[JAVA API] java.util.ArrayList<E>

java.util

Class ArrayList<E>

• java.lang.Object
• • java.util.AbstractCollection<E>
  • • java.util.AbstractList<E>
    • • java.util.ArrayList<E>

• All Implemented Interfaces:

  Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess

  Direct Known Subclasses:

  AttributeList, RoleList, RoleUnresolvedList

  ---

  
  

  public class ArrayList<E>
  extends AbstractList<E>
  implements List<E>, RandomAccess, Cloneable, Serializable

  Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null. In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. (This class is roughly equivalent to Vector, except that it is unsynchronized.)

  The size, isEmpty, get, set, iterator, and listIterator operations run in constant time. The add operation runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.

  Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.

  An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.

  Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be "wrapped" using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

     List list = Collections.synchronizedList(new ArrayList(...));

  The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

  Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

  This class is a member of the Java Collections Framework.

  Since:

  1.2

  See Also:

  Collection, List, LinkedList, Vector, Serialized Form

• • Field Summary

    • Fields inherited from class java.util.AbstractList

      modCount

  • Constructor Summary

    Constructors 

    Constructor and Description
    ArrayList() Constructs an empty list with an initial capacity of ten.
    ArrayList(Collection<? extends E> c) Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.
    ArrayList(int initialCapacity) Constructs an empty list with the specified initial capacity.

  • Method Summary

    Methods 

    Modifier and Type	Method and Description
    boolean	add(E e) Appends the specified element to the end of this list.
    void	add(int index, E element) Inserts the specified element at the specified position in this list.
    boolean	addAll(Collection<? extends E> c) Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's Iterator.
    boolean	addAll(int index, Collection<? extends E> c) Inserts all of the elements in the specified collection into this list, starting at the specified position.
    void	clear() Removes all of the elements from this list.
    Object	clone() Returns a shallow copy of this ArrayList instance.
    boolean	contains(Object o) Returns true if this list contains the specified element.
    void	ensureCapacity(int minCapacity) Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.
    E	get(int index) Returns the element at the specified position in this list.
    int	indexOf(Object o) Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.
    boolean	isEmpty() Returns true if this list contains no elements.
    Iterator<E>	iterator() Returns an iterator over the elements in this list in proper sequence.
    int	lastIndexOf(Object o) Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element.
    ListIterator<E>	listIterator() Returns a list iterator over the elements in this list (in proper sequence).
    ListIterator<E>	listIterator(int index) Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list.
    E	remove(int index) Removes the element at the specified position in this list.
    boolean	remove(Object o) Removes the first occurrence of the specified element from this list, if it is present.
    boolean	removeAll(Collection<?> c) Removes from this list all of its elements that are contained in the specified collection.
    protected void	removeRange(int fromIndex, int toIndex) Removes from this list all of the elements whose index is between fromIndex, inclusive, and toIndex, exclusive.
    boolean	retainAll(Collection<?> c) Retains only the elements in this list that are contained in the specified collection.
    E	set(int index, E element) Replaces the element at the specified position in this list with the specified element.
    int	size() Returns the number of elements in this list.
    List<E>	subList(int fromIndex, int toIndex) Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive.
    Object[]	toArray() Returns an array containing all of the elements in this list in proper sequence (from first to last element).
    <T> T[]	toArray(T[] a) Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.
    void	trimToSize() Trims the capacity of this ArrayList instance to be the list's current size.

    • Methods inherited from class java.util.AbstractList

      equals, hashCode

    • Methods inherited from class java.util.AbstractCollection

      containsAll, toString

    • Methods inherited from class java.lang.Object

      finalize, getClass, notify, notifyAll, wait, wait, wait

    • Methods inherited from interface java.util.List

      containsAll, equals, hashCode

ArrayList 사용 예제

import java.util.*;

class Ex103_ArrayList 
{
	public static void main(String[] args)
	{
		//Ex103_ArrayList.java
		//m1();					//1. 기본 ArrayList
		//m2();					//2. 제넥릭 ArrayList
		//m3();					//3. 항목 추가,삽입, 삭제
		m4();					//4. ArrayList의 배열 생성 개념
	}

	//1. 기본 ArrayList
	public static void m1() {
		//컬렉션(배앨과 유사)
		//생성(길이미정) -> 데이터를 집어넣을때마다 
		//	자동으로 배열의 길이가 증가

		ArrayList list1 = new ArrayList();

		list1.add(100);	//데이터 입력
		list1.add(200);
		list1.add(300);

		//리스트의 길이(배열의 length)
		//	-> ArrayList의 요소의 갯수(들어가있는 데이터의 총 갯수)
		System.out.println(list1.size());

