使用数组和复制数组的效率--优先使用数组

    数组的使用可以被证明为是相当快的，这里指的是与ArrayList的比较，但是array不能伸缩，上来就要固定长度，这个是它的局限性，也是它速度快的原因。
    ArrayList可以变换长度，会自动帮你扩展，现在想一下，它是怎么扩展的呢？很容易想到，它就是新建一个数组，把现有的东西复制到一个这个更大的数组中去，实现的，这样的效率极其低下。 

引用

array在某些时候的表现，可能比ArrayList快上10倍---
Peter hagger

    有个问题，我事先要是不知道长度呢，我怎么定，可能会想，我估个值，尽可能大，就行了，这样的做法虽然浪费了许多空间，但是

引用

性能上的收益可能超过内存方面的代价，只有通过细致的性能测评和对系统的详尽分析，才能做出正确的选择---Peter hagger

    当然数组不能存储对象类型，ArrayList在大多数时是很不错的，但是如果有机会有数组实现，还是用数组实现。
    数组的复制也是经常遇到的问题，通常是这样做的：

public void copyArray(int[] src,int[] target){
		for(int i:src){
			target[i]=src[i];
		}
	}

如果知道有个函数是System.arraycopy(src, srcPos, dest, destPos, length),就会这样

public void copyArray2(int[] src,int[] target){
		int length = src.length;
		System.arraycopy(src, 0, target, 0, length);
	}

打印结果：
用普通复制方法的时间32
用System.arraycopy的时间15

原因：System.arraycopy是以本机函数实现的，执行速度更快。

评论

13 楼 DoubleEO 2009-01-10  

抛出异常的爱 写道

如果想要多次增加数组大小
用list比array快.....因为:

int newCapacity = (oldCapacity * 3)/2 + 1;  
//.......
 elementData = Arrays.copyOf(elementData, newCapacity);  

如果想要快速遍历所有点.
list比array快
因为:

//当remove时
System.arraycopy(elementData, index+1, elementData, index,  numMoved);  

如果想要快速定位
用hashmap会不会更快一点呢.

是啊，像你学习了~

12 楼 DoubleEO 2009-01-10  

hurricane1026 写道

/*
 * @(#)ArrayList.java	1.56 06/04/21
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util;

/**
 * Resizable-array implementation of the <tt>List</tt> interface.  Implements
 * all optional list operations, and permits all elements, including
 * <tt>null</tt>.  In addition to implementing the <tt>List</tt> interface,
 * this class provides methods to manipulate the size of the array that is
 * used internally to store the list.  (This class is roughly equivalent to
 * <tt>Vector</tt>, except that it is unsynchronized.)<p>
 *
 * The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>,
 * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant
 * time.  The <tt>add</tt> operation runs in <i>amortized constant time</i>,
 * that is, adding n elements requires O(n) time.  All of the other operations
 * run in linear time (roughly speaking).  The constant factor is low compared
 * to that for the <tt>LinkedList</tt> implementation.<p>
 *
 * Each <tt>ArrayList</tt> instance has a <i>capacity</i>.  The capacity is
 * the size of the array used to store the elements in the list.  It is always
 * at least as large as the list size.  As elements are added to an ArrayList,
 * its capacity grows automatically.  The details of the growth policy are not
 * specified beyond the fact that adding an element has constant amortized
 * time cost.<p>
 *
 * An application can increase the capacity of an <tt>ArrayList</tt> instance
 * before adding a large number of elements using the <tt>ensureCapacity</tt>
 * operation.  This may reduce the amount of incremental reallocation.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access an <tt>ArrayList</tt> instance concurrently,
 * and at least one of the threads modifies the list structurally, it
 * <i>must</i> be synchronized externally.  (A structural modification is
 * any operation that adds or deletes one or more elements, or explicitly
 * resizes the backing array; merely setting the value of an element is not
 * a structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the list:<pre>
 *   List list = Collections.synchronizedList(new ArrayList(...));</pre>
 *
 * <p>The iterators returned by this class's <tt>iterator</tt> and
 * <tt>listIterator</tt> methods are <i>fail-fast</i>: if the list is
 * structurally modified at any time after the iterator is created, in any way
 * except through the iterator's own <tt>remove</tt> or <tt>add</tt> methods,
 * the iterator will throw a {@link ConcurrentModificationException}.  Thus, in
 * the face of concurrent modification, the iterator fails quickly and cleanly,
 * rather than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.<p>
 *
 * Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i><p>
 *
 * This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @version 1.56, 04/21/06
 * @see	    Collection
 * @see	    List
 * @see	    LinkedList
 * @see	    Vector
 * @since   1.2
 */

