Java - How to count duplicated items in a List
In this section, we will show you how to count duplicated items in a List,using Collections.frequency and Map.
Main.java
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 Main {
public static void main(String[] args) {
List<String> list = new ArrayList<>();
list.add("Java");
list.add("Kotlin");
list.add("Python");
list.add("PHP");
list.add("Angular");
list.add("Java");
list.add("Kotlin");
list.add("React");
list.add("PHP");
System.out.println("\nExample 1 - Count 'Java' with frequency");
System.out.println("Java : " + Collections.frequency(list, "Java"));
System.out.println("\nExample 2 - Count all with frequency");
Set<String> uniqueSet = new HashSet<>(list);
for (String temp : uniqueSet) {
System.out.println(temp + ": " + Collections.frequency(list, temp));
}
System.out.println("\nExample 3 - Count all with Map");
Map<String, Integer> 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("\nSorted Map");
Map<String, Integer> treeMap = new TreeMap<>(map);
printMap(treeMap);
}
public static void printMap(Map<String, Integer> map){
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println("Key : " + entry.getKey() + " Value : "
+ entry.getValue());
}
}
}
Console Output:
Example 1 - Count 'Java' with frequency a : 2 Example 2 - Count all with frequency Java: 2 PHP: 2 React: 1 Angular: 1 Kotlin: 2 Python: 1 Example 3 - Count all with Map Key : Java Value : 2 Key : PHP Value : 2 Key : React Value : 1 Key : Angular Value : 1 Key : Kotlin Value : 2 Key : Python Value : 1 Sorted Map Key : Angular Value : 1 Key : Java Value : 2 Key : Kotlin Value : 2 Key : PHP Value : 2 Key : Python Value : 1 Key : React Value : 1
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