		//데이터 출력
		//	- get(index)
		System.out.println(list1.get(0));	//0 ~ size()-1

		//ArrayList 탐색
		for(int i=0; i< list1.size(); i++) {
			System.out.print(list1.get(i) + "\t");
		}
		System.out.println();

		for (Object n : list1) {
			System.out.println(n);
		}

		int num = (int)list1.get(1);	//200
		System.out.println(num*2);
	}//end m1()

	//2. 제넥릭 ArrayList
	public static void m2() {
		//컬렉션
		//제네릭컬렉션
		ArrayList list1 = new ArrayList();	//Object배열
		ArrayList list2 = new ArrayList(); //int배열
		
		/*
			class ArrayList
			{
				private Object[] list;
			}
			
			class ArrayList
			{
				private T[] list;
			}
		*/

		for(int i=1; i<= 10; i++) {
			list2.add(i);	//int형
		}

		for(int i=0; i< 10; i++) {
			System.out.println(list2.get(i) * 2);
		}

		for( int result : list2 )
			System.out.println(result * 2);

		ArrayList list3 = new ArrayList();
		list3.add("홍길동");
		list3.add("하하하");

		ArrayList list4 = new ArrayList();
		list4.add(true);
		list4.add(false);

		System.out.println(list2.size());	//요소의 총갯수

		list2.clear();	//Removes all of the elements from this list

		for(int i=1; i<= 10; i++)
			list2.add(i);

		//boolean contains(element)
		//	- 배열안에 찾고자하는 요소가 들어있습니까?
		System.out.println(list2.contains(11));
		System.out.println(userContains(list2, 5));

		//int indexOf(Object o)
		System.out.println(list2.indexOf(5));
		System.out.println(list2.lastIndexOf(8));

		//size() -> 0
		//boolean isEmpty()
		System.out.println(list2.isEmpty());
		list2.clear();
		System.out.println(list2.isEmpty());

	}//end m2()

	public static boolean userContains(ArrayList list, int item) {
		for(int i=0; i< list.size(); i++) {
			if(list.get(i) == item) {
				return true;
			}
		}
		return false;
	}

	//3. 항목 추가,삽입, 삭제
	public static void m3() {
		ArrayList list = new ArrayList();

		list.add("홍길동");
		list.add("아무게");
		list.add("하하하");
		list.add("이순신");
		list.add("호호호");
		list.add("하하하");

		
		//void remove(Object o)
		//	- 배열에서 항목을 삭제
		for(int i=0; i< list.size(); i++) {
			System.out.printf("list.get(%d) = \"%s\"\n",i,list.get(i));
		}

		//"하하하"가 2개 이상있으면 제일 빠른 인덱스 하나만 삭제!
		list.remove("하하하");

		System.out.println("하하하 삭제 후");
		for(int i=0; i< list.size(); i++) {
			System.out.printf("list.get(%d) = \"%s\"\n",i,list.get(i));
		}
		
		System.out.println();
		System.out.println("클리어");
		System.out.println();

		list.add("가가가");
		list.add("나나나");
		list.add("다다다");
		list.add("라라라");
		//void add(Object o) : Append모드(추가하기)
		//void add(int index, Object o) : Insert모드(삽입하기)
		list.add(2,"추가");
		for(int i=0; i< list.size(); i++) {
			System.out.printf("list.get(%d) = \"%s\"\n",i,list.get(i));
		}

		//수정하기
		//void set(int index, Object o)
		System.out.println();
		System.out.println("수정하기");

		list.set(2, "수정");
		for(int i=0; i< list.size(); i++) {
			System.out.printf("list.get(%d) = \"%s\"\n",i,list.get(i));
		}

		System.out.println();
		System.out.println("배열로 변환하기");
		//ArrayList -> 배열로 변환
		//toArray() 메서드

		//1. 미리 배열 생성 후 복사
		String[] copy = new String[list.size()];	//원본길이
		list.toArray(copy);	//ArrayList => Array로 복사

		for(String name : copy)
			System.out.println(name);

		//2. Object 배열로 복사 후 다운캐스팅
		Object[] copy2 = list.toArray();
		System.out.println((String)copy2[0]);

	}//end m3()
	
	//4. ArrayList의 배열 생성 개념
	public static void m4() {

		//용량 지정시 ArrayList내부의 배열이 생성되고 복사되며
		//	낭비되는 배열들을 아낄수 있다.
		ArrayList list = new ArrayList(100);

		for(int i=1; i<=100; i++) {
			list.add(i);
		}

		//size() - 배열의 전체 크기가 아님
		//	- 이미 존재하는 방에 채워져 있는 요소의 갯수

		System.out.println(list.size());

		//void trimToSize()
		//	- 현재 요소의 갯수의 크기로 배열의 크기를 재조정
		list.trimToSize();

	}//end m4
}