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer.
     */
    private transient Object[] elementData;

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param   initialCapacity   the initial capacity of the list
     * @exception IllegalArgumentException if the specified initial capacity
     *            is negative
     */
    public ArrayList(int initialCapacity) {
	super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
	this.elementData = new Object[initialCapacity];
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
	this(10);
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    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);
    }

    /**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
     */
    public void trimToSize() {
	modCount++;
	int oldCapacity = elementData.length;
	if (size < oldCapacity) {
            elementData = Arrays.copyOf(elementData, size);
	}
    }

    /**
     * Increases the capacity of this <tt>ArrayList</tt> instance, if
     * necessary, to ensure that it can hold at least the number of elements
     * specified by the minimum capacity argument.
     *
     * @param   minCapacity   the desired minimum capacity
     */
    public void ensureCapacity(int minCapacity) {
	modCount++;
	int oldCapacity = elementData.length;
	if (minCapacity > oldCapacity) {
	    Object oldData[] = elementData;
	    int newCapacity = (oldCapacity * 3)/2 + 1;
    	    if (newCapacity < minCapacity)
		newCapacity = minCapacity;
            // minCapacity is usually close to size, so this is a win:
            elementData = Arrays.copyOf(elementData, newCapacity);
	}
    }

    /**
     * Returns the number of elements in this list.
     *
     * @return the number of elements in this list
     */
    public int size() {
	return size;
    }

    /**
     * Returns <tt>true</tt> if this list contains no elements.
     *
     * @return <tt>true</tt> if this list contains no elements
     */
    public boolean isEmpty() {
	return size == 0;
    }

    /**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested
     * @return <tt>true</tt> if this list contains the specified element
     */
    public boolean contains(Object o) {
	return indexOf(o) >= 0;
    }

    /**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */



    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;
    }

    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */
    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;
    }

    /**
     * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The
     * elements themselves are not copied.)
     *
     * @return a clone of this <tt>ArrayList</tt> instance
     */
    public Object clone() {
	try {
	    ArrayList<E> v = (ArrayList<E>) 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();
	}
    }

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list in
     *         proper sequence
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * Returns an array containing all of the elements in this list in proper
     * sequence (from first to last element); the runtime type of the returned
     * array is that of the specified array.  If the list fits in the
     * specified array, it is returned therein.  Otherwise, a new array is
     * allocated with the runtime type of the specified array and the size of
     * this list.
     *
     * <p>If the list fits in the specified array with room to spare
     * (i.e., the array has more elements than the list), the element in
     * the array immediately following the end of the collection is set to
     * <tt>null</tt>.  (This is useful in determining the length of the
     * list <i>only</i> if the caller knows that the list does not contain
     * any null elements.)
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of the list
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this list
     * @throws NullPointerException if the specified array is null
     */
    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;
    }

    // Positional Access Operations

    /**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
	RangeCheck(index);

	return (E) elementData[index];
    }

    /**
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
	RangeCheck(index);

	E oldValue = (E) elementData[index];
	elementData[index] = element;
	return oldValue;
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
	ensureCapacity(size + 1);  // Increments modCount!!
	elementData[size++] = e;
	return true;
    }

    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
	if (index > size || index < 0)
	    throw new IndexOutOfBoundsException(
		"Index: "+index+", Size: "+size);

	ensureCapacity(size+1);  // Increments modCount!!
	System.arraycopy(elementData, index, elementData, index + 1,
			 size - index);
	elementData[index] = element;
	size++;
    }

    /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
	RangeCheck(index);

	modCount++;
	E oldValue = (E) elementData[index];

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

	return oldValue;
    }

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    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;
    }

    /*
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
    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; // Let gc do its work
    }

    /**
     * Removes all of the elements from this list.  The list will
     * be empty after this call returns.
     */
    public void clear() {
	modCount++;

