Java Listで重複した項目を数える方法
Collections.frequencyとMapを使ってList内の重複エントリの総数を数える方法を示すJavaの例です。
CountDuplicatedList.java
package com.mkyong; 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<String> list = new ArrayList<String>(); 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("\nExample 1 - Count 'a' with frequency"); System.out.println("a : " + Collections.frequency(list, "a")); System.out.println("\nExample 2 - Count all with frequency"); Set<String> uniqueSet = new HashSet<String>(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<String, Integer>(); 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<String, Integer>(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()); } } }
出力
Example 1 - Count 'a' with frequency a : 4 Example 2 - Count all with frequency d: 1 b: 2 c: 2 a: 4 Example 3 - Count all with Map Key : d Value : 1 Key : b Value : 2 Key : c Value : 2 Key : a Value : 4 Sorted Map Key : a Value : 4 Key : b Value : 2 Key : c Value : 2 Key : d Value : 1
参考文献
JavaDoc]。リンク://java/how-to-sort-a-map-in-java/[Javaでマップをソートする方法]