Collections.frequency() in Java

Java's Collections class provides a frequency() method that returns the number of times an element appears in a Collection. This method works for collections that implement the Collection interface, such as ArrayList, LinkedList, ArrayDeque, etc.

The method's declaration is:

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

Where c is the collection in which to determine the frequency of o, and o is the object whose frequency is to be determined.

The method returns the number of elements in the specified collection equal to the specified object.

Let's look at a few examples to see how the frequency() method works:

Example 1:

csharp
import java.util.*; 

public class Example1 { 
    public static void main(String[] args) 
    { 
        List<Integer> list = new ArrayList<Integer>(); 
        list.add(10); 
        list.add(20); 
        list.add(10); 

        // Count the frequency of 10
        System.out.println(Collections.frequency(list, 10)); 
    } 
} 

Output:

2

Example 2:

java
import java.util.*; 

class Point {
    int x, y;
    Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
    
    @Override
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof Point))
            return false;
        Point p = (Point)o;
        return p.x == x && p.y == y;
    }
}

public class Example2 { 
    public static void main(String[] args) 
    { 
        List<Point> list = new ArrayList<Point>(); 

        // Adding element to list
        list.add(new Point(10, 20));
        list.add(new Point(5, 25));
        list.add(new Point(10, 20));
        
        Point p = new Point(10, 20);
        
        int freq = Collections.frequency(list, p);

        // prining the frequency
        System.out.println(freq); 
    } 
} 

Output:

2

In Example 2, we are using a user-defined type Point and overriding the equals() method to ensure that the frequency() method can properly compare the elements in the list.

Example 3:

java
import java.util.*; 

class Example3 { 
    public static void main(String[] args) 
    { 
        Integer arr[] = {10, 15, 10, 20};
        
        // asList() creates a wrapper over the same array and converts the
        // array to a list
        int res = Collections.frequency(Arrays.asList(arr), 10);
        
        // Count the frequency of 10
        System.out.println(res); 
    } 
}

Output:

2

In Example 3, we are using an array of Integers and converting it to a List using the asList() method before calling the frequency() method.

Overall, the Collections.frequency() method is a useful tool for determining the frequency of elements in a collection, and can be used with both built-in and user-defined types.