TreeMap: 정렬된 키-값 쌍

TreeMap이란 무엇인가요?

import java.util.TreeMap;

TreeMap<String, Integer> scores = new TreeMap<>();
scores.put("Charlie", 85);
scores.put("Alice", 92);
scores.put("Bob", 78);

// Iteration is in key order: Alice, Bob, Charlie
for (var entry : scores.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

firstKey, lastKey, floorKey, ceilingKey

TreeMap<Integer, String> map = new TreeMap<>();
map.put(10, "ten"); map.put(20, "twenty"); map.put(30, "thirty"); map.put(40, "forty");

System.out.println(map.firstKey());       // 10
System.out.println(map.lastKey());        // 40
System.out.println(map.floorKey(25));     // 20 (largest key ≤ 25)
System.out.println(map.ceilingKey(25));   // 30 (smallest key ≥ 25)
System.out.println(map.lowerKey(20));     // 10 (strictly less)
System.out.println(map.higherKey(20));    // 30 (strictly greater)

Entry 탐색

TreeMap<Integer, String> prices = new TreeMap<>();
prices.put(100, "Budget"); prices.put(300, "Standard"); prices.put(700, "Premium");

var entry = prices.floorEntry(350);
System.out.println(entry.getKey() + ": " + entry.getValue()); // 300: Standard

var top = prices.lastEntry();
System.out.println(top.getValue()); // Premium

subMap, headMap, tailMap

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 1; i <= 10; i++) map.put(i, "v"+i);

// Keys from 3 (inclusive) to 7 (exclusive)
System.out.println(map.subMap(3, 7));   // {3=v3, 4=v4, 5=v5, 6=v6}

// Keys strictly less than 5
System.out.println(map.headMap(5));     // {1=v1, 2=v2, 3=v3, 4=v4}

// Keys >= 7
System.out.println(map.tailMap(7));     // {7=v7, 8=v8, 9=v9, 10=v10}

포함 및 제외 경계

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 1; i <= 10; i++) map.put(i*10, "v"+i);

// From 30 (inclusive) to 60 (inclusive)
System.out.println(map.subMap(30, true, 60, true));
// {30=v3, 40=v4, 50=v5, 60=v6}

내림차순

TreeMap<String, Integer> tm = new TreeMap<>();
tm.put("A", 1); tm.put("C", 3); tm.put("B", 2);

for (String key : tm.descendingKeySet()) {
    System.out.print(key + " "); // C B A
}

pollFirstEntry와 pollLastEntry

TreeMap<Integer, String> events = new TreeMap<>();
events.put(8, "Breakfast");
events.put(12, "Lunch");
events.put(18, "Dinner");

var first = events.pollFirstEntry(); // removes 8=Breakfast
System.out.println(first.getValue() + " removed");
System.out.println(events.firstKey()); // 12

사용 사례: 순위표

TreeMap<Integer, String> leaderboard = new TreeMap<>(Comparator.reverseOrder());
leaderboard.put(1200, "Alice");
leaderboard.put(1500, "Bob");
leaderboard.put(900, "Carol");

int rank = 1;
for (var e : leaderboard.entrySet()) {
    System.out.println(rank++ + ". " + e.getValue() + " (" + e.getKey() + ")");
}
// 1. Bob (1500)
// 2. Alice (1200)
// 3. Carol (900)

사용 사례: 이벤트 스케줄러

import java.time.LocalTime;
TreeMap<LocalTime, String> schedule = new TreeMap<>();
schedule.put(LocalTime.of(9,0), "Standup");
schedule.put(LocalTime.of(14,0), "Review");
schedule.put(LocalTime.of(17,0), "Retro");

LocalTime now = LocalTime.of(11, 30);
var next = schedule.ceilingEntry(now);
System.out.println("Next: " + next.getValue()); // Review

TreeMap과 HashMap의 성능 비교

스레드 안전성

빠른 확인

복습: TreeMap