In this article, we will discuss how to sort elements of List using Collections class’s utility sort() method
Sorting ArrayList using Comparable/Comparator interfaces:
- Default natural sorting order for String, Wrapper types
- java.lang.Comparable interface –> for natural ordering of Objects
- java.util.Comparator interface –> for customized sorting of Objects
Cautions:
- Objects should be of Comparable type like String or Wrapper class or else override compareTo() method, otherwise ClassCastException will be thrown
- Specified list to the sort() method shouldn’t contain any NULL value, otherwise NullPointerException will be thrown
1. Natural ordering of String type
- Method signature : public static void sort(List<String> strList)
- String and wrapper classes already implements comparable interface, so when we store elements of String type then we can use Collections sort() method to sort elements in ascending order of alphabets
- Note: there shouldn’t be any NULL elements, otherwise NullPointerException will be thrown
ArrayListNaturalSortingOfStringType.java
package in.bench.resources.java.collection.sorting.arraylist;
import java.util.ArrayList;
import java.util.Collections;
public class ArrayListNaturalSortingOfStringType {
public static void main(String[] args) {
// creating ArrayList object of type String
ArrayList<String> al = new ArrayList<String>();
// adding elements to ArrayList object
al.add("Narayan Murthy");
al.add("Dinesh");
al.add("Nandan Nilekeni");
al.add("Ashok Arora");
al.add("Shibulal");
al.add("Kris Gopalakrishnan");
al.add("Raghavan");
System.out.println("Before Sorting:"
+ " Iterating ArrayList\n");
// Iterating using enhanced for-loop
for(String str : al){
System.out.println(str);
}
// sorting using Collections.sort(al);
Collections.sort(al);
System.out.println("\n\nAfter Sorting:"
+ " Iterating ArrayList\n");
// Iterating using enhanced for-loop
for(String str : al){
System.out.println(str);
}
}
}
Output:
Before Sorting: Iterating ArrayList
Narayan Murthy
Dinesh
Nandan Nilekeni
Ashok Arora
Shibulal
Kris Gopalakrishnan
Raghavan
After Sorting: Iterating ArrayList
Ashok Arora
Dinesh
Kris Gopalakrishnan
Nandan Nilekeni
Narayan Murthy
Raghavan
Shibulal
2. Comparable interface and Collections.sort() method
- Method signature: public static void sort(List<Object> list);
Customer.java
- Customer POJO with 2 member variables of Integer and String type
- which implements Comparable interface to provide natural ordering of Customer objects on the basis of customer name
package in.bench.resources.java.collection.sorting.arraylist;
public class Customer implements Comparable<Customer> {
// member variables
int customerId;
String customerName;
// 2-arg parameterized constructor
// override toString() method
// override compareTo() method
@Override
public int compareTo(Customer o) {
return this.customerName.compareTo(o.customerName);
}
}
Main class
- This class uses above customer POJO to store objects inside ArrayList and
- prints customer objects in ascending order of customer name
ArrayListNaturalSortingOfObjectType.java
package in.bench.resources.java.collection.sorting.arraylist;
import java.util.ArrayList;
import java.util.Collections;
public class ArrayListNaturalSortingOfObjectType {
public static void main(String[] args) {
// creating ArrayList object of type Customer
ArrayList<Customer> al = new ArrayList<Customer>();
// adding elements to TreeSet object
al.add(new Customer(101, "Narayan Murthy"));
al.add(new Customer(107, "Dinesh"));
al.add(new Customer(103, "Nandan Nilekeni"));
al.add(new Customer(102, "Ashok Arora"));
al.add(new Customer(104, "Shibulal"));
al.add(new Customer(106, "Kris Gopalakrishnan"));
al.add(new Customer(105, "Raghavan"));
System.out.println("Before Sorting:"
+ " Insertion Order\n");
// insertion order
for(Customer cust : al){
