Interview_Collection Framework, Thread

✅ JCF란 무엇인가요?

• Java collection framework
• handle data efficiently

✅ JCF의 계층 구조를 설명해주세요

• Collection interface: List, Queue, Set
• Map: Map

✅ Map 인터페이스는 왜 Collection 인터페이스에 상속을 받지 않았나요?

• Map has key + value
• List , Queue, Set does not

✅ List 인터페이스는 무엇이고, 구현체의 종류는 무엇이 있나요?

• has order
• allow duplication
• used for order

• flexible in size

• List 🆚 Array: List is flexible in size

• Arraylist
• LinkedList
• Vector

✅ ArrayList에 대해 설명해주세요

List ➕ Array

• list that uses array, but flexible in size
• default size: 10
• if bigger size ArrayList is needed, increase capacity by 50%
• 👎🏻 some space might not be used ➡️ waste memory

✅ ArrayList의 시간 복잡도

• get with index: O(1)
• add, delete last: O(1)
• add, delete middle: O(n), 추가/삭제하고 나머지 다 이동시켜야 함
• search: O(n)

✅ ArrayList는 어떻게 동적으로 사이즈가 늘어나나요?

• default size: 10
• if bigger size ArrayList is needed, increase capacity by 50%
• 👎🏻 some space might not be used ➡️ waste memory

✅ LinkedList에 대해 설명해주세요

• List ➕ Node
• has data + pointer
• pointer references before, next node
• 👍🏻 does not waste memory

✅ LinkedList의 시간 복잡도

• get with index: O(n): node is not linear
• add, delete: O(1), 추가/삭제하고 앞, 뒤만 참조값 바꾸면 끝
• search: O(n)

✅ 언제 ArrayList를 사용하고, 언제 LinkedList를 사용할까요?

• ArrayList: fast for get value with index
• LinkedList: add, delete data is important, make perfect size of list, when a lot of change is made to List

✅ add를 보면 시간복잡도가 똑같은데 왜 LinkedList가 효율적이나

• ArrayList has to change the whole list after adding/deleting
• LinkedList only has to change surrounding(before, next) nodes

✅ ArrayList와 Vector는 어떠한 차이가 있나요?

• Vector: all methods are synchronized, multi thread safe
• ArrayList: not synchronized, single thread

✅ Stack과 Queue가 무엇인가요?

• Stack: LIFO, push(), pop()
• Queue: FIFO, offer(), poll()
• Deque: double end Queue, has all methods of Stack and Queue
• Deque is faster than Stack

✅ Set이 무엇이고, 구현 클래스가 무엇이 있는지 설명해 주세요.

• no order
• no duplication

• used for search

• HashSet
• LinkedHashSet
• TreeSet

✅ Set에서 중복 요소를 어떻게 걸러내는지 설명해 주세요

• if do equals(), hashCode() is same, then considered as same

• before saving data, compare hashCode with equals()

• if hashCode is the same, does not add to set

• hashCode(): get the hashCode of object
• equals(): check if hashCode is same or not

✅ What is hash?

• use hash function to take an input, and change into fixed-size value(called hash code)
• hash code is used to decide where in memory the object should be saved

💥 What is hash collision?

• when hash code is same
• 💊 create another new array inside array

✅ What is Hash set?

• no duplication
• no order
• must override equals(), hashCode()

• 💥 Rehashing: if hash collision occurs frequently ➡️ create bigger array

• add, delete: O(1)
• search: O(1)
• get all hash set: return in random order

✅ What is Linked Hash set?

• Hash set ➕ LinkedList(order)
• has node, know reference of before, next node
• get all linked hash set: return in input order

✅ What is Tree set?

• binary tree
• can have order
• add, delete: O(logN)
• search: O(logN)
• get all: has ascending order, uses Comparator

✅ Map이 무엇이고, 구현 클래스가 무엇이 있는지 설명해 주세요.

• key ➕ value
• no order
• key cannot be duplicate

• value can be duplicate

• HashMap
• LinkedHashMap
• TreeMap

✅ HashMap은 어떻게 동작하나요?

• use hash code() as key

• must override equals(), hashCode()

• add, delete: O(1)
• search: O(1)
• get all: return in random order

✅ HashMap의 최악의 시간 복잡도를 설명해 주세요

• when Hash Collision occurs
• HashMap has hash chaining
• hash chaining: when hashCode is same, new array is created to save data
• when hash collision occurs, has to check all array ➡️ O(n)

Key 값은 다르지만 해시 함수에 의해 같은 출력 값을 갖는 현상을 해시 충돌이라고 하는데, HashMap은 이를 해시 체이닝을 통해 해결하고 있습니다. 만일, 한 개의 버킷에 대해 모두 해시 충돌이 발생하면 연결 리스트로 모두 탐색해야함으로 조회 시간 복잡도가 O(N)이 될 수 있습니다.

✅ Linked Hash Map

• HashMap ➕ LinkedList(order)
• add, delete: O(1)
• search: O(1)
• get all: input order

✅ Tree Map

• HashMap ➕ Tree
• add, delete: O(logN)
• search: O(logN)
• get all: key ascending order(SortedMap)

✅ iterable과 iterator의 차이

• iterable interface: collection implements iterable
• for(int i=0; i<n; i++)
• array: with index
• node: with reference

• tree: with parent, child node

• iterator: tool to iterate collection
• hasNext(): 다음 값이 있니?
• next(): 다음으로 간다

✅ Collection과 Collections의 차이는 무엇인가요?

• Collection interface: highest parent of JCF

• Collections: utility class to manipulate classes that implement Collection interface
• max()
• shuffle()
• sort()
• reverse

✅ Java에서 스레드를 만드는 방법을 설명해 주세요.

• 두 가지 방법이 있다
• 1️⃣ with thread class class MyThread extends Thread
• 2️⃣ with runnable interface static class MyRunnable implements Runnable

Thread class 🆚 Runnable interface

• thread class: only can extend thread class
• runnable interface: can implement several interfaces

✅ 스레드를 생성하여 start 메소드를 호출했는데 왜 run 메소드가 호출되는지 이유

• start(): create new thread, and call run() method
• run(): new thread is not created, but run on thread that is running now

✅ 스레드 풀이란 무엇이고, 왜 사용할까요?

• limit the number thread
• prevent repreated thread creation/removal
• provide safe multi threading environment

✅ 스프링과 같은 프레임워크에서는 스레드 풀의 스레드 개수를 수백 개 이상으로 운영합니다. 이는 Context Switching이 일어남에도 불구하고도 이런 선택을 내린 것인데, 왜 그럴까요?

• in web applications network calls, database requests are handled in Blocking I/O method
• with many threads, even when one thread is blocked waiting would be minimized
• can respond several client requests at the same time
• make efficient use of CPU

✅ Context Switching란?

• CPU can only do one job at once
• context switching to look like it is doing many jobs at the same time

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