ArrayList in Java (equivalent to vector in C++) having a dynamic size. It can be shrinked or expanded based on size. ArrayList is a part of the collection framework and is present in java.util package.
--> java.util Package
--> ArrayList Class
Syntax: Creating an empty ArrayList
ArrayList <E> list = new ArrayList <> ();
Different Ways to Sort an ArrayList of Objects by Property
- Using Comparator interface
- Using Comparable interface
Approach 1:
In this case, the comparator is a lambda which defines the below parameters as follows:
- Takes two objects from the list o1 and o2.
- Compares the two object’s customProperty using compareTo() method.
- And finally returns a positive number if o1’s property is greater than o2’s, negative if o1’s property is lesser than o2’s, and zero if they are equal.
- Based on this, the list is sorted based on the least property to the greatest and stored back on to list.
Procedure:
- In the below program, we’ve defined a CustomObject class with a String property, customProperty.
- We’ve also added a constructor that initializes the property, and a getter function getCustomProperty() which returns customProperty.
- In the main() method, we’ve created an array list of custom objects list, initialized with 5 objects.
- For sorting the list with the given property, we use the list‘s sort() method.
- The sort() method takes the list to be sorted (final sorted list is also the same) and a comparator.
Example
Java
// Java Program to Sort ArrayList of Objects by Property// Importing required classesimport java.util.*;// Class 1// Custom class with custom property// It takes and stores custom objectsclass CustomObject { // Class data member private String customProperty; // Method public CustomObject(String property) { this.customProperty = property; } // Getter public String getCustomProperty() { return this.customProperty; }}// Class 2public class GFG { // Method 1 // To print sorted ArrayList objects // using enhanced for loop public static void print(ArrayList<CustomObject> list) { for (CustomObject obj : list) { System.out.println(obj.getCustomProperty()); } } // Method 2 // Comparing two list // using compareTo() method public static void sort(ArrayList<CustomObject> list) { list.sort((o1, o2) -> o1.getCustomProperty().compareTo( o2.getCustomProperty())); } // Method 3 // Adding custom objects public static void add(ArrayList<CustomObject> list) { // Adding elements to list // using add() method list.add(new CustomObject("Z")); list.add(new CustomObject("A")); list.add(new CustomObject("B")); list.add(new CustomObject("X")); list.add(new CustomObject("Aa")); } // Method 4 // Main driver method public static void main(String[] args) { // Creating an empty ArrayList of custom class type ArrayList<CustomObject> list = new ArrayList<>(); // Calling above methods defined inside class // in main() method add(list); sort(list); print(list); }} |
Output
A Aa B X Z
Approach 2: Using Comparable and comparator
When the ArrayList is of a custom object type, then, in this case, we use two sorting methods by either Comparator or Comparable and in this case Collections.sort() cannot be used directly as it will give an error because it sorts only specific data-types and not user-defined types.
2-A: Sorting ArrayList with Comparable
- The custom object type class which is Student here will implement the Comparable class<Student>.
- This will override the compareTo() method of Comparable class which takes the object of class Student as a parameter, and we need to compare the values/ attributes by which we want to sort the list and return accordingly in the compareTo() function.
