java ArrayList与Vector

Posted 2022-11-13 StubbornAnt

本文基于JDK1.7

Vector与ArrayList

• Vector用同步实现了线程安全，ArrayList是线程不安全的，但是可以通过Collections.synchronizedList(list)来实现线程安全，不要求线程安全的场景用ArrayList效率更高
• Vector与ArrayList一样是一种可以根据需要自动伸缩的列表，它们都是基于数组实现的，加入了自动扩容机制
• Vector在初始化时除了提供initialCapacity外还可以额外提供capacityIncrement参数，从而在扩容时ArrayList一般会扩展到原来的1.5倍，而Vector在capacityIncrement不为零时会扩展capacityIncrement个元素
• 对Vector与ArrayList相同，在批量插入元素前，调整列表的容量到合理值，可以避免频繁的扩容引起的开销
• Vector是一个比较老的类，为了适配新的List接口对许多原有方法进行了包装，使得其代码较为臃肿，例如：

    public synchronized boolean add(E e) 
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
      
    public synchronized void addElement(E obj) 
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
      

• 在序列化方面，Vector只提供了WriteObject方法，没有提供ReadObject方法，没有完全实现定制化
  java中一个类实现了Seriaizable就可以实现序列化，ArrayList中保存元素的数组被声明为transient，即elementData数组不会被默认的序列化写入到输出中。ArrayList实现了ReadObject和WriteObject两个方法来进行定制的序列化。这样做的优势在于elementData数组中可能存在很大的空间是没有元素的，如果在序列化过程中这部分也被序列化会耗费大量的资源，因此ArrayList定制序列化，writeObject时只输出实体元素。在对端恢复ArrayList时，ReadObject先确定数组大小，然后将元素拷贝到对应数组中去，避免了添加元素时频繁的扩容，提高了效率

    private transient Object[] elementData;
    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();
        
    
    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();
            
        
      

Vector全部源码

• Vector的多数方法实现机制与ArrayList是相同的，只是在方法上添加了synchronized约束
• 由于Vector比List接口出现要早，很多方法名比较长如addElement等，为了适配新的接口List，又添加了许多很类似的方法

public class Vector<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable 
    protected Object[] elementData;
    protected int elementCount;
    protected int capacityIncrement;
    private static final long serialVersionUID = -2767605614048989439L;
    public Vector(int initialCapacity, int capacityIncrement) 
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: " + initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    
    public Vector(int initialCapacity) 
        this(initialCapacity, 0);
    
    public Vector() 
        this(10);
    
    public Vector(Collection<? extends E> c) 
        elementData = c.toArray();
        elementCount = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
    
    public synchronized void copyInto(Object[] anArray) 
        System.arraycopy(elementData, 0, anArray, 0, elementCount);
    
    public synchronized void trimToSize() 
        modCount++;
        int oldCapacity = elementData.length;
        if (elementCount < oldCapacity) 
            elementData = Arrays.copyOf(elementData, elementCount);
        
    
    public synchronized void ensureCapacity(int minCapacity) 
        if (minCapacity > 0) 
            modCount++;
            ensureCapacityHelper(minCapacity);
        
    
    private void ensureCapacityHelper(int minCapacity) 
        // 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 + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity);
        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;
    
    public synchronized void setSize(int newSize) 
        modCount++;
        if (newSize > elementCount) 
            ensureCapacityHelper(newSize);
         else 
            for (int i = newSize; i < elementCount; i++) 
                elementData[i] = null;
            
        
        elementCount = newSize;
    
    public synchronized int capacity() 
        return elementData.length;
    
    public synchronized int size() 
        return elementCount;
    
    public synchronized boolean isEmpty() 
        return elementCount == 0;
    
    public Enumeration<E> elements() 
        return new Enumeration<E>() 
            int count = 0;
            public boolean hasMoreElements() 
                return count < elementCount;
            
            public E nextElement() 
                synchronized (Vector.this) 
                    if (count < elementCount) 
                        return elementData(count++);
                    
                
                throw new NoSuchElementException("Vector Enumeration");
            
        ;
    
    public boolean contains(Object o) 
        return indexOf(o, 0) >= 0;
    
    public int indexOf(Object o) 
        return indexOf(o, 0);
    
    public synchronized int indexOf(Object o, int index) 
        if (o == null) 
            for (int i = index; i < elementCount; i++)
                if (elementData[i] == null)
                    return i;
         else 
            for (int i = index; i < elementCount; i++)
                if (o.equals(elementData[i]))
                    return i;
        
        return -1;
    
    public synchronized int lastIndexOf(Object o) 
        return lastIndexOf(o, elementCount - 1);
    
    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;
    
    public synchronized E elementAt(int index) 
        if (index >= elementCount) 
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        
        return elementData(index);
    
    public synchronized E firstElement() 
        if (elementCount == 0) 
            throw new NoSuchElementException();
        
        return elementData(0);
    
    public synchronized E lastElement() 
        if (elementCount == 0) 
            throw new NoSuchElementException();
        
        return elementData(elementCount - 1);
    
    public synchronized void setElementAt(E obj, int index) 
        if (index >= elementCount) 
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        
        elementData[index] = obj;
    
