Vector源碼學習

 

安全的可增加數組結構

實現：
1. 內部採用數組的方式。
　　1.1 添加元素，會每次校驗容量是否知足， 擴容規則有兩種，1.增長擴容補償的長度，2.按照現有數組長度翻一倍。容量上限是Integer.MAX_VALUE。 copy使用Arrays.copy的api
　　1.2 刪除元素
　　　　1.2.1 經過對象刪除。遍歷數組，刪除第一個匹配的對象
　　　　1.2.3 經過下標刪除。判斷下標是否越界。
　　　　　　使用 System.arraycopy進行copy, 並將元素的最後一位設置爲null.供gc回收
2. 內部是同步[modCount]
　　2.1 ArrayList數據結構變化的時候，都會將modCount++。
　　2.2 採用Iterator遍歷的元素， next()會去檢查集合是否被修改[checkForComodification]，若是集合變動會拋出異常
　　相似於數據庫層面的 樂觀鎖 機制。 能夠經過 Iterator的api去刪除元素
3. 數組結構，內部存儲數據是有序的，而且數據能夠爲null,支持添加劇複數據
4. 線程安全的, 關於數組的增刪方法都採用了synchronized標註。

 

源碼學習

// 自動增加的對象數組
public class Vector<E> {

    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    // 元素數組
    protected Object[] elementData;
    
    // 元素長度
    protected int elementCount;
    
    // 擴容步長[增加容量]
    protected int capacityIncrement;
    
    // 集合變動次數
    private int modCount = 0;
    
    
    public Vector(int initialCapacity) {
        this(initialCapacity, 0);     // 10個長度，步長爲0 
    }
    
    public Vector() {
        this(10);  // 默認10個長度
    }
    
    public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)      // 初始化容量 小於0  拋異常
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];   // 建立指定長度數組
        this.capacityIncrement = capacityIncrement;   // 增加容量大小
    }
    
    // 添加元素
    public synchronized boolean add(E element) {

        modCount++;
        ensureCapacityHelper(elementCount + 1);  // 校驗當前容器容量是否知足
        elementData[elementCount++] = element;
        return true;
    }
    
    public synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }
    
    private void ensureCapacityHelper(int minCapacity) {
        if (minCapacity - elementData.length > 0)   // 當前下標  > 數組長度
            grow(minCapacity);
    }

　　 // 擴容方法
    private void grow(int minCapacity) {
        
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);   // 若是步長大於0, 每次擴容步長大小，不然按數組的長度翻一倍
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);    // 拷貝原來的內容
        
    }
    
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
    
    
    public boolean remove(Object o) {
        return removeElement(o);
    }
    
    public synchronized E remove(int index) {
        modCount++;
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        E oldValue = elementData(index);

        int numMoved = elementCount - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--elementCount] = null; // Let gc do its work

        return oldValue;
    }
    
    // 從結尾開始查找元素
    public synchronized int lastIndexOf(Object o) {
        return lastIndexOf(o, elementCount-1);
    }
    
    // 指定位置，從結尾查找元素
    public synchronized int lastIndexOf(Object o, int index) {
        if (index >= elementCount)
            throw new IndexOutOfBoundsException(index + " >= "+ elementCount);

        if (o == null) {
            for (int i = index; i >= 0; i--)
                if (elementData[i]==null)   
                    return i;
        } else {
            for (int i = index; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }
    
    private synchronized boolean removeElement(Object obj) {
        if (obj == null) {
            for (int index = 0; index < elementCount; index++)
                if (elementData[index] == null) {  // 刪除 null
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < elementCount; index++)
                if (obj.equals(elementData[index])) {   // eqals比較
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }
    
    public synchronized void removeElementAt(int index) {
        modCount++;
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                     elementCount);
        }
        else if (index < 0) {
            throw new ArrayIndexOutOfBoundsException(index);
        }
        int j = elementCount - index - 1;
        if (j > 0) {
            System.arraycopy(elementData, index + 1, elementData, index, j);
        }
        elementCount--;
        elementData[elementCount] = null; /* to let gc do its work */
    }
    
    private void fastRemove(int index) {
        modCount++;
        int numMoved = elementCount - index - 1;      // 當前size - index - 1    數組從0開始
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);   // system arraycopy
        elementData[--elementCount] = null; // clear to let GC do its work   gc回收   數組最後一個元素設置爲null
    
    }

    public synchronized Iterator<E> iterator() {
        return new Itr();
    }
    
    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        public boolean hasNext() {
            // Racy but within spec, since modifications are checked
            // within or after synchronization in next/previous
            return cursor != elementCount;
        }

        public E next() {
            synchronized (Vector.this) {
                checkForComodification();
                int i = cursor;
                if (i >= elementCount)
                    throw new NoSuchElementException();
                cursor = i + 1;
                return elementData(lastRet = i);
            }
        }

        public void remove() {
            if (lastRet == -1)
                throw new IllegalStateException();
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.remove(lastRet);
                expectedModCount = modCount;
            }
            cursor = lastRet;
            lastRet = -1;
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }
    
    public synchronized E get(int index) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

        return elementData(index);
    }
    
    public synchronized E elementAt(int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }

        return elementData(index);
    }

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    public int size() {
        return elementCount;
    }
    
}