Java Collection Classes

Java Collections class is a part of collection framework. This class is designed to provide methods for searching, sorting, copying etc. It consists exclusively of built-in static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections.

This class is located into java.util package. The declaration of this class is given below.

Collections Class Declaration

public class Collections extends Object

It inherits Object class and all the methods of this class throw a NullPointerException if the object is null.

Collections Methods

This table contains commonly used methods of Collections class.

Example: Sorting List

In this example, we are using sort() method of Collections class that is used to sort elements of a collection. Here we are using Arralist class that stores integer type object and sorted.

  
import java.util.*;  
public class Demo {  
    public static void main(String a[]){
      // Creating ArrayList
        ArrayList<Integer> list = new ArrayList<>();
        // Adding elements
        list.add(100);  
        list.add(2);  
        list.add(66); 
        list.add(22);
        list.add(10);
        // Displaying list
        System.out.println(list);
        // Sorting list
        Collections.sort(list);
        // Displaying sort data
        System.out.println(list);
    }  
}
  

[100, 2, 66, 22, 10] [2, 10, 22, 66, 100]

Example: Finding min and max elements

The collections class provides two methods max() and min() that can be used to fetch max and min values from a collection. See the below example.

  
import java.util.*;  
public class Demo {  
    public static void main(String a[]){
      // Creating ArrayList
        ArrayList<Integer> list = new ArrayList<>();
        // Adding elements
        list.add(100);  
        list.add(2);  
        list.add(66); 
        list.add(22);
        list.add(10);
        // Displaying list
        System.out.println(list);
        // Find min element
        int min = Collections.min(list);
        // Find max element
        int max = Collections.max(list);
        // Displaying data
        System.out.println("Minimum element : "+ min);
        System.out.println("Maximum element : "+ max);
    }  
}
  

[100, 2, 66, 22, 10] Minimum element : 2 Maximum element : 100

Example: Swapping Elements

To swap elements, we don’t need write logic code. Collections class provides built-in swap method that can be used to swap elements from one position to another in a collection. The swap() method takes three arguments: first is reference of object, second is index of first elements and third is index of second elements to be swapped. See the below example.

  
import java.util.*;  
public class Demo {  
    public static void main(String a[]){
      // Creating ArrayList
        ArrayList<Integer> list = new ArrayList<>();
        // Adding elements
        list.add(100);  
        list.add(2);  
        list.add(66); 
        list.add(22);
        list.add(10);
        // Displaying list
        System.out.println(list);
        // Swapping elements
        Collections.swap(list, 0, 4); // 100 is swapped by 10
        System.out.println("List after swapping : "+ list);
    }  
}
  

[100, 2, 66, 22, 10] List after swapping : [10, 2, 66, 22, 100]

Example: Reverse the list

Collections class provides a static method reverse() that is used to get a collection in reverse order. In the below example, we are getting list in reverse order using the reverse() method.

  
import java.util.*;  
public class Demo {  
    public static void main(String a[]){
      // Creating ArrayList
        ArrayList<Integer> list = new ArrayList<>();
        // Adding elements
        list.add(100);  
        list.add(2);  
        list.add(66); 
        list.add(22);
        list.add(10);
        // Displaying list
        System.out.println(list);
        // Reverse the list
        Collections.reverse(list);
        // Displaying data
        System.out.println("List in reverse order "+list);
    }  
}
  

[100, 2, 66, 22, 10] List in reverse order [10, 22, 66, 2, 100]