Java.util.Collections.frequency() in Java

Last Updated : 26 Dec, 2025

Collections.frequency() method in Java is a utility method provided by the java.util.Collections class. It is used to count how many times a specified element occurs in a given collection.

  • This method compares elements using the equals() method, so proper implementation of equals() is important, especially for custom objects.
  • Throws NullPointerException if the specified collection is null.

Example:

Java 
import java.util.*;
public class GFG {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("code");
        list.add("code");
        list.add("quiz");
        list.add("code");
        int freq = Collections.frequency(list, "code");
        System.out.println("The frequency of the word code is: " + freq);
    }
}

Output
The frequency of the word code is: 3

Explanation:

  • The list contains the word "code" three times.
  • Collections.frequency(list, "code") counts the number of occurrences of "code" in the list using the equals() method.

Syntax

public static int frequency(Collection<?> c, Object o)

Parameters:

  • c: The collection in which the frequency of the object is to be determined.
  • o: The object whose frequency is to be counted.

Return Value: Returns an int representing the number of times the specified object appears in the collection.

Using Collections.frequency() with Custom Objects

For custom-defined objects, Collections.frequency() works only if the equals() method is properly overridden. This ensures that two objects are compared based on their content rather than their memory reference.

Example: This code demonstrates how to use Collections.frequency() to count how many times a specific element appears in a list.

Java 
import java.util.*;
public class GFG {
    public static void main(String[] args) {
        ArrayList<Student> list = new ArrayList<>();
        list.add(new Student("Ram", 19));
        list.add(new Student("Ashok", 20));
        list.add(new Student("Ram", 19));
        list.add(new Student("Ashok", 19));

        int freq = Collections.frequency(list, new Student("Ram", 19));
        System.out.println("The frequency of the Student Ram, 19 is: " + freq);
    }
}

class Student {
    private String name;
    private int age;

    Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
    @Override
    public boolean equals(Object o) {
        if (!(o instanceof Student)) {
            return false;
        }
        Student s = (Student) o;
        return this.name.equals(s.name) && this.age == s.age;
    }
}

Output
The frequency of the Student Ram, 19 is: 2

Explanation:

  • list contains two Student objects with name "Ram" and age 19.
  • equals() method is overridden to compare students based on name and age.
  • Collections.frequency() uses this logic to count matching objects and returns 2.
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