Sorting in Java

what we’ll talk about today

            An overview of sorting

            Sorting methods in the Java library

            The enhanced for statement

            Ways to compare objects so they can be sorted

            Intuitive sorting algorithm (how you would sort)

            Selection sort

An overview

            Searching large data lists mandates sorting

            We assume we can sort once, then search many times

            Sorting comes in two main categories

                        O(n²) and O(n log n)

            The O(n2) sorts are faster for shorter lists, or with certain input patterns

The O(n log n) algorithms have quite a bit of overhead, so are more efficient only for longer lists.

Java Sorting methods in the library

            In the class Array are several static sort methods that can be used to sort arrays of different types.

import java.util.*;

public class problem2

{

public static void main(String[] args)

{

int[] ar = {10,30,50,40,20};

Arrays.sort(ar);

for(int num=0; num<ar.length; num++)

System.out.println(ar[num]);

} // end main

}

sThe enhanced for statement

            Since iterating through an array always takes the same form

                for(int num=0; num<ar.length; num++)

System.out.println(ar[num]);

Java 5.0 has included another form of the for statement0

for( int number : ar)

System.out.println(number);

          String[] st = {"cat","at","zebra",”henry", "bay"};

          Arrays.sort(st);

          for(String number : st)

          System.out.println(number);

When sorting objects, there must be a way to compare the objects

            Usually, the object class will implement the Comparable interface

            Sometimes, an alterative method of sorting may be wanted

                        For instance, a class Person may compare objects by name, so they can be sorted by name

                        So you call Arrays.sort(personArray)

                        At times, it list may want to be sorted by SS#

In this case you use a comparator, which is an object of a class that implements the Comparator interface. This has only one method, compare which has two arguments, the two things to be compared. The call to compart obj.compare(a, b) returns an int just as compareTo does.

            public class ComparePersons implements Comparator<Person>

         public int compare(Person a, Person b)

         { if ( a.getSSN( ) < b.getSSN( ) return -1;

               if ( a.getSSN( ) = = b.getSSN( ) return 0;

               return 1;

         }

then, to sort the list by SSN, call

         Arrays.sort(personArray, new ComparePersons)

Let’s look at some sorting algorithms

Sort this list in place, paying careful attention to the steps you used to sort, and count the number of comparisons you must make

            42 95 16 63 28 74

If there are 6 items in the list, how many comparisons?

If there are n items in the list, how many comparisons?

n*(n-1)/2

Code for selection sort

    public void sort()

    {

        for (int pos = 0; pos < a.length-1; pos++)

        {

                int minPos = pos;

                for(int i = pos +1; i < a.length; i++)

                    if(a.[i] < a[minPos]

                        minPos= i;

                swap(minPos, pos);

         }

     }

         private void swap (int i, int j)

         { int temp = a[i];

             a[i] = a[j];

             a[j] = temp;

          }

}