Java Collections frequency() Method

The frequency() method of Java Collections class is used to get the number of elements in the specified collection equal to the specified object.

Syntax

Following is the declaration of frequency() method:

Parameter

Parameter	Description	Required/Optional
c	It is the collection in which to determine the frequency of obj.	Required
obj	It is the object whose frequency is to be determined.	Required

Returns

The frequency() method returns the number of elements in the collections c equal to the specified object obj.

Exceptions

NullPointerException- This exception will be thrown if collection c is null.

Compatibility Version

Java 1.5 and above

Example 1

import java.util.*;
public class CollectionsFrequencyExample1 {
	public static void main(String[] args) {
	    //Create a list object       
	    List<String> arrlist = new ArrayList<String>();
	    //Add elements in the list
	    arrlist.add("Java");
	    arrlist.add("COBOL");
          arrlist.add("Java");
          arrlist.add("C++");
	    arrlist.add("Java");
	    System.out.println("List of elements: "+arrlist);
	    //Count the frequency of the given word    
	    System.out.println("Frequency of the Word: "+Collections.frequency(arrlist,"Java"));
	    }	  
}

Test it Now

Output:

List of elements: [Java, COBOL, Java, C++, Java]
Frequency of the Word: 3

Example 2

import java.util.*;
public class CollectionsFrequencyExample2 {
	public static void main(String[] args) {
	      //Create a list object       
		List<Integer> list = new ArrayList<>();
            list.add(1);
            list.add(2);
            list.add(1);
            list.add(3);
            list.add(2);
            list.add(3);
            list.add(4);     
	      System.out.println("List of elements: "+list);
	      System.out.println("\nCount all with frequency:");
		Set<Integer> uniqueSet = new HashSet<Integer>(list);
		for (Integer i : uniqueSet) {
		     System.out.println(i + ": " + Collections.frequency(list, i));
		}
	}	  
}

Test it Now

Output:

List of elements: [1, 2, 1, 3, 2, 3, 4]
Count all with frequency:
1: 2
2: 2
3: 2
4: 1

Example 3

import java.util.*;
public class CollectionsFrequencyExample3 {
	public static void main(String[] args) {
		// Let us create an array of integers
            Integer arr[] = {20, 10, 20, 30, 20, 40, 20};        
            int freq = Collections.frequency(Arrays.asList(arr), 20);
            System.out.println("Frequency of 20 is: "+freq);		
	     }	  
}

Test it Now

Output: