[백준] 1181번 단어 정렬

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;

public class Main {
	public static void main(String[] args) throws NumberFormatException, IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int N = Integer.parseInt(br.readLine());
		
		HashSet<String> set = new HashSet<>();
		for (int i = 0; i < N; i++) {
			set.add(br.readLine());
		}
		ArrayList<String> list = new ArrayList<>(set);
		Collections.sort(list, new Comparator<String>() {
			@Override
			public int compare(String o1, String o2) {
				if(o1.length()>o2.length()) {
					return 1;
				}else if(o1.length()<o2.length()) {
					return -1;
				}else {
					return o1.compareTo(o2);
				}
			}
		});
		for(String s : list) {
			System.out.println(s);
		}
	}
}

포인트 : Arraylist는 Collections.sort(list, new Comparator<String>){} 으로 처리할 수 있다.