Sort the list alphabetically using the compareTo () method

Question

I am writing a phone book program in java and I need to list the people in the list alphabetically, and for this I need to write a sorting algorithm for the list in java, and it should use only the compareTo () method. So can anyone help me do this?

public void listAlpha() { Node tempNode = head; for(int i = 0; i <= size; i++) { for(int j = 0; j <= i; j++) { int comparison = ((tempNode.getNext().getElement().getName()).compareTo(tempNode.getElement().getName())); if(comparison < 0) { Person tempPerson = tempNode.getElement(); tempNode.setElement(tempNode.getNext().getElement()); tempNode.getNext().setElement(tempPerson); tempNode = tempNode.getNext(); } } } 

(By the way, this is homework, and I use my own data structures.)

This is the class that belongs to the method described above:

 import java.util.*; /** Singly linked list implementation .*/ public class SLinkedList<E> implements LinkedList<E>, Iterable<E> { protected Node<E> head; // head node of the list protected Node<E> tail; // tail node of the list protected int size; // number of nodes in the list public Iterator<E> iterator() { return new LinkedListIterator(head); } /** Default constructor that creates an empty list */ public SLinkedList() { head = null; tail = null; size = 0; } public int size() { return size; } public boolean isEmpty() { return size == 0; } public void addFirst(E newElement) { Node<E> newNode = new Node(newElement,null); if(size == 0) //if list is empty tail = newNode; newNode.setNext(head); head = newNode; size++; } public void addLast(E newElement) { Node<E> newNode = new Node(newElement,null); if(size == 0) //if list is empty head = newNode; if (size != 0) //if list is not empty tail.setNext(newNode); tail = newNode; size++; } public E removeFirst() { Node<E> tempNode = null; if (size != 0) { if(size == 1) tail = null; tempNode = head; head = head.getNext(); tempNode.setNext(null); size--; } //if list is empty then return null return tempNode.getElement(); } public E removeLast() { Node<E> tempNode = head; if(size == 0) return null; if(size == 1) { head = null; tail = null; size--; return tempNode.getElement(); } //size is greater than 1 for(int i=1; i<=size-2; i++) { tempNode = tempNode.getNext(); //go to element that before the tail } Node<E> tempNode2 = tail; tail = tempNode; tail.setNext(null); size--; return tempNode2.getElement(); } public void remove(E element){ int index = 0; boolean found = false; Node<E> temp = head; for(int i=1; i<=size; i++) {//find the node with element index++; if(temp.getElement().equals(element)){ found = true; break; } temp = temp.getNext(); } if(found){ if(index == 1) removeFirst(); else if(index == size) removeLast(); else{ //find the previous node Node<E> prev = head; for(int i=1; i<index-1; i++) { prev = prev.getNext(); } prev.setNext(temp.getNext()); temp.setNext(null); size--; } } } public int searchList(E searchKey) { if(size == 0) return -1; Node tempNode = head; for(int i=1; i<=size; i++) { if(tempNode.getElement().equals(searchKey)) return i; //return index of the node tempNode = tempNode.getNext(); } return -1; //not found } public void printList() { Node tempNode = head; for(int i=1; i<=size; i++) { System.out.print(tempNode.getElement()); if(i!=size) //if it is not last element System.out.print(" - "); tempNode = tempNode.getNext(); } System.out.println(); } 

Character class:

 public class Person { private String name; private String surname; private String address; private PhoneNumber phone1; private PhoneNumber phone2; private PhoneNumber phone3; public Person() { name = null; surname = null; address = null; phone1.setPhone(0); phone1.setType(""); phone2.setPhone(0); phone2.setType(""); phone3.setPhone(0); phone3.setType(""); } public Person(String n, String s, String a,PhoneNumber p1, PhoneNumber p2, PhoneNumber p3) { name = n; surname = s; address = a; phone1 = p1; phone2 = p2; phone3 = p3; } public String getName() { return name; } public void setName(String n) { name = n; } public String getSur() { return surname; } public void setSur(String s) { surname = s; } public void insertPhone(PhoneNumber phone) { if(phone2 == null) phone2 = phone; else if(phone3 == null) phone3 = phone; } public PhoneNumber getPhone1() { return phone1; } public PhoneNumber getPhone2() { return phone2; } public PhoneNumber getPhone3() { return phone3; } public String getAdd() { return address; } public void setAdd(String a) { address = a; } 

Answer 1

Like everyone else, compareTo is part of Comparable .

How you implement it depends on whether you want to order first by last name or first name, and if you want them to be sorted in ascending order.

For example, if you want to order first by last name, in ascending order:

 public class Person implements Comparable<Person> { // the parts of Person you already have would go here public int compareTo(Person person) { if (person == null) { return -1; } if (surname != null && person.getSur() == null) { return -1; } else if (surname == null && person.getSur() != null) { return 1; } else if (surname != null && person.getSur() != null) { int compare = surname.compareToIgnoreCase(person.getSur()); if (compare != 0) { return compare; } } // Note that we did nothing if both surnames were null or equal if (name == null && person.getName() == null) { return 0; } else if (name != null && person.getName() == null) { return -1; } else if (name == null && person.getName() != null) { return 1; } else { return name.compareToIgnoreCase(person.getName()); } } } 

(I really have not tested this code)

It depends on the implementation of the compareToIgnoreCase string.

Note that this also moves all null objects and objects with null names to the end of the list.

Having said all this, if you are implementing Comparable, you can make Collections API for you using sort .

If you find that you need several different sorting methods for an object, you can create a set of Comparator objects to sort instead.

Answer 2

You can implement your Person class Comparable and define the following method:

  public class Person implements Comparable<Person> { // Your previous code public int compareTo(Person other) { if (other == null) { // throw exception for example } return this.name.toLowerCase().compareTo(other.name.toLowerCase()); } } 

Answer 3

Add Comparable to your Person class.

Your compareTo () method will look something like this:

 public int compareTo(Person other) { return name.compareTo(other.getName()) } 

Then use Collections.sort(<your list of Person>);

Answer 4

The signature of the Person class should be like this:

 public class Person implements Comparable<Person> 

Add the compareTo-method class to the Person class and use Collections.sort (personList) as starf suggested.