Java HashMap의 자주 사용되는 메소드

HashMap은 카운터가 필요할 때 매우 유용합니다.

HashMap<String, Integer> countMap = new HashMap<String, Integer>();   //.... a lot of a's like the following if(countMap.keySet().contains(a)){ countMap.put(a, countMap.get(a)+1); }else{ countMap.put(a, 1); }

1. HashMap을 통한 루프

Iterator it = mp.entrySet().iterator(); while (it.hasNext()) { Map.Entry pairs = (Map.Entry)it.next(); System.out.println(pairs.getKey() + " = " + pairs.getValue()); }

Map<Integer, Integer> map = new HashMap<Integer, Integer>(); for (Map.Entry<Integer, Integer> entry : map.entrySet()) { System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue()); }

2. HashMap 인쇄하기

public static void printMap(Map mp) { Iterator it = mp.entrySet().iterator(); while (it.hasNext()) { Map.Entry pairs = (Map.Entry)it.next(); System.out.println(pairs.getKey() + " = " + pairs.getValue()); it.remove(); // avoids a ConcurrentModificationException } }

3. 값으로 HashMap 정렬

class ValueComparator implements Comparator<String> {   Map<String, Integer> base;   public ValueComparator(Map<String, Integer> base) { this.base = base; }   public int compare(String a, String b) { if (base.get(a) >= base.get(b)) { return -1; } else { return 1; } // returning 0 would merge keys } }

HashMap<String, Integer> countMap = new HashMap<String, Integer>(); //add a lot of entries countMap.put("a", 10); countMap.put("b", 20);   ValueComparator vc = new ValueComparator(countMap); TreeMap<String,Integer> sortedMap = new TreeMap<String,Integer>(vc);   sortedMap.putAll(countMap);   printMap(sortedMap);