Lecture 20 SYS-1S22 / MTH-1A66. Sorting Objects. Stuart Gibson S01.09A. Sorting Arrays of Objects

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1 Lecture 20 SYS-1S22 / MTH-1A66 Sorting Objects Stuart Gibson sg@sys.uea.ac.uk S01.09A 1 Sorting Arrays of Objects It is common that we want to sort arrays of objects It is obvious what we mean by one integer being less than another. It is not necessarily obvious what we mean by one object being less than another. Before we can sort an array of objects, we have to define what it means for one object to be: less than greater than or equal to another object. 2

2 compareto Method The general form of a compareto method is: public int compareto (Object that) if (/*Define this greater than that /*) return 1; if (/*Define this less than that /*) return -1; else // must be equal return 0; The return type is an int. The argument passed in is any Java Object. 3 compareto should compareto Method return a positive number if this is greater than that. return a negative number if this is less than that. return 0 if this is equal to that. Note: do not rely on the result being -1 or 1! To call the compareto method of an object is the same as calling any method of an object, we use the dot operator. For example: int a = object1.compareto(object2); 4

3 String Class compareto Method String objects have a compareto method built in One String is said to be less than another if it comes before it Alphabetically, thus: AAA is less than ABB CC is less than CCDDDD BB is less than BBA Is the String compareto method case sensitive? What would the ordering of aaa and AAA be? Hint: Remember that a String is an array of characters and characters are really numbers. 5 For example: String Class compareto Method String name1 = new String( Alice ); String name2 = new String( Bob ); String name3 = new String( Bob ); int a,b,c; a = name1.compareto(name2); b = name2.compareto(name3); c = name3.compareto(name1); System.out.println( a: +a+ b: +b+ c: +c); a: -1 b: 0 c: 1 6

4 a: -1 b: 0 c: 1 String Class compareto Method a is -1 therefore name1 less than name2 Alice less than Bob b is 0 therefore name2 equal to name3 Bob equal to Bob c is 1 therefore name3 greater than name1 Bob greater than Alice 7 For example: String Class compareto Method if(name3.compareto("bobby")!= 0) System.out.println( Bob is not equal to Bobby"); if(name3.compareto("bob") == 0) System.out.println( Bob is equal to Bob"); if("robert".compareto("bob") > 0) System.out.println("Robert is greater than Bob!"); Bob is not equal to Bobby Bob is equal to Bob Robert is greater than Bob 8

5 String Class compareto Method What happens when we change cases? String name = fred ; if(name.compareto( Fred )!= 0) System.out.println(name+ is not equal Fred ); if(name.compareto( Fred ) > 0) System.out.println(name+ is greater than Fred ); if(name.compareto( Fred ) < 0) System.out.println(name+ is less than Fred ); fred is not equal to Fred fred is greater than Fred 9 String Class compareto Method The String compareto method works by comparing characters of the array, starting from index 0 until it finds a difference. Remember that characters are compared by their numeric (ASCII / Unicode) value. So the numeric representations are: F is 70 whereas f is 105 Hence f is greater than F and therefore.. fred is greater than Fred. Is there a way to compare Strings ignoring case? 10

6 String Class comparetoignorecase Method The String comparetoignorecase works by comparing the characters of the String, but first converts them to lowercase. String name = fred ; if(name.comparetoignorecase( Fred )!= 0) System.out.println(name+ is not equal Fred ); else System.out.println(name+ is equal to Fred ); fred is equal to Fred 11 Comparing Student Objects If we define a Student object as: public class Student private String forename; private String surname; private Date dob; private String school; private int year; // Constructor that takes three arguments public Student(String fn, String sn, Date db) forename = fn; sn = surname; dob = new Date(db); // other methods here 12

7 Comparing Student Objects (by name) Our first attempt would be to compare them using surname // in Student class public int compareto(student that) return surname.comparetoignorecase(that.surname); What happens if the surnames are the same, we would to compare first names as well: public int compareto(student that) int c = surname.comparetoignorecase(that.surname); if (c == 0) c = forename.comparetoignorecase(that.forename); return c; 13 Comparing Student Objects (by dob) It is important to note that we can not have two compareto methods with the same method signature, so we have to alter the compareto method each time. It is up to the person that designs the class how they want to sort the Objects using the compareto method. We might want to sort Student objects by date of birth (dob), and hence ALTER the compareto method to be: // in Student class public int compareto(student that) return dob.compareto(that.dob); This assumes we have written a compareto method for the class Date. 14

