared for the collection and that type implements Comparable

• The collection will contain superclass references to subclass objects

• Polymorphism and dynamic binding mean that comparison will be done on the basis of the comparability of the common superclass

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Flexibility of the Template Pattern—Changing the Comparison Operation

• The discussion of Java sorting so far has been based on natural order as defined by implementing the Comparable interface

• The other aspect of the template pattern is the ability to change the pairwise comparison to sort different kinds of objects or in a different order

• How that is accomplished in Java sorting will be addressed next

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Sorting in a Different Order with a Different Comparison Operation

• Suppose you want to sort objects which are of the same kind, but you want to define a new sort order

• The Comparator interface in the Java API is the basis for this

• There are sort() methods in the Arrays and Collections classes which are defined to work on parameters which implement this interface

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The Comparator Interface for “Non-Natural Order” Sorting

• Let a class implement the Comparator interface• This means that it implements a method named

compare() with this signature• compare(o1:Object, o2:Object):int• Given two explicit parameters that are objects, it

will compare the two• It will return an integer value as the result of the

comparison

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• compare() isn’t declared static in the interface definition, but in a practical sense it is

• It takes two instances of a class as explicit parameters and compares them with each other

• Just like with compareTo(), we don’t call the method ourselves

• A sorting method that uses Comparator will call the compare() method

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compare()’s Return Values

• The meaning of the integer return value of compare() is like that of compareTo(),

• If the return value is -1, the first explicit parameter is less than the second explicit parameter

• If the return value is 0, the first explicit parameter is equal to the second explicit parameter

• If the return value is 1, the first explicit parameter is greater than the second explicit parameter

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Using Comparator/compare() in Arrays and Collections

• The Arrays and Collections classes both contain sort() methods based on Comparator

• The methods take two parameters:• An array or collection of elements to be sorted• An instance of a class that implements the

Comparator interface for the elements of the array or collection

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• Notice that this is one step deeper than plain Comparable

• With Comparable, the element class itself implemented the interface and contained compareTo(), which defined its natural order

• A Comparator is external to the element class• In theory, you could have many different

comparators for the same element class

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The sort() Method with a Comparator Parameter in the Arrays Class

• Here is part of the Java API documentation for the sort() method with a Comparator parameter in the Arrays class:

• public static <T> void sort(T[] a, Comparator<? super T> c)

• Sorts the specified array of objects according to the order induced by the specified comparator.

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The sort() Method with a Comparator Parameter in the Collections Class

• Here is part of the Java API documentation for the sort() method with a Comparator parameter in the Collections class:

• public static <T> void sort(List<T> list, Comparator<? super T> c)

• Sorts the specified list according to the order induced by the specified comparator.

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• Recall that in Collections there are only two sort() methods

• The first was the one on natural order• The second is this one

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• This one uses some advanced syntactical notation

• Luckily, we don’t have to understand the notation in detail in order to use the method

• Pass in a parameter of type List containing elements of type T

• Pass in another parameter of type Comparator on elements of type T

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• The List will be sorted by the Comparator• You can have as many different comparators

as you would like

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Review of the Overall Idea

• Stated in very general terms, the idea is this:• What sorting is doesn’t change• What sort order you get depends on the

objects you’re sorting and their implementation of Comparable and Comparator

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• With careful design it is possible to write flexible and general code for sorting

• Different sorting algorithms are possible• Different pairwise comparisons are possible

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• Flexibility and generality are what the template pattern is about

• The pattern doesn’t just apply to sorting• It can bring the same benefits if applied in

other domains

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UML for the Pattern

• The book provides the UML diagram shown on the following overhead to summarize the Comparable and Comparator interfaces along with the Collections class

• The diagram only shows the sort() method that depends on Comparator

• The diagram would be more complete if it included the sort() method that depends on Comparable

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The Book’s Concrete Example of Sorting

• The book illustrates sorting based on objects in the fireworks set of classes

• The code constructs an array of rockets• It then uses the sort() method of the Arrays

class and a comparator named ApogeeComparator to sort the rockets

• It prints out the sorted array

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• Then it sorts the array again using a NameComparator object as the comparator parameter to the sort method

