unit 16 template

1

Unit 16Template

Summary prepared by Kirk Scott

2

Design Patterns in JavaChapter 21

Template Method

Summary prepared by Kirk Scott

3

The Introduction Before the Introduction

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

• I will not cover the additional material where the authors try to come up with examples out of whole cloth based on the fireworks scenario

• As a student, you are only responsible for the first example

4

• 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

5

• 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

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

6

• 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 chaning the overall outline of the algorithm

7

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.

8

A Classic Example: Sorting

• The book observes that there are many different sorting algorithms

• However, fundamentally, each algorithm ultimately depends on the ability to do pairwise comparisons between two values or objects

9

• From the perspective of the Template design pattern, this leads to two conclusions

• 1. You could, for example, implement a template for quick sort

• You could also implement a template for merge sort

• Each template could make use of a call to a method that supported the pairwise comparison of objects to be sorted

10

• 2. You could also just be concerned with an implementation of merge sort

• However, you might be interested in applying it to different kinds of objects

• Or you might be interested in applying it to the same kinds of objects, but comparing those objects based on different characteristics at different times

11

• The book now tackles the question of how sorting is done in at least some of the classes in the Java API

• I will cover the same material• My presentation order and the details I

choose to bring out may differ from the book’s presentation

12

• The first thing to notice is that 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 sort order”

• In essence the second track is an extension that makes it possible to easily compare objects on some order other than the so-called natural order

13

• In discussing both tracks, I will be considering the business of doing a pairwise comparison between objects first

• Then I will consider the question of a template method that makes use of the comparison

• I will take up the question of natural order first• Just to try to make things clear from the outset,

“natural order” is the order that will be defined by an implementation of the Comparable interface

14

• There is an interface named Comparable in the Java API

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

compareTo() with this signature and return type:• compareTo(obj:Object):int• In other words, given one instance of a class, it’s

possible to compare it to another instance of that class

15

• The compareTo() method contains the logic for comparison

• In straightforward cases, you would expect the comparison to be based on the value of (the most important) one of the instance variables of the object

16

compareTo()’s Return Values

• If the return value is -1, the implicit parameter is less than the explicit parameter according to the natural order

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

• If the return value is 1, the implicit parameter is greater than the explicit parameter according to the natural order

17

The Arrays and Collections Classes

• Now, on to the question of a template method• Not only does the Java API contain the

Comparable interface, it also contains classes named Arrays and Collections

• The first thing to be careful about is not to confuse these classes with the class Array or the interface Collection

18

• It is moderately inconvenient that the API contains a mixture of classes and interfaces where the names differ only in that one may be singular and another may be plural

• For background review, I note that the Array class is a concrete class where instances of the class are plain old arrays

• The Collection interface is at the root of the collection hierarchy, which contains all of the implementing classes like ArrayList, HashMap, etc.

19

The Arrays and Collections Classes Contain Static Methods

• So what are the classes Arrays and Collections?

• They are classes that contain methods that can be used to work on instances of arrays or collections

20

• 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 scanned the methods listed for the class and found them all to be static

21

• 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.”

22

• This is a simplistic analogy, but it might be helpful

• The Math class contains static methods for performing mathematical operations

• The Arrays and Collections classes contain static methods for performing operations on instances of these classes, both of which are characterized by having multiple elements

23

The sort() Methods in the Arrays Class

• Take the Arrays class for example• It has 18 different sort methods in it• Most of these methods differ according to the

type of element in the array that is passed in as the explicit parameter to be sorted

• If the elements of the array are a simple type, the sort order is clear

24

• For example, if an array contains integers, the call to the sort method with that kind of explicit parameter will sort in numerical order based on the numerical comparison operator <

• For what it’s worth, if you look further into the documentation, you will find that the sorting algorithm that is implemented is a version of quick sort

25

Sorting Arrays

• The question is, how will an array be sorted if it contains references to objects?

• To make this a reasonable question, remember that for a garden variety array, you have to declare the type of the object in the array

• In other words, each of an array’s elements has to be an instance of the same class

26

Natural Order Sorting is Based on Implementing the Comparable Interface

• Here is part of the Java API documentation for that sort() method:

• 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.