Example:
Java
// Java program to sort ArrayList of Custom Object// Using Comparable class// Importing required classesimport java.util.*;// Class 1// Main class// ArrayListSortingclass GFG { // Main driver method public static void main(String args[]) { // Creating an empty ArrayList of Student type ArrayList<Student> arraylist = new ArrayList<Student>(); // Adding elements to above List arraylist.add(new Student(12, "Riya", 15)); arraylist.add(new Student(14, "Mahima", 16)); arraylist.add(new Student(13, "Shubhi", 15)); // Sorting above list using sort() method // of Collections class Collections.sort(arraylist); // Iterating over list via for each loop and // printing all elements inside the List for (Student str : arraylist) { System.out.println(str); } }}// Class 2// Implementing Comparable interfacepublic class Student implements Comparable<Student> { // Class data members private String studentname; private int rollno; private int studentage; // Constructor of Student class public Student(int rollno, String studentname, int studentage) { // this keyword refers to current instance itself this.rollno = rollno; this.studentname = studentname; this.studentage = studentage; } // Getter and Setter methods public String getStudentname() { return studentname; } public void setStudentname(String studentname) { this.studentname = studentname; } public int getRollno() { return rollno; } public void setRollno(int rollno) { this.rollno = rollno; } public int getStudentage() { return studentage; } public void setStudentage(int studentage) { this.studentage = studentage; } // overriding the compareTo method of Comparable class @Override public int compareTo(Student comparestu) { int compareage = ((Student)comparestu).getStudentage(); // For Ascending order return this.studentage - compareage; // For Descending order do like this // return compareage-this.studentage; } @Override public String toString() { return "[ rollno=" + rollno + ", name=" + studentname + ", age=" + studentage + "]"; }} |
Output
[ rollno=12, name=Riya, age=15] [ rollno=13, name=Shubhi, age=15] [ rollno=14, name=Mahima, age=16]
2-B: Sorting ArrayList with Comparator
- We will define another class that will implement the Comparator class of the type of our custom object. For eg, in the below code, our custom class is Student so another class that we have defined will implement Comparatot<Student>.
- This class will override the compare method of the Comparator class which accepts two objects of the Student class as parameters and returns the comparison value according to our requirement whether we want to sort the array in ascending or descending order and on which attribute, we want to sort the list.
Example:
Java
// Java Program to Sort ArrayList of Custom Object// Using Comparator class// Importing required classesimport java.util.*;import java.util.Comparator;// Classpublic class Student { // Class data members private String studentname; private int rollno; private int studentage; // Constructor public Student(int rollno, String studentname, int studentage) { // this keyword refers to current instance itself this.rollno = rollno; this.studentname = studentname; this.studentage = studentage; } // Getters and Setters method public String getStudentname() { return studentname; } public void setStudentname(String studentname) { this.studentname = studentname; } public int getRollno() { return rollno; } public void setRollno(int rollno) { this.rollno = rollno; } public int getStudentage() { return studentage; } public void setStudentage(int studentage) { this.studentage = studentage; } // Usage of comparator public static Comparator<Student> StuNameComparator = new Comparator<Student>() { // Comparing attributes of students public int compare(Student s1, Student s2) { String StudentName1 = s1.getStudentname().toUpperCase(); String StudentName2 = s2.getStudentname().toUpperCase(); // Returning in ascending order return StudentName1.compareTo( StudentName2); // descending order // return // StudentName2.compareTo(StudentName1); } }; // Comparator for sorting the list by roll no public static Comparator<Student> StuRollno = new Comparator<Student>() { // Method public int compare(Student s1, Student s2) { int rollno1 = s1.getRollno(); int rollno2 = s2.getRollno(); // For ascending order return rollno1 - rollno2; // For descending order // rollno2-rollno1; } }; // Overriding toString() method to list out student details @Override public String toString() { return "[ rollno=" + rollno + ", name=" + studentname + ", age=" + studentage + "]"; }}// Class 2// Main classclass Details { // Main driver method public static void main(String args[]) { // Creating an empty ArrayList of Student type ArrayList<Student> arraylist = new ArrayList<Student>(); // Adding elements to ArrayList arraylist.add(new Student(101, "Zues", 26)); arraylist.add(new Student(505, "Abey", 24)); arraylist.add(new Student(809, "Vignesh", 32)); // Sorting based on Student Name System.out.println("Student Name Sorting:"); // Using sort() method of Collection class Collections.sort(arraylist, Student.StuNameComparator); for (Student str : arraylist) { System.out.println(str); } // Now, sorting on Rollno property System.out.println("RollNum Sorting:"); Collections.sort(arraylist, Student.StuRollno); // Iterating over list via for each and // printing the elements for (Student str : arraylist) { System.out.println(str); } }} |
Output
Student Name Sorting: [ rollno=505, name=Abey, age=24] [ rollno=809, name=Vignesh, age=32] [ rollno=101, name=Zues, age=26] RollNum Sorting: [ rollno=101, name=Zues, age=26] [ rollno=505, name=Abey, age=24] [ rollno=809, name=Vignesh, age=32]