• It prints out the re-sorted array• The code is shown on the following overheads

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• public class ShowComparator • {• public static void main(String args[]) • {• Rocket r1 = new • Rocket("Sock-it", 0.8, new Dollars(11.95), 320, 25);• Rocket r2 = new • Rocket("Sprocket", 1.5, new Dollars(22.95), 270, 40);• Rocket r3 = new • Rocket("Mach-it", 1.1, new Dollars(22.95), 1000, 70);• Rocket r4 = new • Rocket("Pocket", 0.3, new Dollars(4.95), 150, 20);• • Rocket[] rockets = new Rocket[] { r1, r2, r3, r4 };

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• System.out.println("Sorted by apogee: ");• Arrays.sort(rockets, new ApogeeComparator());• for (int i = 0; i < rockets.length; i++) • System.out.println(rockets[i]);

• System.out.println();• System.out.println("Sorted by name: ");• Arrays.sort(rockets, new NameComparator());• for (int i = 0; i < rockets.length; i++) • System.out.println(rockets[i]); • }• }

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• The book gives the output shown on the following overhead for the program

• It’s clear that the toString() method for the Rocket class isn’t very complete

• It would be helpful if it showed the apogee values so that that sort could be verified

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• Sorted by apogee:• Pocket• Sprocket• Sock-it• Mach-it

• Sorted by name:• Mach-it• Pocket• Sock-it• sprocket

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The Comparator Classes for the Example

• Clearly, how the code sorts is based on the template pattern

• In other words, everything depends on how the ApogeeComparator and NameComparator are implemented

• The book does the code as a challenge• As usual, the solutions are simply shown

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Solution 21.1• Notice how the implementation relies on the compare()

method for the Double class.

• public class ApogeeComparator implements Comparator • {• public int compare(Object o1, Object o2) • {• Rocket r1 = (Rocket) o1;• Rocket r2 = (Rocket) o2;• return Double.compare(r1.getApogee(), r2.getApogee());• }• }

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Solution 21.1• Notice how the implementation relies on the compareTo()

method for the String class.

• public class NameComparator implements Comparator • {• public int compare(Object o1, Object o2) • {• Rocket r1 = (Rocket) o1;• Rocket r2 = (Rocket) o2;• return r1.toString().compareTo(r2.toString());• }• }

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How the Example Illustrates the Pattern

• The book reiterates the idea behind the Template pattern with this observation:

• Sorting, per se, doesn’t have anything to do with rocket apogees

• Rocket apogees are a problem domain specific concept• It is extremely convenient to be able to implement

sorting with a general algorithm that would apply to many things

• Then for each different type of thing, all you have to do is provide the comparison step

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Another Example

• This is another example based on a Cup class and a Seed class

• The template design pattern can be used to sort an ArrayList of Cups

• The Cup class has two attributes, an owner's name and an ArrayList of seeds

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• An ArrayList of cups can be sorted on either of these attributes

• Sorting on seed count is the natural order and is implemented with compareTo() in the Cup class

• Sorting by name is accomplished with a comparator which implements compare()

• The sort() methods in the API can then be used on the ArrayList of cups

• Code is given on the following overheads

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The Cup Class• import java.util.ArrayList;• import java.awt.Color;

• public class Cup implements Comparable• {• private String ownersName;• private ArrayList<Seed> seedArrayList;

• public Cup()• {• seedArrayList = new ArrayList<Seed>();• }

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• public Cup(String ownersNameIn, int seedCountIn)

• {• Seed newSeed;

• ownersName = ownersNameIn;• seedArrayList = new ArrayList<Seed>();• for(int i = 0; i < seedCountIn; i++)• {• newSeed = new Seed(Color.BLUE);• seedArrayList.add(newSeed);• }• }

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• public void setOwnersName(String ownersNameIn)• {• ownersName = ownersName;• }

