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Java集合系列

閱讀目錄

  • 一、ArrayList簡介
  • 二、ArrayList源碼分析
  • 三、ArrayList遍歷方式

一、ArrayList簡介

  ArrayList是可以動態增長和縮減的索引序列,它是基於數組實現的List類。

  該類封裝了一個動態再分配的Object[]數組,每一個類對象都有一個capacity屬性,表示它們所封裝的Object[]數組的長度,當向ArrayList中添加元素時,該屬性值會自動增加。如果想ArrayList中添加大量元素,可使用ensureCapacity方法一次性增加capacity,可以減少增加重分配的次數提高性能。

  ArrayList的用法和Vector向類似,但是Vector是一個較老的集合,具有很多缺點,不建議使用。另外,ArrayList和Vector的區別是:ArrayList是線程不安全的,當多條線程訪問同一個ArrayList集合時,程序需要手動保證該集合的同步性,而Vector則是線程安全的。

  ArrayList與Collection關系如下圖:

  

二、ArrayList源碼分析

  下面就ArrayList的源代碼進行簡單的分析:

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;
    //默認的初始容量為10
    private static final int DEFAULT_CAPACITY = 10;
    private static final Object[] EMPTY_ELEMENTDATA = {};
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    transient Object[] elementData; 
    // ArrayList中實際數據的數量
    private int size;
    public ArrayList(int initialCapacity) //帶初始容量大小的構造函數
    {
        if (initialCapacity > 0)   //初始容量大於0,實例化數組
        {
            this.elementData = new Object[initialCapacity];
        } 
        else if (initialCapacity == 0) //初始化等於0,將空數組賦給elementData
        {
            this.elementData = EMPTY_ELEMENTDATA;  
        } 
        else    //初始容量小於,拋異常
        {
            throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
        }
    }
    public ArrayList()  //無參構造函數,默認容量為10
    {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    public ArrayList(Collection<? extends E> c)  //創建一個包含collection的ArrayList
    {
        elementData = c.toArray(); //返回包含c所有元素的數組
        if ((size = elementData.length) != 0)
        {
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);//復制指定數組,使elementData具有指定長度
        } 
        else
        {
            //c中沒有元素
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }
    //將當前容量值設為當前實際元素大小
    public void trimToSize()
    {
        modCount++;
        if (size < elementData.length) 
        {
            elementData = (size == 0)? EMPTY_ELEMENTDATA:Arrays.copyOf(elementData, size);
        }
    }
    
