String alphabetical order using List or Arraylist

Question

In this program I sorted strings in Alphabetic way in Simple Array but how can I do the same thing using List or ArrayList, Suppose I have a class Students and I want to order names in an alphabetically way, so how can I do that?

public class CityData{
 public static String citynames[]= {"Hyderabad","Karachi","Abtabad","AzadKashmir"};

    public static void main(String args[]) {
        int size=citynames.length;
        String temp=null;

            for(int i=0; i<size; i++) {
                for(int j=i+1; j<size; j++) {
                    if(citynames[i].compareTo(citynames[j])>0) {
                     temp=citynames[i];
                     citynames[i]=citynames[j];
                     citynames[j]=temp; } 
        } 
                System.out.println(citynames[i]);
      }
    }

 }  

Result:

Abtabad
AzadKashmir
Hyderabad
Karachi

Answer 1

I do not have the rights to comment yet, hence adding at as an answer. You can google this easily and also lookup API for sorting collections.

Code in java 8

public class ArrayListSort {
    public static String citynames[] = { "Hyderabad", "Karachi", "Abtabad", "AzadKashmir" };

    public static void main(String args[]) {
        Student stud1 = new Student("Alex", 20);
        Student stud2 = new Student("Bryan", 21);
        Student stud3 = new Student("Chris", 22);
        Student stud4 = new Student("Dan", 23);

        List<Student> students = new ArrayList<>(Arrays.asList(stud4, stud3, stud2, stud1));
        Collections.sort(students);
        for(Student stud: students){
            System.out.println(stud.getName());
        }
    }
}

class Student implements Comparable<Student> {
    private String name;
    private int age;

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return this.name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public int compareTo(Student other) {
        return this.name.compareTo(other.name);

    }
}

Edit 1 to answer your comment, look up Comparable interface

Answer 2

What you are looking for is actually pretty simple to do:

Instead of using citynames[i] or citynames[j], for Lists and ArrayLists, you use citynames.get(i) or citynames.get(j).

Just think of List.get() is the same as the brackets you put before for the simple arrays.

Just remember, when you want to set a value, you actually have to use the List.set(int index, Object value).

The same can be said if you are trying to get the length of the List or ArrayList. You can simply replace citynames.length to citynames.size();

public static void main(String args[]) {
    int size=citynames.size();
    String temp=null;

        for(int i=0; i<size; i++) {
            for(int j=i+1; j<size; j++) {
                if(citynames.get(i).compareTo(citynames.get(j))>0) {
                 temp=citynames.get(i);
                 citynames.set(i, citynames.get(j));
                 citynames.set(j, temp); } 
    } 
            System.out.println(citynames.get(i));
  }
}

Update: Better Solution:

Note: When using Collections.sort() it is important to make sure the object type of the array implements Comparable. the Collections.sort() uses the compareTo() within the object class to sort the elements.

public class Main(){
import java.util.Collections;
    public static void main(String args[]) {
        ArrayList<City> citynames = new ArrayList<City>();
        citynames.add("Hyderabad");
        citynames.add("Karachi");
        citynames.add("Abtabad");
        citynames.add("AzadKashmir");
        Collections.sort(citynames);
        for(cityname : citynames)
            System.out.println(cityname);
    }
}

public class City implements Comparable{
    private String name;
    // Constructor
    public City(String name){
        this.name = name;
    }

    // Getter method
    public String getName(){
        return name;
    }

    // compareTo Method
    public int compareTo(City other){
        return name.compareTo(other.getName());
    }

    // Other methods may exist
}

For more information: https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html

Answer 3

You can sort the collections based on your requirements.

If the input objects for collections is implementing the Comparable interface like String, Integer, Double classes, then you can directly use Collections.sort() method from Collections util class

List<String> al = new ArrayList<>();
    al.add("Hyderabad");
    al.add("Karachi");
    al.add("Abtabad");
    al.add("AzadKashmir");
    al.add("Udupi");
    al.add("Jammu");

    Collections.sort(al);

Or you can sort the list based on your requirement.(Reverse alphabetical Order)

Collections.sort(al, (str1, str2) -> str2.compareTo(str1));

If you don't want to use the Collections class, then directly use the sort() method present in List interface.

i. Albhabetical Order

al.sort((str1, str2) -> str1.compareTo(str1));

ii. Reverse Albhabetical Order

al.sort((str1, str2) -> str2.compareTo(str1));

The above solution is for the Objects where the class implements the Comparable Interface like String, Integer, Double, etc...

When to sort the Custom Classes, you need to implement sort by Comparator class or Lambda expression for the Comparator.

Consider you have a Student Class, and need to sort by city names. You can use below code.

Student Class

public class Student {
    private String name;
    private String city;

    public Student() {}

    public Student(String name, String city) {
        this.name = name;
        this.city = city;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    @Override
    public String toString() {
        return "Student -> [name=" + name + ", city=" + city + "]";
    }
}

Sorting By Collection Class sort() method

Student st1 = new Student("Samir", "Hyderabad");
Student st2 = new Student("Akbar", "Karachi");
Student st3 = new Student("Ramu", "Abtabad");
Student st4 = new Student("Rahim", "AzadKashmir");
Student st5 = new Student("Sardar", "Udupi");
Student st6 = new Student("Fahad khan", "Jammu");

List<Student> al2 = new ArrayList<>();
al2.add(st1);
al2.add(st2);
al2.add(st3);
al2.add(st4);
al2.add(st5);
al2.add(st6);

//Alphabetical Order
Collections.sort(al2, (std1, std2) -> std1.getCity().compareTo(std2.getCity()));

//Reverse Alphabetical Order
Collections.sort(al2, (std1, std2) -> std2.getCity().compareTo(std1.getCity()));

By using List.sort() method

//Alphabetical Order
al2.sort((std1, std2) -> std1.getCity().compareTo(std2.getCity()));

//Reverse Alphabetical Order
al2.sort((std1, std2) -> std1.getCity().compareTo(std2.getCity()));