• public String getOwnersName()• {• return ownersName;• }

• public int getSeedCount()• {• return seedArrayList.size();• }

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• public void addSeed(Seed aSeed)• {• seedArrayList.add(aSeed);• }• public Seed removeSeed()• {• return (Seed) seedArrayList.remove(seedArrayList.size() -

1);• }

• public String toString()• {• return ("Cup[ownersName=" + ownersName• + ", seedCount=" + seedArrayList.size() + "]");• }

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• public int compareTo(Object objIn) throws ClassCastException• {• if(objIn instanceof Cup)• {• Cup that = (Cup) objIn;• if(this.seedArrayList.size() < that.seedArrayList.size())• {• return -1;• }• else if(this.seedArrayList.size() > that.seedArrayList.size())• {• return 1;• }• else• {• return 0;• }• }• else• {• throw new ClassCastException();• }• }• }

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The Seed Class• import java.awt.Color;

• public class Seed• {• private Color seedColor;

• public Seed()• {• }

• public Seed(Color aColor)• {• seedColor = aColor;• }

• public Color getColor()• {• return seedColor;• }• }

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The CupNameComparator Class• import java.util.Comparator;

• public class CupNameComparator implements Comparator• {• public int compare(Object obj1In, Object obj2In) throws

ClassCastException• {• if(!((obj1In instanceof Cup) && (obj2In instanceof Cup)))• {• throw new ClassCastException();• }• else• {• Cup cupOne = (Cup) obj1In;• Cup cupTwo = (Cup) obj2In;• return (cupOne.getOwnersName().compareTo(cupTwo.getOwnersName()));• }• }• }

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The Test Program• import java.util.ArrayList;• import java.util.Collections;

• public class TestCupAndSeed• {• public static void main(String[] args)• {• MyTerminalIO myterminal = new MyTerminalIO();

• ArrayList<Cup> cupArrayList = new ArrayList<Cup>();

• Cup cup1 = new Cup("Bob", 2);• Cup cup2 = new Cup("Sue", 4);• Cup cup3 = new Cup("Al", 3);

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• cupArrayList.add(cup1);• cupArrayList.add(cup2);• cupArrayList.add(cup3);

• myterminal.println(cupArrayList);

• Collections.sort(cupArrayList);

• myterminal.println(cupArrayList);

• CupNameComparator myComparator = new CupNameComparator();

• Collections.sort(cupArrayList, myComparator);

• myterminal.println(cupArrayList);• }• }

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Lasater’s UML for the Pattern

• Lasater’s UML for the pattern is given on the following overhead

• Keep in mind that this is general, not specific to sorting

• The idea is that an operation may implement a general algorithm

• What the algorithm accomplishes depends on the specific operations implemented in the concrete subclass

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UML for the Pattern

• On the following overhead I give a UML diagram where I attempt to summarize my understanding of the pattern.

• The Template class contains a method with a general implementation of an algorithm

• That implementation relies on calls to the specific “other” methods defined in the other classes

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Conclusion

• As pointed out at the beginning, the overhead presentation ends with the example taken from sorting

• The rest of the chapter will not be covered and you will not be responsible for it

• A subset of the book’s summary is given on the following overheads

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Summary

• The intent of the Template Method design pattern is to define an algorithm in a method

• Some of the steps are left abstract, stubbed out, or defined in an interface

• Other classes then fill in the functionality for those missing steps

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• The Template Method design pattern can be useful in a large scale development environment

• One developer is responsible for setting up the outline of an algorithm (like sorting for example)

• Then other developers only need to fill in the missing steps in order to use the general outline

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• The book also notes that the development of the Template Method pattern may go in the opposite direction

• You may find that you are repeating similar code in different places, where the only difference seems to be in certain small steps

• It would then be possible to refactor and generalize, taking the commonality out into a template

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• Elsewhere in the chapter the authors observe that there is some similarity between a template and an adapter

• In the end, all of the patterns start looking alike• The idea here is that with a few well-designed

interfaces it is possible for one piece of code (sorting for example) to make use of another (comparable objects) in order to achieve its overall goal

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The End