Java Software Solutions - Chapter 10: Polymorphism

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Chapter 10Polymorphism Java Software Solutions Foundations of Program Design Seventh Edition John Lewis William Loftus Polymorphism Polymorphism is an object-oriented concept that allows us to create versatile software designs Chapter 10 focuses on: defining polymorphism and its benefits using inheritance to create polymorphic references using interfaces to create polymorphic references using polymorphism to implement sorting and searching algorithms 9-2 Outline 9-3 Polymorphic References Polymorphism via Inheritance Polymorphism via Interfaces Sorting Searching Binding Consider the following method invocation: obj.doIt(); At some point, this invocation is bound to the definition of the method that it invokes If this binding occurred at compile time, then that line of code would call the same method every time However, Java defers method binding until run time -- this is called dynamic binding or late binding Late binding provides flexibility in program design 9-4 Polymorphism The term polymorphism literally means "having many forms" A polymorphic reference is a variable that can refer to different types of objects at different points in time The method invoked through a polymorphic reference can change from one invocation to the next All object references in Java are potentially polymorphic 9-5 Polymorphism Suppose we create the following reference variable: 	Occupation job; Java allows this reference to point to an Occupation object, or to any object of any compatible type This compatibility can be established using inheritance or using interfaces Careful use of polymorphic references can lead to elegant, robust software designs 9-6 Outline 9-7 Polymorphic References Polymorphism via Inheritance Polymorphism via Interfaces Sorting Searching References and Inheritance An object reference can refer to an object of its class, or to an object of any class related to it by inheritance For example, if the Holiday class is used to derive a class called Christmas, then a Holiday reference could be used to point to a Christmas object 9-8 Holiday day; day = new Christmas(); Holiday Christmas References and Inheritance Assigning a child object to a parent reference is considered to be a widening conversion, and can be performed by simple assignment Assigning an parent object to a child reference can be done also, but it is considered a narrowing conversion and must be done with a cast The widening conversion is the most useful 9-9 Polymorphism via Inheritance It is the type of the object being referenced, not the reference type, that determines which method is invoked Suppose the Holiday class has a method called celebrate, and the Christmas class overrides it Now consider the following invocation: day.celebrate(); If day refers to a Holiday object, it invokes the Holiday version of celebrate; if it refers to a Christmas object, it invokes the Christmas version 9-10 Polymorphism via Inheritance Consider the following class hierarchy: 9-11 StaffMember Executive Hourly Volunteer Employee Polymorphism via Inheritance Now let's look at an example that pays a set of diverse employees using a polymorphic method See Firm.java (page 486) See Staff.java (page 487) See StaffMember.java (page 489) See Volunteer.java (page 491) See Employee.java (page 492) See Executive.java (page 493) See Hourly.java (page 494) 9-12 Outline 9-13 Polymorphic References Polymorphism via Inheritance Polymorphism via Interfaces Sorting Searching Polymorphism via Interfaces An interface name can be used as the type of an object reference variable Speaker current; The current reference can be used to point to any object of any class that implements the Speaker interface The version of speak that the following line invokes depends on the type of object that current is referencing current.speak(); 9-14 Polymorphism via Interfaces Suppose two classes, Philosopher and Dog, both implement the Speaker interface, providing distinct versions of the speak method In the following code, the first call to speak invokes one version and the second invokes another: 	Speaker guest = new Philospher(); 	guest.speak(); 	guest = new Dog(); 	guest.speak(); 9-15 Outline 9-16 Polymorphic References Polymorphism via Inheritance Polymorphism via Interfaces Sorting Searching Event Processing Revisited File Choosers and Color Choosers Sliders Sorting Sorting is the process of arranging a list of items in a particular order The sorting process is based on specific value(s) sorting a list of test scores in ascending numeric order sorting a list of people alphabetically by last name There are many algorithms, which vary in