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Unit 17
Template
Summary prepared by Kirk Scott
1
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 Java
Chapter 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 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 “NonNatural 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]);
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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
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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.
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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.
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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
68
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;
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public Cup()
{
seedArrayList = new
ArrayList<Seed>();
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}
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•
public Cup(String ownersNameIn, int
seedCountIn)
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{
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Seed newSeed;
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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;
}
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public String getOwnersName()
{
return ownersName;
}
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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
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import java.awt.Color;
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public class Seed
{
private Color seedColor;
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public Seed()
{
}
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public Seed(Color aColor)
{
seedColor = aColor;
}
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public Color getColor()
{
return seedColor;
}
}
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The CupNameComparator Class
•
import java.util.Comparator;
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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
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import java.util.ArrayList;
import java.util.Collections;
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public class TestCupAndSeed
{
public static void main(String[] args)
{
MyTerminalIO myterminal = new MyTerminalIO();
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ArrayList<Cup> cupArrayList = new
ArrayList<Cup>();
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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);
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myterminal.println(cupArrayList);
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Collections.sort(cupArrayList);
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myterminal.println(cupArrayList);
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CupNameComparator myComparator = new
CupNameComparator();
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Collections.sort(cupArrayList, myComparator);
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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
80
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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
84
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
85
• 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
86
• 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
88
The End
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