Count the number of times an element is repeated in ArrayList in Java

To get the number of the repetition or the frequency of appearance of an existing element in ArrayList, use the method frequency(Collection c, Object o) of the Collections.

int  frequency(Collection c, Object o): this method returns the number of elements in ArrayList that equal to the object o. It throws an exception NullPointerException if c is null.

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

public class ArrayListFrequency {

 public static void main(String[] args) {
 
 // Create an ArrayList of type String
 ArrayList< String> aList = new ArrayList< String> (); 
 
 //populate ArrayList with String
 aList.add("aa"); 
 aList.add("ab"); 
 aList.add("ac"); 
 aList.add("aa"); 
 aList.add("ab"); 
 aList.add("ab"); 
 aList.add("ac"); 
 
 System.out.println("ArrayList Elements "); 
 for(String e:aList)
 System.out.println(e); 

 /*frenquency() counts the number of occurrences*/
 int frequenceaa = Collections.frequency(aList, "aa"); 
 int frequenceab = Collections.frequency(aList, "ab"); 
 int frequenceac = Collections.frequency(aList, "ac"); 
 
 System.out.println("frequency(aa): "+frequencyaa); 
 System.out.println("frequency(ab): "+frequenceab); 
 System.out.println("frequency(ac): "+frequencyac); 
 }
}

Runtime:

ArrayList Elements 
aa
ab
ac
aa
ab
ab
ac
frequency(aa):  2
frequency(ab): 3
frequency(ac): 2

References:
Javadoc: the Collections.frenquency()