Problem Solving 5 Using Java API for Searching and Sorting Applications

1. Problem Solving 5Using Java API for Searching and Sorting Applications ICS-201 Introduction to Computing II Semester 071

2. Searching Algorithm: Binary Search • Java provides two binary search (02) methods: • Arrays.binarySearch (for an array) • Collections.binarySearch (for a List) • Note that the values in the array / list are in sorted order. If they are not, binarySearch() method is not guaranteed to work properly.

3. Searching Algorithm: Binary Search (Cont’d)

4. Searching Algorithm: Binary Search (Cont’d) • Using Arrays.binarySearch(): int[] numbers = {-3, 2, 8, 12, 17, 29, 44, 79}; int index = Arrays.binarySearch(numbers, 29); System.out.println("29 is found at index " + index);

5. Searching Algorithm: Binary Search (Cont’d) • Using Collections.binarySearch (for a List) // binary search on ArrayList with the same values: int index = Collections.binarySearch(list, 29); System.out.println("29 is found at index " + index);

6. Sorting Algorithms: Merge Sort Algorithm • Java provides two sorting methods: • Arrays.sort() (for an array) Arrays.sort(strings); • Collections.sort() (for a List)

7. Sorting Algorithms: Merge Sort Algorithm (Cont’d)

8. Sorting Algorithms: Merge Sort Algorithm (Cont’d) • Using Arrays.sort() // demonstrate the Arrays.sort method String[] strings = {"c", "b", "g", "h", "d", "f", "e", "a"}; // Printing the strings before sorting…. System.out.println(Arrays.toString(strings)); Arrays.sort(strings); • // Printing the strings after sorting…. System.out.println(Arrays.toString(strings)); • Output: [c, b, g, h, d, f, e, a] [a, b, c, d, e, f, g, h]

9. Sorting User-Defined Types • Let’s assume we have an array of Student type objects that we want to sort. The type Student is defined as follows: class Student { private int id; public Student(int id) { this.id = id; } public intgetId() { return id; } // other code follows… }

10. Sorting User-Defined Types (Cont’d) public class TestSort { public static void main(String[] args) { Student[] students = new Student[5]; students[0] = new Student(555555); students[1] = new Student(444444); students[2] = new Student(333333); students[3] = new Student(111111); students[4] = new Student(222222); System.out.println(Arrays.toString(students)); Arrays.sort(students); // Will this work? System.out.println(Arrays.toString(students)); } }

11. Sorting User-Defined Types (Cont’d) The previous code compiles correctly. However, at runtime, we get the following exception: The method Arrays.sort() assumes that the class Student implements the Comparable interface!  ClassCastException

12. Sorting User-Defined Types (Cont’d) • To fix this problem, let’s have the class Student implements the Comparable interface as follows: public intcompareTo(Object o) { if(o == null) throw new NullPointerException(); else { if(!(o instanceof Student)) throw new ClassCastException(); else { Student st = (Student) o; return id – st.getId(); } } } // End of compareTo() method } // End of class Student class Student implements Comparable { private int id; public Student(int id) { this.id = id; } public intgetId() { return id; } // other code follows… // toString() method should be here

13. Sorting User-Defined Types • Now the code is running correctly and the output should be:

14. Sorting User-Defined Types (Cont’d) • Now, let’s revisit the compareTo() method: public intcompareTo(Object o) { if(o == null) throw new NullPointerException(); else { if(!(o instanceof Student)) throw new ClassCastException(); else { Student st = (Student) o; return id – st.getId(); } } } // End of compareTo() method Searching/Sorting based on id field! So to use Arrays.sort(), we have to restrict our comparison key on a single key!

15. Sorting User-Defined Types (Cont’d) • To use different fields as keys for searching and sorting purposes, we have to use the following methods: • Java provides two sorting methods: • Arrays.sort(Object[], Comparator) (for an array) using a specific Comparator object • Collections.sort(List, Comparator) (for a List) • using a specific Comparator object

16. Comparator Interface • The Comparator (java.util) interface defines the following two methods: • int compare(Object o1, Object o2) • boolean equals(Object o)

17. Comparator Interface (Cont’d) • Example: Let’s define a Comparator class to compare two Student objects based on the id field. • class ComparatorId implements Comparator { • public intcompare(Object o1, Object o2) { • if(o1 == null | o2 == null) • throw new NullPointerException(); • else { • if(!(o1 instanceof Student) | !(o2 instanceof Student)) • throw new ClassCastException(); • else { • Student st1 = (Student) o1; • Student st2 = (Student) o2; • return st1.getId() – st2.getId(); • } • } • } } // End of ComparatorId class

18. Comparator Interface (Cont’d) • Important Note: The class ComparatorId implements the Comparator interface and it overrides only one method and the compiler does NOT complain! • Answer: ?????

19. Sorting Using a Comparator object • The code below illustrates the use of ComparatorId object for sorting purposes: • import java.util.*; • public class TestSort2 { • public static void main(String[] args) { • Student[] students = new Student[5]; • students[0] = new Student(555555); • students[1] = new Student(444444); • students[2] = new Student(333333); • students[3] = new Student(111111); • students[4] = new Student(222222); • System.out.println(Arrays.toString(students)); • Arrays.sort(students, new ComparatorId()); • System.out.println(Arrays.toString(students)); • } • }

20. Comparator Interface (Cont’d) • Drill Exercise: Now try to define a Comparator class to compare two Student objects based on the gpa field. • class ComparatorGpa implements Comparator { • public intcompare(Object o1, Object o2) { • } // End of ComparatorGpa class