27

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

• If that’s the case, then the sorting of the array will correspond to the results obtained by a pairwise comparison of its elements using the compareTo() method of that class

28

• For what it’s worth, if you look further into the documentation, you will find that the sorting algorithm that is implemented is a version of merge sort

29

Sorting Object References in Collections

• The situation with the Collections class is similar, but simpler

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

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

sort() method for the Arrays class

30

Again, Natural Order Sorting is Based on Implementing the Comparable Interface

• Here is part of the Java API documentation for that sort() method:

• 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.

31

• The signature of this sort() method differs from the signature of the sort() method in the Arrays class in two ways:

• 1. In general a collection can contain references to different kinds of objects

• Angle bracket notation can be used to specify a single object type that a collection contains

32

• 2. In the Arrays class, the sort() method was void and it sorted the explicit parameter directly

• In the Collections class, the sort() method returns a sorted version of the explicit parameter

33

• Aside from those two differences, the sort() methods in Arrays and Collections 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

34

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

• Recall this observation made earlier:• 1. You could, for example, implement a

template for quick sort• You could also implement a template for

merge sort• Each template could make use of a call to a

method that supported the pairwise comparison of objects to be sorted

35

• If you factor out the individual comparison operation, different sorting algorithms can rely on that

• In the Java supplied sorting methods, simple types are sorted using quick sort and objects are sorted using merge sort

• But all sorting relies on the ability to compare pairs of elements

36

Flexibility of the Template Pattern—Changing the Comparison Operation

• Now recall this observation, made earlier:• 2. You could also just be concerned with an

implementation of merge sort• However, you might be interested in applying it

to different kinds of objects• Or you might be interested in applying it to the

same kinds of objects, but comparing those objects based on different characteristics at different times

37

Sorting in a Different Order with a Different Comparison Operation

• Specifically, consider applying the same sorting algorithm to the same kinds of objects, but wanting to sort them on a different characteristic

• This is the motivation behind a different interface in the Java API and a different kind of sort() method that appears in the Arrays and Collections classes

38

The Comparator Interface for “Non-natural Order” Sorting

• There is an interface named Comparator in the Java API

• Let a class implement the Comparator interface

• This means that it implements a method named compare() with this signature and return type:

• compare(o1:Object, o2:Object):int

39

• 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 parameter 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

40

Return Values of compare()

• The meaning of the integer return value is the same as for 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

41

• The results of compare() are not considered the natural order

• The Arrays and Collections classes both contain a sort() method that takes as parameters both an array or collection to be sorted, as well as an instance of a class that implements the Comparator interface for the elements of the array or collection

42

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

• Here is part of the Java API documentation for that sort() method 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.

43

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

• Here is part of the Java API documentation for that sort() method 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.

44

• Notice that the definitions use the angle bracket notation like the definition for the other sort() method in Collections

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

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

45

• As the documentation states, using the Comparator interface, you can sort objects on any of their characteristics, not just their natural order, defined using the Comparable interface

46

Review of the Overall Idea

• Finally, this is the grand conclusion in the context of the Template design pattern

• For arrays and collections containing objects, there is one underlying sorting algorithm, merge sort

• However, the sort order that is obtained can be changed by changing the pairwise comparison that is implemented using the Comparator interface

47

• 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• With careful design and reliance on

polymorphism, it is possible to write flexible and general code

• That’s what the template pattern is about, and it doesn’t just apply to sorting

48

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 would be more complete if it included the sort() method on the natural order along with the sort() method with the comparator parameter

49

50

The Book’s Concrete Example

• 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

51

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

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

52

• 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 };

53

• 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]); • }• }

54

• 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

55

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

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

56

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 gives the partial code shown on the following overhead for these classes

57

• public class ApogeeComparator implements Comparator• {• // Challenge!• }

• public class NameComparator implements Comparator• {• // Challenge!• }

58

• Challenge 21.1• Fill in the missing code in the

ApogeeComparator and NameComparator classes so that the program will sort a collection of rockets correctly.

59

• Solution 21.1• Your completed program should look

something like:• [See the following overheads]

60

• 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());• }• }

61

• 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());• }• }

62

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

63

Another Example

• No other example will be presented in class• This overhead is just a reminder that an

application of sorting the Cup and Seed classes is given in the Web directory of this unit

64

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

65

66

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

67

68

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 overhead

• The rest of the diagrams for the chapter have been tipped in after that for future reference, but there is no need to look at them

69

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

70

• The Template Method design pattern can 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

71

• 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

72

• 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

73

74

75

76

The End