    //將集合的capacit增加minCapacity
    public void ensureCapacity(int minCapacity) 
    {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)?0:DEFAULT_CAPACITY;
        if (minCapacity > minExpand)
        {
            ensureExplicitCapacity(minCapacity);
        }
    }
    private void ensureCapacityInternal(int minCapacity)
    {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
        {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }
    private void ensureExplicitCapacity(int minCapacity)
    {
        modCount++;
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    private void grow(int minCapacity)
    {
        int oldCapacity = elementData.length;
     //注意此處擴充capacity的方式是將其向右一位再加上原來的數,實際上是擴充了1.5倍 int newCapacity = oldCapacity + (oldCapacity >> 1); if (newCapacity - minCapacity < 0) newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); 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的大小 public int size() { return size; } //判斷ArrayList是否為空 public boolean isEmpty() { return size == 0; } //判斷ArrayList中是否包含Object(o) public boolean contains(Object o) { return indexOf(o) >= 0; } //正向查找,返回ArrayList中元素Object(o)的索引位置 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; } //逆向查找,返回返回ArrayList中元素Object(o)的索引位置 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實例的淺拷貝。 public Object clone() { try { ArrayList<?> v = (ArrayList<?>) super.clone(); 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(e); } } //返回一個包含ArrayList中所有元素的數組 public Object[] toArray() { return Arrays.copyOf(elementData, size); } @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { if (a.length < size) return (T[]) Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; } @SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; } //返回至指定索引的值 public E get(int index) { rangeCheck(index); //檢查給定的索引值是否越界 return elementData(index); } //將指定索引上的值替換為新值,並返回舊值 public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; } //將指定的元素添加到此列表的尾部 public boolean add(E e) { ensureCapacityInternal(size + 1); elementData[size++] = e; return true; } // 將element添加到ArrayList的指定位置 public void add(int index, E element) { rangeCheckForAdd(index); ensureCapacityInternal(size + 1); //從指定源數組中復制一個數組,復制從指定的位置開始,到目標數組的指定位置結束。 //arraycopy(被復制的數組, 從第幾個元素開始復制, 要復制到的數組, 從第幾個元素開始粘貼, 一共需要復制的元素個數) //即在數組elementData從index位置開始,復制到index+1位置,共復制size-index個元素 System.arraycopy(elementData, index, elementData, index + 1,size - index); elementData[index] = element; size++; } //刪除ArrayList指定位置的元素 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; //將原數組最後一個位置置為null return oldValue; } //移除ArrayList中首次出現的指定元素(如果存在)。 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; } //快速刪除指定位置的元素 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; } //清空ArrayList,將全部的元素設為null public void clear() { modCount++; for (int i = 0; i < size; i++) elementData[i] = null; size = 0; } //按照c的迭代器所返回的元素順序,將c中的所有元素添加到此列表的尾部 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; } //從指定位置index開始,將指定c中的所有元素插入到此列表中 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) //先將ArrayList中從index開始的numMoved個元素移動到起始位置為index+numNew的後面去 System.arraycopy(elementData, index, elementData, index + numNew, numMoved); //再將c中的numNew個元素復制到起始位置為index的存儲空間中去 System.arraycopy(a, 0, elementData, index, numNew); size += numNew; return numNew != 0; } //刪除fromIndex到toIndex之間的全部元素 protected void removeRange(int fromIndex, int toIndex) { modCount++; //numMoved為刪除索引後面的元素個數 int numMoved = size - toIndex; //將刪除索引後面的元素復制到以fromIndex為起始位置的存儲空間中去 System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved); int newSize = size - (toIndex-fromIndex); //將ArrayList後面(toIndex-fromIndex)個元素置為null for (int i = newSize; i < size; i++) { elementData[i] = null; } size = newSize; } //檢查索引是否越界 private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } 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; } //刪除ArrayList中包含在c中的元素 public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, false); } //刪除ArrayList中除包含在c中的元素,和removeAll相反 public boolean retainAll(Collection<?> c) { Objects.requireNonNull(c); //檢查指定對象是否為空 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) //判斷c中是否有elementData[r]元素 elementData[w++] = elementData[r]; } finally { 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; } //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 { int expectedModCount = modCount; s.defaultWriteObject(); //寫入數組大小 s.writeInt(size); //寫入所有數組的元素 for (int i=0; i<size; i++) { s.writeObject(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } //先將ArrayList的“大小”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; s.defaultReadObject(); 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(); } } }

三、ArrayList遍歷方式

  ArrayList支持3種遍歷方式

  1、通過迭代器遍歷:

    Iterator iter = list.iterator();
    while (iter.hasNext())
    {
        System.out.println(iter.next());
    }            

  2、隨機訪問,通過索引值去遍歷,由於ArrayList實現了RandomAccess接口

    int size = list.size();
    for (int i=0; i<size; i++) 
    {
        System.out.println(list.get(i));        
    }        

  3、for循環遍歷:

    for(String str:list)
    {
    System.out.println(str);
   }    

  完整的代碼示例如下:

public class DemoMain 
{
    public static void main(String[] args)
    {
        List<String> list=new ArrayList<String>();
        list.add("nihao");
        list.add("xujian");
        list.add("wang");
        System.out.println("--------通過迭代器遍歷---------");
        Iterator iter = list.iterator();
        while (iter.hasNext())
        {
           System.out.println(iter.next());
        }
        
        System.out.println("--------通過隨機訪問---------");
        int size = list.size();
        for (int i=0; i<size; i++) 
        {
            System.out.println(list.get(i));        
        }
        
        System.out.println("--------通過for循環訪問---------");
        for(String str:list)
        {
            System.out.println(str);
        }
    }
}

  運行結果如圖示:

  

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