8 Comparing Date Objects What if we had stored the date as a String? String date1 = "12/02/2003"; String date2 = "12/03/1977"; if (date1.compareto(date2) > 0) System.out.println(date1+ is after +date2); else if (date1.compareto(date2) == 0) System.out.println(date1+ is the same as +date2); else System.out.println(date1+ is before +date2); 12/02/2003 is before 12/03/1977 Obviously this is wrong, so it is better to define a Date class: public class Date private int day; private int month; private int year; 15 Comparing Date Objects // in Date class public int compareto(date other) if(year > other.year) return 1; else if (year < other.year) return -1; else if (month > other.month) return 1; else if (month < other.month) return -1; else if (day > other.day) return 1; else if (day < other.day) return -1; else return 0; 16

9 Sorting an Array of Objects Suppose we want to write a selection sort method for the Sort class to sort an array of Student Objects public class Sort // see Lecture 19 public static void selectionsort(int[] t) // implementation here // overload the selectionsort method public static void selectionsort(student[] t) // implementation here 17 Selection Sorting an Array of Student objects The algorithm to perform the sort is identical: Task: Selection Sort: Sorts Array T[0 n-1] into ascending order for i = 0 to n-2 loop find position of smallest element in T[i..n-1] pos = i min = T[i] for j=i+1 to n-1 loop if T[j] < min pos = j min = T[j] swap element at smallest position with element at i T[pos] = T[i] T[i] = min 18

10 Selection sort previous implementation for (int i=0; i<t.length-1; i++) pos = i; for(int j=i+1; j< t.length; j++) if(t[j] < t[pos]) pos = j; if (pos!= i) temp = t[pos]; t[pos] = t[i]; t[i] = temp; 19 Selection Sorting an Array of Student objects The two key differences in the implementation are that we can not compare Student objects using the < operator in the line: if(t[j] < t[pos]) we need to replace it to use the compareto method: if(t[j].compareto(t[pos]) < 0) In the swap routine, temp is no longer an int but should be a Student reference instead so the line: int temp; becomes Student temp; 20

11 Selection sort Student implementation public static void selectionsort(student[] t) int pos; Student temp; for (int i=0; i<t.length-1; i++) pos = i; for(int j=i+1; j< t.length; j++) if(t[j].compareto(t[pos]) < 0) pos = j; if (pos!= i) temp = t[pos]; t[pos] = t[i]; t[i] = temp; 21 Some more on compareto So far for all our objects we have created the compareto method in the following format: public int compareto (ourtype that) but at the start we said that it had the general form: public int compareto(object that) Why should it be in that form? The code for both our selectionsort routines is very similar. It is only the type of object that is changing. Repeating this code for every possible object is very tedious! Is there another way? 22

12 Selection sort Generic case??? Why not try and make a generic case for all objects? We know that all user defined objects are java Objects so why not try making the selectionsort like this: public static void selectionsort(object[] t) Object temp; // implementation here This seems great in theory but what happens at the line: if(t[j].compareto(t[pos]) < 0) 23 Selection sort Generic case??? According to our method declaration t[j] is a Java Object, so the compiler looks for a compareto method in the Java Object class. Check API listing for Object no compareto! Or try compiling our code and we get the following error: Sort.java:59: cannot resolve symbol symbol : method compareto (java.lang.object) location: class java.lang.object if(t[j].compareto(t[pos]) < 0) ^ 1 error Finished 24

13 Selection sort Generic case??? The only way we know of solving this would be to cast the Object into a specific object type like a Student. However we would need to cast the java Object for every type of object we want to sort. So we are back to the same problem as before. What we need is a way of making our object belong to two different categories, primarily we want it to be a Student object, but for generic algorithms we want it to be a comparable object. We say that we want the Student object to be polymorphic, i.e. have many forms. So how could we define each of these forms or categories? 25 One Object - Many Categories For example, depending upon what the situation is, objects belong to different categories. As a species I would be classed as Human When questioned about my job, I would be classed as a student When playing football I would be classed as a footballer. I can be placed in a certain category because I function in a certain way at a certain time. For example to class myself as a footballer, I must have the ability to play football. 26

