Java中的ArrayList詳解
阿新 • • 發佈:2019-01-10
ArrayList簡介
ArrayList
是一個其容量能夠動態增長的動態陣列。它繼承了AbstractList
,實現了List
、RandomAccess
, Cloneable
, java.io.Serializable
。
基本的ArrayList
,長於隨機訪問元素,但是在List
中間插入和移除元素時較慢。同時,ArrayList
的操作不是執行緒安全的!一般在單執行緒中才使用ArrayList
,而在多執行緒中一般使用Vector
或者CopyOnWriteArrayList
。
ArrayList遍歷方式
ArrayList
有三種遍歷方式
迭代器遍歷
Iterator<Integer> it = arrayList.iterator();
while (it.hasNext()){
System.out.print(it.next() + " ");
}
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索引值遍歷
for(int i = 0; i < arrayList.size(); i++){
System.out.print(arrayList.get(i) + " ");
}
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for迴圈遍歷
for(Integer number : arrayList){
System.out.print(number + " ");
}
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需要說明的是,遍歷ArrayList
時,通過索引值遍歷效率最高,for迴圈遍歷次之,迭代器遍歷最低。
toArray()的使用
有時候,當我們呼叫ArrayList
中的 toArray()
,可能遇到過丟擲java.lang.ClassCastException
異常的情況,這是由於toArray()
返回的是 Object[]
陣列,將 Object[]
轉換為其它型別(如,將Object[]
轉換為的Integer[]
)則會丟擲java.lang.ClassCastException
異常,因為Java
不支援向下轉型。
所以一般更常用的是使用另外一種方法進行使用:
<T> T[] toArray(T[] a)
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呼叫toArray(T[] a)
返回T[]
可通以下方式進行實現:
// toArray用法
// 第一種方式(最常用)
Integer[] integer = arrayList.toArray(new Integer[0]);
// 第二種方式(容易理解)
Integer[] integer1 = new Integer[arrayList.size()];
arrayList.toArray(integer1);
// 丟擲異常,java不支援向下轉型
//Integer[] integer2 = new Integer[arrayList.size()];
//integer2 = arrayList.toArray();
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ArrayList用法示例
/**
* @author GongchuangSu
* @since 2016.05.10
*/
import java.util.ArrayList;
import java.util.Iterator;
public classArrayListDemo {
public static void main(String[] srgs){
ArrayList<Integer> arrayList = new ArrayList<Integer>();
System.out.printf("Before add:arrayList.size() = %d\n",arrayList.size());
arrayList.add(1);
arrayList.add(3);
arrayList.add(5);
arrayList.add(7);
arrayList.add(9);
System.out.printf("After add:arrayList.size() = %d\n",arrayList.size());
System.out.println("Printing elements of arrayList");
// 三種遍歷方式列印元素
// 第一種:通過迭代器遍歷
System.out.print("通過迭代器遍歷:");
Iterator<Integer> it = arrayList.iterator();
while(it.hasNext()){
System.out.print(it.next() + " ");
}
System.out.println();
// 第二種:通過索引值遍歷
System.out.print("通過索引值遍歷:");
for(int i = 0; i < arrayList.size(); i++){
System.out.print(arrayList.get(i) + " ");
}
System.out.println();
// 第三種:for迴圈遍歷
System.out.print("for迴圈遍歷:");
for(Integer number : arrayList){
System.out.print(number + " ");
}
// toArray用法
// 第一種方式(最常用)
Integer[] integer = arrayList.toArray(new Integer[0]);
// 第二種方式(容易理解)
Integer[] integer1 = new Integer[arrayList.size()];
arrayList.toArray(integer1);
// 丟擲異常,java不支援向下轉型
//Integer[] integer2 = new Integer[arrayList.size()];
//integer2 = arrayList.toArray();
System.out.println();
// 在指定位置新增元素
arrayList.add(2,2);
// 刪除指定位置上的元素
arrayList.remove(2);
// 刪除指定元素
arrayList.remove((Object)3);
// 判斷arrayList是否包含5
System.out.println("ArrayList contains 5 is: " + arrayList.contains(5));
// 清空ArrayList
arrayList.clear();
// 判斷ArrayList是否為空
System.out.println("ArrayList is empty: " + arrayList.isEmpty());
}
}
/**
Before add:arrayList.size() = 0
After add:arrayList.size() = 5
Printing elements of arrayList
通過迭代器遍歷:1 3 5 7 9
通過索引值遍歷:1 3 5 7 9
for迴圈遍歷:1 3 5 7 9
ArrayList contains 5 is: true
ArrayList is empty: true
*/
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ArrayList原始碼解析
package java.util;
public classArrayList<E> extendsAbstractList<E>
implementsList<E>, RandomAccess, Cloneable, java.io.Serializable
{
// 序列版本號
private static final long serialVersionUID = 8683452581122892189L;
// 預設容量大小
private static final int DEFAULT_CAPACITY = 10;
// 空陣列
private static final Object[] EMPTY_ELEMENTDATA = {};
// 用於儲存ArrayList中資料的陣列
private transient Object[] elementData;
// ArrayList中所包含元素的個數
private int size;
// 帶初始容量引數的建構函式
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}
// 預設建構函式,其預設初始容量為10
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
// 帶Collection引數的建構函式
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
// 將此 ArrayList 例項的容量調整為列表的當前大小(實際元素個數)
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}
// 如有必要,增加此 ArrayList 例項的容量,以確保它至少能夠容納最小容量引數所
// 指定的元素數
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != EMPTY_ELEMENTDATA)
// any size if real element table
? 0
// larger than default for empty table. It's already supposed to be
// at default size.
