Java.util.Collections.frequency() in Java

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:

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.

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.

Related Topics:

• Java Collections Framework
• equals() method in Java
• hashCode() method in Java