ArrayList 的一些認識:
- 非線程安全的動態數組(Array升級版),支持動態擴容
- 實現 List 接口、底層使用數組保存所有元素,其操作基本上是對數組的操作,允許null值
- 實現了 RandmoAccess 接口,提供了隨機訪問功能
- 線程安全可見Vector,實時同步
- 適用於訪問頻繁場景,頻繁插入或刪除場景請選用linkedList
■ 類定義
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Java.io.Serializable
- 繼承 AbstractList,實現了 List,它是一個數組隊列,提供了相關的添加、刪除、修改、遍歷等功能
- 實現 RandmoAccess 接口,實現快速隨機訪問:通過元素的序號快速獲取元素對象
- 實現 Cloneable 接口,重寫 clone(),能被克隆(淺拷貝)
- 實現 java.io.Serializable 接口,支持序列化
■ 全局變量
/** * The array buffer into which the elements of the ArrayList are stored. * The capacity of the ArrayList is the length of this array buffer. Any * empty ArrayList with elementData =http://www.cnblogs.com/romanjoy/p/= EMPTY_ELEMENTDATA will be expanded to * DEFAULT_CAPACITY when the first element is added. * ArrayList底層實現為動態數組; 對象在存儲時不需要維持,java的serialzation提供了持久化
* 機制,我們不想此對象被序列化,所以使用 transient */ private transient Object[] elementData;
/** * The size of the ArrayList (the number of elements it contains). * 數組長度 :註意區分長度(當前數組已有的元素數量)和容量(當前數組可以擁有的元素數量)的概念 * @serial */ private int size; /** * The maximum size of array to allocate.Some VMs reserve some header words in an array. * Attempts to allocate larger arrays may result in OutOfMemoryError: * Requested array size exceeds VM limit * 數組所能允許的最大長度;如果超出就會報`內存溢出異常` -- 可怕後果就是宕機 */ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
■ 構造器
/** * Constructs an empty list with the specified initial capacity. * 創建一個指定容量的空列表 * @param initialCapacity the initial capacity of the list * @throws IllegalArgumentException if the specified initial capacity is negative */ public ArrayList(int initialCapacity) { super(); if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); this.elementData = http://www.cnblogs.com/romanjoy/p/new Object[initialCapacity]; }
/** * Constructs an empty list with an initial capacity of ten. * 默認容量為10 */ public ArrayList() { this(10); }
/** * Constructs a list containing the elements of the specified collection, * in the order they are returned by the collection's iterator. * 接受一個Collection對象直接轉換為ArrayList * @param c the collection whose elements are to be placed into this list * @throws NullPointerException if the specified collection is null 萬惡的空指針異常 */ public ArrayList(Collection<? extends E> c) { elementData = c.toArray();//獲取底層動態數組 size = elementData.length;//獲取底層動態數組的長度 // c.toArray might (incorrectly) not return Object[] (see 6260652) if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); }
■主要方法
- add()
- 從源碼上看,ArrayList 一般在尾部插入元素,支持動態擴容
- 不推薦使用頻繁插入/刪除是因為在執行add()/remove() 會調用非常耗時的 system.arraycopy(),頻繁插入/刪除場景請選用 LinkedList
/** * Appends the specified element to the end of this list. * 使用尾插入法,新增元素插入到數組末尾 * 由於錯誤檢測機制使用的是拋異常,所以直接返回true * @param e element to be appended to this list * @return <tt>true</tt> (as specified by {@link Collection#add}) */ public boolean add(E e) { //調整容量,修改elementData數組的指向; 當數組長度加1超過原容量時,會自動擴容 ensureCapacityInternal(size + 1); // Increments modCount!! add屬於結構性修改 elementData[size++] = e;//尾部插入,長度+1 return true; }
/** * Inserts the specified element at the specified position in this list. * Shifts the element currently at that position (if any) and any subsequent elements to * the right (adds one to their indices). * 支持插入一個新元素到指定下標 * 該操作會造成該下標之後的元素全部後移(使用時請慎重,避免數組長度過大) * @param index index at which the specified element is to be inserted * @param element element to be inserted * @throws IndexOutOfBoundsException {@inheritDoc} */ public void add(int index, E element) { //下標邊界校驗,不符合規則 拋出 `IndexOutOfBoundsException` rangeCheckForAdd(index); //調整容量,修改elementData數組的指向; 當數組長度加1超過原容量時,會自動擴容 ensureCapacityInternal(size + 1); // Increments modCount!! //註意是在原數組上進行位移操作,下標為 index+1 的元素統一往後移動一位 System.arraycopy(elementData, index, elementData, index + 1,size - index); elementData[index] = element;//當前下標賦值 size++;//數組長度+1 }
- ensureCapacity() : 擴容,1.8 有個默認值的判斷
//1.8 有個默認值的判斷 public void ensureCapacity(int minCapacity) { int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) // any size if not default element table ? 0 // larger than default for default empty table. It's already // supposed to be at default size. : DEFAULT_CAPACITY; if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); } } private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); }
- set() / get(): 直接操作下標指針
/** * Replaces the element at the specified position in this list with * the specified element. * * @param index index of the element to replace * @param element element to be stored at the specified position * @return the element previously at the specified position * @throws IndexOutOfBoundsException {@inheritDoc} */ public E set(int index, E element) { rangeCheck(index); //檢測插入的位置是否越界 E oldValue = elementData(index); elementData[index] = element; return oldValue; }
/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
- remove(): 移除其實和add差不多,也是用的是 System.arrayCopy(...)