efficiency, for sorting a list of items We will examine two specific algorithms: Selection Sort Insertion Sort 9-17 Selection Sort The approach of Selection Sort: select a value and put it in its final place into the list repeat for all other values In more detail: find the smallest value in the list switch it with the value in the first position find the next smallest value in the list switch it with the value in the second position repeat until all values are in their proper places 9-18 Selection Sort An example: 	original: 3 9 6 1 2 	smallest is 1: 1 9 6 3 2 	smallest is 2: 1 2 6 3 9 	smallest is 3: 1 2 3 6 9 	smallest is 6: 1 2 3 6 9 Each time, the smallest remaining value is found and exchanged with the element in the "next" position to be filled 9-19 Swapping The processing of the selection sort algorithm includes the swapping of two values Swapping requires three assignment statements and a temporary storage location: 	temp = first; 	first = second; 	second = temp; 9-20 Polymorphism in Sorting Recall that an class that implements the Comparable interface defines a compareTo method to determine the relative order of its objects We can use polymorphism to develop a generic sort for any set of Comparable objects The sorting method accepts as a parameter an array of Comparable objects That way, one method can be used to sort a group of People, or Books, or whatever 9-21 Selection Sort The sorting method doesn't "care" what it is sorting, it just needs to be able to call the compareTo method That is guaranteed by using Comparable as the parameter type Also, this way each class decides for itself what it means for one object to be less than another See PhoneList.java (page 500) See Sorting.java (page 501), specifically the selectionSort method See Contact.java (page 503) 9-22 Insertion Sort The approach of Insertion Sort: pick any item and insert it into its proper place in a sorted sublist repeat until all items have been inserted In more detail: consider the first item to be a sorted sublist (of one item) insert the second item into the sorted sublist, shifting the first item as needed to make room to insert the new addition insert the third item into the sorted sublist (of two items), shifting items as necessary repeat until all values are inserted into their proper positions 9-23 Insertion Sort An example: 	original: 3 9 6 1 2 	insert 9: 3 9 6 1 2 	insert 6: 3 6 9 1 2 	insert 1: 1 3 6 9 2 	insert 2: 1 2 3 6 9 See Sorting.java (page 501), specifically the insertionSort method 9-24 Comparing Sorts The Selection and Insertion sort algorithms are similar in efficiency They both have outer loops that scan all elements, and inner loops that compare the value of the outer loop with almost all values in the list Approximately n2 number of comparisons are made to sort a list of size n We therefore say that these sorts are of order n2 Other sorts are more efficient: order n log2 n 9-25 Outline 9-26 Polymorphic References Polymorphism via Inheritance Polymorphism via Interfaces Sorting Searching Event Processing Revisited Searching Searching is the process of finding a target element within a group of items called the search pool The target may or may not be in the search pool We want to perform the search efficiently, minimizing the number of comparisons Let's look at two classic searching approaches: linear search and binary search As we did with sorting, we'll implement the searches with polymorphic Comparable parameters 9-27 Linear Search A linear search begins at one end of a list and examines each element in turn Eventually, either the item is found or the end of the list is encountered See PhoneList2.java (page 508) See Searching.java (page 509), specifically the linearSearch method 9-28 Binary Search A binary search assumes the list of items in the search pool is sorted It eliminates a large part of the search pool with a single comparison A binary search first examines the middle element of the list -- if it matches the target, the search is over If it doesn't, only one half of the remaining elements need be searched Since they are sorted, the target can only be in one half of the other 9-29 Binary Search The process continues by comparing the middle element of the remaining viable candidates Each comparison eliminates approximately half of the remaining data Eventually, the target is found or the data is exhausted See PhoneList2.java (page 508) See Searching.java (page 509), specifically the binarySearch method 9-30 Summary Chapter 10 has focused on: defining polymorphism and its benefits using inheritance to create polymorphic references using interfaces to create polymorphic references using polymorphism to implement sorting and searching algorithms 9-31