Use collections.frequency and map to compute the number of occurrences of duplicates in the Java list

This article demonstrates how to use collections.frequency and map to calculate the number of occurrences of a duplicate item. (Collections.frequency is supported after JDK version 1.5)

Package Com.qiyadeng.core;
Import java.util.ArrayList;
Import java.util.Collections;
Import Java.util.HashMap;
Import Java.util.HashSet;
Import java.util.List;
Import Java.util.Map;
Import Java.util.Set;
     
Import Java.util.TreeMap; public class Countduplicatedlist {public static void main (string[] args) {List List = new Arrayl
        IST ();
        List.add ("a");
        List.add ("B");
        List.add ("C");
        List.add ("D");
        List.add ("B");
        List.add ("C");
        List.add ("a");
        List.add ("a");
     
        List.add ("a");
        System.out.println ("\ n Example 1-Calculates the number of ' a ' occurrences");
     
        System.out.println ("A:" + collections.frequency (list, "a"));
        System.out.println ("\ n Example 2-calculates the number of occurrences of all objects");
        Set uniqueset = new HashSet (list);
        for (String temp:uniqueset) {System.out.println (temp + ":" + collections.frequency (list, temp));
    } System.out.println ("\ n Example 3-the number of occurrences of the object is computed with a map");    Map map = new HashMap ();
            for (String temp:list) {Integer count = map.get (temp);
        Map.put (temp, (count = = null)? 1:count + 1);
     
        } printmap (map);
        System.out.println ("\nmap sort-Sort by key");
        Map TreeMap = new TreeMap (map);
     
    Printmap (TREEMAP); public static void Printmap (map map) {for (Map.entry Entry:map.entrySet ()) {System.out
        . println ("Key-value:" + entry.getkey () + "-" + entry.getvalue ()); }
    }
}

Output results

Example 1-Calculates the number of ' a ' occurrences
a:4
Example 2-Calculates the number of occurrences of all objects
d:1
b:2
c:2
a:4
Example 3-Calculates the number of occurrences of an object with a map
Key-valu e:d-1
key-value:b-2
key-value:c-2
key-value:a-4
Map sort-sorted by key
key-value:a-4
key -value:b-2
key-value:c-2
key-value:d-1

Source: http://www.qiyadeng.com/