/** * Removes the element at the specified position in this list. * Shifts any subsequent elements to the left (subtracts one from their indices). * * 移除指定下標元素,同時大於該下標的所有數組元素統一左移一位 * * @param index the index of the element to be removed * @return the element that was removed from the list 返回原數組元素 * @throws IndexOutOfBoundsException {@inheritDoc} */ public E remove(int index) { rangeCheck(index);//下標邊界校驗 E oldValue = http://www.cnblogs.com/romanjoy/p/elementData(index);//獲取當前坐標元素 fastRemove(int index);//這裏我修改了一下源碼,改成直接用fastRemove方法,邏輯不變 return oldValue;//返回原數組元素 }
/** * Removes the first occurrence of the specified element from this list,if it is present. * If the list does not contain the element, it is unchanged. * More formally, removes the element with the lowest index <tt>i</tt> such that * <tt>(o==null?get(i)==null:o.equals(get(i)))</tt> (if such an element exists). * Returns <tt>true</tt> if this list contained the specified element * (or equivalently, if this list changed as a result of the call). * 直接移除某個元素: * 當該元素不存在,不會發生任何變化 * 當該元素存在且成功移除時,返回true,否則false * 當有重復元素時,只刪除第一次出現的同名元素 : * 例如只移除第一次出現的null(即下標最小時出現的null) * @param o element to be removed from this list, if present * @return <tt>true</tt> if this list contained the specified element */ public boolean remove(Object o) { //按元素移除時都需要按順序遍歷找到該值,當數組長度過長時,相當耗時 if (o == null) {//ArrayList允許null,需要額外進行null的處理(只處理第一次出現的null) for (int index = 0; index < size; index++) if (elementData[index] == null) { fastRemove(index); return true; } } else { for (int index = 0; index < size; index++) if (o.equals(elementData[index])) { fastRemove(index); return true; } } return false; }
- 封裝 fastRemove()
/* * Private remove method that skips bounds checking and does not return the value removed. * 私有方法,除去下標邊界校驗以及不返回移除操作的結果 */ private void fastRemove(int index) { modCount++;//remove操作屬於結構性改動,modCount計數+1 int numMoved = size - index - 1;//需要左移的長度 if (numMoved > 0) //大於該下標的所有數組元素統一左移一位 System.arraycopy(elementData, index+1, elementData, index,numMoved); elementData[--size] = null; // Let gc do its work 長度-1,同時加快gc }
■ 遍歷、排序
/** * Created by meizi on 2017/7/31. * List<數據類型> 排序、遍歷 */ public class ListSortTest { public static void main(String[] args) { List<Integer> nums = new ArrayList<Integer>(); nums.add(3); nums.add(5); nums.add(1); nums.add(0); // 遍歷及刪除的操作 /*Iterator<Integer> iterator = nums.iterator(); while (iterator.hasNext()) { Integer num = iterator.next(); if(num.equals(5)) { System.out.println("被刪除元素:" + num); iterator.remove(); //可刪除 } } System.out.println(nums);*/ //工具類(Collections) 進行排序 /*Collections.sort(nums); //底層為數組對象的排序,再通過ListIterator進行遍歷比較,取替 System.out.println(nums);*/ //②自定義排序方式 /*nums.sort(new Comparator<Integer>() { @Override public int compare(Integer o1, Integer o2) { if(o1 > o2) { return 1; } else if (o1 < o2) { return -1; } else { return 0; } } }); System.out.println(nums);*/ //遍歷 since 1.8 Iterator<Integer> iterator = nums.iterator(); iterator.forEachRemaining(obj -> System.out.print(obj)); //使用lambda 表達式 /** * Objects 展示對象各種方法,equals, toString, hash, toString */ /*default void forEachRemaining(Consumer<? super E> action) { Objects.requireNonNull(action); while (hasNext()) action.accept(next()); }*/ } }
■ list 與 Array 的轉換問題
- Object[] toArray(); 可能會產生 ClassCastException
- <T> T[] toArray(T[] contents) -- 調用 toArray(T[] contents) 能正常返回 T[]
public class ListAndArray { public static void main(String[] args) { List<String> list = new ArrayList<String>(); list.add("java"); list.add("C++"); String[] strings = list.toArray(new String[list.size()]); //使用泛型可避免類型轉換的異常,因為java不支持向下轉換 System.out.println(strings[0]); } }
■ 關於Vector 就不詳細介紹了,因為官方也並不推薦使用: (JDK 1.0)
- 矢量隊列,作用等效於ArrayList,線程安全
- 官方不推薦使用該類,非線程安全推薦 ArrayList,線程安全推薦 CopyOnWriteList
- 區別於arraylist, 所有方法都是 synchronized 修飾的,所以是線程安全
Tags: ArrayList 數組 實現 接口 the 序列化
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