Collections.frequency(Collection, Object) in Java

Collections.frequency(Collection, Object): This method is available in java.util.Collections class of Java.

Syntax:

int java.util.Collections.frequency(Collection<?> c, Object o)

This method takes two arguments. This method returns the number of elements in the specified collection equal to the specified object.

Parameters: Two parameters are required for this method.

c: the collection in which to determine the frequency of o.

o: the object whose frequency is to be determined.

Returns: the number of elements in c equal to o.

Throws:

NullPointerException - if c is null.

Approach 1: When no exception

Java

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

public class Collectionsfrequency {
    public static void main(String[] args) {

        List<Integer> arrlist = new ArrayList<Integer>();

        arrlist.add(12);
        arrlist.add(10);
        arrlist.add(10);

        arrlist.add(11);
        arrlist.add(10);

        System.out.println(Collections.frequency(arrlist, 10));

    }
}

Output:

3

Approach 2: NullPointerException 

Java

import java.util.Collections;
import java.util.List;

public class Collectionsfrequency {
    public static void main(String[] args) {

        List<Integer> arrlist = null;

        System.out.println(Collections.frequency(arrlist, 10));

    }
}

Output:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.Collection.iterator()" because "c" is null
	at java.base/java.util.Collections.frequency(Collections.java:5465)