System.out.println(cust.customerId + " "
+ cust.customerName);
}
// sorting using Collections.sort(al);
Collections.sort(al);
System.out.println("\n\nAfter Sorting:"
+ " Natural ordering of Customer Name\n");
// natural ordering of customer name using Comparable
for(Customer cust : al){
System.out.println(cust.customerId + " "
+ cust.customerName);
}
}
}
Output:
Before Sorting: Insertion Order
101 Narayan Murthy
107 Dinesh
103 Nandan Nilekeni
102 Ashok Arora
104 Shibulal
106 Kris Gopalakrishnan
105 Raghavan
After Sorting: Natural ordering of Customer Name
102 Ashok Arora
107 Dinesh
106 Kris Gopalakrishnan
103 Nandan Nilekeni
101 Narayan Murthy
105 Raghavan
104 Shibulal
3. Customized Sorting using Comparator interface
Method signature: public static void sort(List<Object> list, Comparator<Object> c)
Customer.java
- Customer POJO with 2 member variables of Integer and String type
- 2-arg parameterized constructor
- Overriding toString() method
package in.bench.resources.java.collection.sorting.arraylist;
public class Customer implements Comparable<Customer> {
// member variables
int customerId;
String customerName;
// 2-arg parameterized constructor
// override toString() method
}
CustomerIdComparator.java
- This is separate class which implements Comparator interface providing customized sorting logic
- compare(); method provides reverse order sorting logic, according to customer Id
package in.bench.resources.java.collection.sorting.arraylist;
import java.util.Comparator;
public class CustomerIdComparator implements Comparator<Customer> {
@Override
public int compare(Customer o1, Customer o2) {
return o2.customerId - o1.customerId;
}
}
Main class
- This class uses above customer POJO to store/save/add objects inside ArrayList
- which implements comparator interface providing customized sorting logic class
- i.e.; reverse ordering of customer Id
- finally prints customer objects in descending order of customer Id
ArrayListCustomizedSortingOfObjectType.java
package in.bench.resources.java.collection.sorting.arraylist;
import java.util.ArrayList;
import java.util.Collections;
public class ArrayListCustomizedSortingOfObjectType {
public static void main(String[] args) {
// creating ArrayList object of type Customer
ArrayList<Customer> al = new ArrayList<Customer>();
// adding elements to TreeSet object
al.add(new Customer(101, "Narayan Murthy"));
al.add(new Customer(107, "Dinesh"));
al.add(new Customer(103, "Nandan Nilekeni"));
al.add(new Customer(102, "Ashok Arora"));
al.add(new Customer(104, "Shibulal"));
al.add(new Customer(106, "Kris Gopalakrishnan"));
al.add(new Customer(105, "Raghavan"));
System.out.println("Before Sorting:"
+ " Insertion Order\n");
// insertion order
for(Customer cust : al){
System.out.println(cust.customerId + " "
+ cust.customerName);
}
// sorting using Collections.sort(al, comparator);
Collections.sort(al, new CustomerIdComparator());
System.out.println("\n\nAfter Sorting:"
+ " Reverse ordering of Customer Id\n");
// reverse ordering of customer Id using Comparator
for(Customer cust : al){
System.out.println(cust.customerId + " "
+ cust.customerName);
}
}
}
Output:
Before Sorting: Insertion Order
101 Narayan Murthy
107 Dinesh
103 Nandan Nilekeni
102 Ashok Arora
104 Shibulal
106 Kris Gopalakrishnan
105 Raghavan
After Sorting: Reverse ordering of Customer Id
107 Dinesh
106 Kris Gopalakrishnan
105 Raghavan
104 Shibulal
103 Nandan Nilekeni
102 Ashok Arora
101 Narayan Murthy
References:
- https://docs.oracle.com/javase/tutorial/collections/intro/
- https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
- https://docs.oracle.com/javase/tutorial/collections/interfaces/list.html
- https://docs.oracle.com/javase/7/docs/api/java/util/List.html
- https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html
Happy Coding !!
Happy Learning !!