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
// 返回ArrayList中的元素個數
public int size() {
return size;
}
// 判斷ArrayList是否為空
public boolean isEmpty() {
return size == 0;
}
// 判斷ArrayList是否包含Object(o)
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
// 返回ArrayList中首次出現的指定元素的索引,或如果此列表不包含元素,則返回 -1
public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
// 返回ArrayList中最後一次出現的指定元素的索引,或如果此列表不包含索引,則返回 -1
public int lastIndexOf(Object o) {
if (o == null) {
for (int i = size-1; i >= 0; i--)
if (elementData[i]==null)
return i;
} else {
for (int i = size-1; i >= 0; i--)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
// 返回此 ArrayList 例項的淺表副本
public Object clone() {
try {
@SuppressWarnings("unchecked")
ArrayList<E> v = (ArrayList<E>) super.clone();
// 將當前ArrayList的全部元素拷貝到v中
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}
// 按適當順序(從第一個到最後一個元素)返回包含此列表中所有元素的陣列
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
// 返回ArrayList的模板陣列。所謂模板陣列,即可以將T設為任意的資料型別
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
// 位置訪問操作
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
// 返回ArrayList中指定位置上的元素
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
// 用指定的元素替代ArrayList中指定位置上的元素,並返回替代前的元素
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
// 將指定的元素新增到ArrayList的尾部
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
// 將指定的元素插入ArrayList中的指定位置
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
// 移除ArrayList中指定位置上的元素,並返回該位置上的元素
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
// 移除ArrayList中首次出現的指定元素(如果存在則移除並返回true,否則返回false)
public boolean remove(Object o) {
if (o == 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;
}
// 私有方法,用於快速移除
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
// 移除ArrayList中的所有元素
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
// 按照指定 collection 的迭代器所返回的元素順序,
// 將該 collection 中的所有元素新增到ArrayList的尾部
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
// 從指定的位置開始,將指定 collection 中的所有元素插入到ArrayList中
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
// 移除列表中索引在 fromIndex(包括)和 toIndex(不包括)之間的所有元素
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// clear to let GC do its work
int newSize = size - (toIndex-fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}
// 私有方法,用於範圍檢測
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
// 私有方法,用於add和addAll
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
// 移除ArrayList中Collection所包含的所有元素
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}
// 保留所有ArrayList和Collection共有的元素
public boolean retainAll(Collection<?> c) {
return batchRemove(c, true);
}
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try {
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
if (w != size) {
// clear to let GC do its work
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
// java.io.Serializable的寫入函式
// 將ArrayList的“容量,所有的元素值”都寫入到輸出流中
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException{
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// Write out size as capacity for behavioural compatibility with clone()
s.writeInt(size);
// Write out all elements in the proper order.
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
// java.io.Serializable的讀取函式:根據寫入方式讀出
// 先將ArrayList的“容量”讀出,然後將“所有的元素值”讀出
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
elementData = EMPTY_ELEMENTDATA;
// Read in size, and any hidden stuff
s.defaultReadObject();
// Read in capacity
s.readInt(); // ignored
if (size > 0) {
// be like clone(), allocate array based upon size not capacity
ensureCapacityInternal(size);
Object[] a = elementData;
// Read in all elements in the proper order.
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}
// 返回一個從指定位置開始遍歷的ListIterator迭代器
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
// 返回一個ListIterator迭代器
public ListIterator<E> listIterator() {
return new ListItr(0);
}
// 返回一個Iterator迭代器
public Iterator<E> iterator() {
return new Itr();
}
// 返回一個指定範圍的子List列表
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, 0, fromIndex, toIndex);
}
}
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