java ArrayList与Vector
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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; } }} 0 0
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