웹프로그래밍/Java
Java HashMap의 자주 사용되는 메소드
by Seras
2017. 11. 22.
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);
}
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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); |