    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--;
    
    public synchronized void insertElementAt(E obj, int index) 
        modCount++;
        if (index > elementCount) 
            throw new ArrayIndexOutOfBoundsException(index + " > " + elementCount);
        
        ensureCapacityHelper(elementCount + 1);
        System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);
        elementData[index] = obj;
        elementCount++;
    
    public synchronized void addElement(E obj) 
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    
    public synchronized boolean removeElement(Object obj) 
        modCount++;
        int i = indexOf(obj);
        if (i >= 0) 
            removeElementAt(i);
            return true;
        
        return false;
    
    public synchronized void removeAllElements() 
        modCount++;
        // Let gc do its work
        for (int i = 0; i < elementCount; i++)
            elementData[i] = null;
        elementCount = 0;
    
    public synchronized Object clone() 
        try 
            @SuppressWarnings("unchecked")
            Vector<E> v = (Vector<E>) super.clone();
            v.elementData = Arrays.copyOf(elementData, elementCount);
            v.modCount = 0;
            return v;
         catch (CloneNotSupportedException e) 
            // this shouldn't happen, since we are Cloneable
            throw new InternalError();
        
    
    public synchronized Object[] toArray() 
        return Arrays.copyOf(elementData, elementCount);
    
    @SuppressWarnings("unchecked")
    public synchronized <T> T[] toArray(T[] a) 
        if (a.length < elementCount)
            return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());
        System.arraycopy(elementData, 0, a, 0, elementCount);
        if (a.length > elementCount)
            a[elementCount] = null;
        return a;
    
    // Positional Access Operations
    @SuppressWarnings("unchecked")
    E elementData(int index) 
        return (E) elementData[index];
    
    public synchronized E get(int index) 
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        return elementData(index);
    
    public synchronized E set(int index, E element) 
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    
    public synchronized boolean add(E e) 
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    
    public boolean remove(Object o) 
        return removeElement(o);
    
    public void add(int index, E element) 
        insertElementAt(element, index);
    
    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;
    
    public void clear() 
        removeAllElements();
    
    // Bulk Operations
    public synchronized boolean containsAll(Collection<?> c) 
        return super.containsAll(c);
    
    public synchronized boolean addAll(Collection<? extends E> c) 
        modCount++;
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityHelper(elementCount + numNew);
        System.arraycopy(a, 0, elementData, elementCount, numNew);
        elementCount += numNew;
        return numNew != 0;
    
    public synchronized boolean removeAll(Collection<?> c) 
        return super.removeAll(c);
    
    public synchronized boolean retainAll(Collection<?> c) 
        return super.retainAll(c);
    
    public synchronized boolean addAll(int index, Collection<? extends E> c) 
        modCount++;
        if (index < 0 || index > elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityHelper(elementCount + numNew);
        int numMoved = elementCount - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew, numMoved);
        System.arraycopy(a, 0, elementData, index, numNew);
        elementCount += numNew;
        return numNew != 0;
    
    public synchronized boolean equals(Object o) 
        return super.equals(o);
    
    public synchronized int hashCode() 
        return super.hashCode();
    
    public synchronized String toString() 
        return super.toString();
    
    public synchronized List<E> subList(int fromIndex, int toIndex) 
        return Collections.synchronizedList(super.subList(fromIndex, toIndex), this);
    
    protected synchronized void removeRange(int fromIndex, int toIndex) 
        modCount++;
        int numMoved = elementCount - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved);
        // Let gc do its work
        int newElementCount = elementCount - (toIndex - fromIndex);
        while (elementCount != newElementCount)
            elementData[--elementCount] = null;
    
    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException 
        final java.io.ObjectOutputStream.PutField fields = s.putFields();
        final Object[] data;
        synchronized (this) 
            fields.put("capacityIncrement", capacityIncrement);
            fields.put("elementCount", elementCount);
            data = elementData.clone();
        
        fields.put("elementData", data);
        s.writeFields();
    
    public synchronized ListIterator<E> listIterator(int index) 
        if (index < 0 || index > elementCount)
            throw new IndexOutOfBoundsException("Index: " + index);
        return new ListItr(index);
    
    public synchronized ListIterator<E> listIterator() 
        return new ListItr(0);
    
    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();
        
    
    final class ListItr extends Itr implements ListIterator<E> 
        ListItr(int index) 
            super();
            cursor = index;
        
        public boolean hasPrevious() 
            return cursor != 0;
        
        public int nextIndex() 
            return cursor;
        
        public int previousIndex() 
            return cursor - 1;
        
        public E previous() 
            synchronized (Vector.this) 
                checkForComodification();
                int i = cursor - 1;
                if (i < 0)
                    throw new NoSuchElementException();
                cursor = i;
                return elementData(lastRet = i);
            
        
        public void set(E e) 
            if (lastRet == -1)
                throw new IllegalStateException();
            synchronized (Vector.this) 
                checkForComodification();
                Vector.this.set(lastRet, e);
            
        
        public void add(E e) 
            int i = cursor;
            synchronized (Vector.this) 
                checkForComodification();
                Vector.this.add(i, e);
                expectedModCount = modCount;
            
            cursor = i + 1;
            lastRet = -1;