14 One Object - Many Categories In Java we can force an object to belong to another category by ensuring that is has certain behaviours. Remember that in Java behaviours are modelled by methods! Therefore if we want to ensure that an object can be treated as a certain type, we must ensure that it conforms to a set functionality by containing certain methods. This functionality is enforced by specifying a set of methods that must be implemented in the object. This specification is provided by an interface. 27 Interfaces An interface has abstract methods. An abstract method is a method that does not have an implementation. An abstract method does not contain a method body, only a method header, after this it ends in a semi-colon. public interface Footballer public void shot(); public void dribble(); public void pass(); 28

15 Interfaces A class is said to implement an interface. If a class implements an interface, it must define each of the abstract methods in the interface. For example if we want our Student object to be capable of being categorised / used as a Footballer it MUST implement the Footballer interface It MUST define all of the abstract methods from the Footballer interface. 29 Interfaces public class Student implements Footballer // Student variables and methods public void shoot() // implementation here public void dribble() // implementation here public void pass() // implementation here 30

16 Comparable Interface So we require the Student object to be a comparable object, i.e. something we can compare in a generic algorithm. Java provides a Comparable interface! The Comparable interface contains just one abstract method, the compareto method: public interface Comparable public int compareto (Object o); Therefore any class implementing the Comparable interface must define a compareto method. 31 Sorting an array of Comparable objects public static void selectionsort( Comparable[] t ) int pos; Comparable temp; for (int i=0; i<t.length-1; i++) pos = i; for(int j=i+1; j< t.length; j++) if(t[j].compareto(t[pos]) < 0) pos = j; if (pos!= i) temp = t[pos]; t[pos] = t[i]; t[i] = temp; 32

17 Comparable sort of Student Objects So we have to change Student class so that it becomes: public class Student implements Comparable Then we can remove the selectionsort(student[] t) method from the Sort class file, because we can now use the selectionsort(comparable[] t) method to sort the Student objects. There is one slight problem! If we want to sort an array of Student objects: Student[] sys1s22 = new Student[35]; // enter data into array // then sort the data Sort.selectionSort(sys1S22); 33 Comparable sort of Student Objects We get the following compilation error: Student.java:3: Student should be declared abstract; it does not define compareto(java.lang.object) in Student public class Student implements Comparable ^ 1 error Finished This is because our compareto method was declared as follows: public int compareto(student that) Not the required form: public int compareto(object that) 34

18 Student Objects compareto method We can rewrite it as follows: // in Student class public int compareto(object that) // cast object to a student object then // compare as normal. Student s = (Student)that; int c = surname.comparetoignorecase(s.surname); if (c == 0) c = forename.comparetoignorecase(s.forename); return c; 35 Comparable sort in practice // create an array of students Student[] bedrockhigh = new Student[3]; bedrockhigh[0] = new Student( Fred, Flintstone, BRH,4); bedrockhigh[1] = new Student( Barney, Rubble, BRH,2); bedrockhigh[2] = new Student( Wilma, Flintstone, BRH,1); //print out array before sorting System.out.println( BEFORE SORTING ); for (int i = 0; i < bedrockhigh.length; i++) System.out.println(bedrockHigh[i]); //sort array using Comparable sort Sort.selectionSort(bedrockHigh); // print out array after sorting System.out.println(); System.out.println( AFTER SORTING ); for (int i = 0; i < bedrockhigh.length; i++) System.out.println(bedrockHigh[i]); 36

19 Comparable sort in practice BEFORE SORTING Flintstone, Fred : BRH 4 Rubble, Barney : BRH 2 Flintstone, Wilma : BRH 1 AFTER SORTING Flintstone, Fred : BRH 4 Flintstone, Wilma : BRH 1 Rubble, Barney : BRH 2 37 Questions Ok, now we can sort an array of any objects as long as we implement the Comparable interface and in doing so declare a compareto method, but What happens if we want to sort the array using a different way to that which we specified in our compareto method? We know we can t have more than one compareto, so ideally it should provide a natural ordering. However we can sort the array using different fields by creating a class that implements the Comparator interface How do we sort an ArrayList? Surely Java has already implemented sorting? More next lecture. 38