Unit 16Template Summary prepared by Kirk Scott
Design Patterns in JavaChapter 21Template Method Summary prepared by Kirk Scott
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
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
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
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
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.
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
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
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
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
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
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
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
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
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
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
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.
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
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
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.”
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
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
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
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
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.
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
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
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
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.
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
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
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
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
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
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
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
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
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
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
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
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.
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.
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
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
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
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
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
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