Java Collections frequency() Method

Description

The Java Collections frequency(Collection<?>, Object) method is used to get the number of elements in the specified collection equal to the specified object.

Declaration

Following is the declaration for java.util.Collections.frequency() method.

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

Parameters

c − This is the collection in which to determine the frequency of o.
o − This is the object whose frequency is to be determined.

Return Value

Exception

NullPointerException − This is thrown if c is null.

Counting occurence of an Element in a List of Integers Example

The following example shows the usage of Java Collection frequency(Collection,Object) method. We've created a List object with some integers, printed the original list. Using frequency(Collection, T) method, we've printed the count of a particular element.

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CollectionsDemo {

   public static void main(String[] args) {
      List<Integer> list = new ArrayList<>(Arrays.asList(1,2,3,3,5,3));

      System.out.println("Initial collection value: " + list);
      // print the frequency of 3.
      int result = Collections.frequency(list, 3);
      System.out.println("3 is present: "+result + " times.");
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Initial collection value: [1, 2, 3, 3, 5, 3]
3 is present: 3 times.

Counting occurence of an Element in a List of Strings Example

The following example shows the usage of Java Collection frequency(Collection,Object) method. We've created a List object with some strings, printed the original list. Using frequency(Collection, T) method, we've printed the count of a particular element.

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CollectionsDemo {

   public static void main(String[] args) {
      List<String> list = new ArrayList<>(Arrays.asList("A","B","C","D","C","C"));

      System.out.println("Initial collection value: " + list);
      // print the frequency of C.
      int result = Collections.frequency(list, "C");
      System.out.println("C is present: "+result + " times.");
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Initial collection value: [A, B, C, D, C, C]
C is present: 3 times.

Counting occurence of an Element in a List of Objects Example

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CollectionsDemo {

   public static void main(String[] args) {
      List<Student> list = new ArrayList<>(Arrays.asList(new Student(1, "Julie"),
         new Student(2, "Robert"), new Student(3, "Adam"),new Student(1, "Julie")));

      System.out.println("Initial collection value: " + list);
      // print the frequency of Julie.
      int result = Collections.frequency(list, new Student(1, "Julie"));
      System.out.println("Julie is present: "+result + " times.");
   }
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   
   @Override
   public boolean equals(Object obj) {
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Initial collection value: [[ 1, Julie ], [ 2, Robert ], [ 3, Adam ], [ 1, Julie ]]
Julie is present: 2 times.

java_util_collections.htm