Problem Solving 5

Using Java API for Searching and Sorting Applications

ICS-201 Introduction to Computing II

Semester 071

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.

Searching Algorithm: Binary Search (Cont’d)

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);

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);

Sorting Algorithms: Merge Sort Algorithm

Java provides two sorting methods:– Arrays.sort() (for an array)

Arrays.sort(strings);

– Collections.sort() (for a List)

Sorting Algorithms: Merge Sort Algorithm (Cont’d)

Sorting Algorithms: Merge Sort Algorithm (Cont’d)

Using Arrays.sort()// demonstrate the Arrays.sort methodString[] 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]

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 int getId() { return id; }// other code follows…}

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)); }}

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

Sorting User-Defined Types (Cont’d)

To fix this problem, let’s have the class Student implements the Comparable interface as follows:

class Student implements Comparable { private int id;

public Student(int id) { this.id = id; } public int getId() { return id; }// other code follows…// toString() method should be here

public int compareTo(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

Sorting User-Defined Types

Now the code is running correctly and the output should be:

Sorting User-Defined Types (Cont’d)

Now, let’s revisit the compareTo() method: public int compareTo(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!

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

Comparator Interface

The Comparator (java.util) interface defines the following two methods:– int compare(Object o1, Object o2)

– boolean equals(Object o)

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 int compare(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

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: ?????

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));

}

}

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 int compare(Object o1, Object o2) {

} // End of ComparatorGpa class