	// Let gc do its work
	for (int i = 0; i < size; i++)
	    elementData[i] = null;

	size = 0;
    }

    /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the
     * specified collection's Iterator.  The behavior of this operation is
     * undefined if the specified collection is modified while the operation
     * is in progress.  (This implies that the behavior of this call is
     * undefined if the specified collection is this list, and this
     * list is nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
	Object[] a = c.toArray();
        int numNew = a.length;
	ensureCapacity(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
	return numNew != 0;
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
	if (index > size || index < 0)
	    throw new IndexOutOfBoundsException(
		"Index: " + index + ", Size: " + size);

	Object[] a = c.toArray();
	int numNew = a.length;
	ensureCapacity(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;
    }

    /**
     * Removes from this list all of the elements whose index is between
     * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.
     * Shifts any succeeding elements to the left (reduces their index).
     * This call shortens the list by <tt>(toIndex - fromIndex)</tt> elements.
     * (If <tt>toIndex==fromIndex</tt>, this operation has no effect.)
     *
     * @param fromIndex index of first element to be removed
     * @param toIndex index after last element to be removed
     * @throws IndexOutOfBoundsException if fromIndex or toIndex out of
     *              range (fromIndex &lt; 0 || fromIndex &gt;= size() || toIndex
     *              &gt; size() || toIndex &lt; fromIndex)
     */
    protected void removeRange(int fromIndex, int toIndex) {
	modCount++;
	int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

	// Let gc do its work
	int newSize = size - (toIndex-fromIndex);
	while (size != newSize)
	    elementData[--size] = null;
    }

    /**
     * Checks if the given index is in range.  If not, throws an appropriate
     * runtime exception.  This method does *not* check if the index is
     * negative: It is always used immediately prior to an array access,
     * which throws an ArrayIndexOutOfBoundsException if index is negative.
     */
    private void RangeCheck(int index) {
	if (index >= size)
	    throw new IndexOutOfBoundsException(
		"Index: "+index+", Size: "+size);
    }

    /**
     * Save the state of the <tt>ArrayList</tt> instance to a stream (that
     * is, serialize it).
     *
     * @serialData The length of the array backing the <tt>ArrayList</tt>
     *             instance is emitted (int), followed by all of its elements
     *             (each an <tt>Object</tt>) in the proper order.
     */
    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 array length
        s.writeInt(elementData.length);

	// 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();
        }

    }

    /**
     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
	// Read in size, and any hidden stuff
	s.defaultReadObject();

        // Read in array length and allocate array
        int arrayLength = s.readInt();
        Object[] a = elementData = new Object[arrayLength];

	// Read in all elements in the proper order.
	for (int i=0; i<size; i++)
            a[i] = s.readObject();
    }
}

就这么点代码，是个人都能看出来arraylist就是封装了数组在里面，你说封装个数组，还比数组快，可能么。
这也就罢了，您换个方式研究咱没意见，最后的结论竟然是为了性能请用数组。
要是这么喜欢性能，您用java干嘛阿。

PS.而且凡是读过书，读过代码，哪怕写过1k行代码的最菜的人，也知道大多数时候，维护一个定长数组需要付出多大的功夫，连cpp里面都漫天vector了。还有人发现了一块新大陆，你让别人能不吐么？
长得丑无所谓，但是出来吓人还是有所谓的。对于不懂的事情，别做总结性发言。

    我也没让你来看啊，你自己非要进来看，还回帖一直在这冷嘲热讽，不过还是给谢谢你的建议。

11 楼 hurricane1026 2009-01-10  

抛出异常的爱 写道

如果想要多次增加数组大小
用list比array快.....因为:

int newCapacity = (oldCapacity * 3)/2 + 1;  
//.......
 elementData = Arrays.copyOf(elementData, newCapacity);  

如果想要快速遍历所有点.
list比array快
因为:

//当remove时
System.arraycopy(elementData, index+1, elementData, index,  numMoved);  

如果想要快速定位
用hashmap会不会更快一点呢.

arraylist是随机存储介质，适合于需要访问某个特定位置的内容，而list（linkedlist）一般也就是增删容易，add没有额外的负担，但是遍历很快
需要快速定位，除非是需要key的，否则没人用hashmap，因为hasher本身也是很大的计算复杂度。

10 楼 抛出异常的爱 2009-01-10  

如果想要多次增加数组大小
用list比array快.....因为:

int newCapacity = (oldCapacity * 3)/2 + 1;  
//.......
 elementData = Arrays.copyOf(elementData, newCapacity);  

如果想要快速遍历所有点.
list比array快
因为:

//当remove时
System.arraycopy(elementData, index+1, elementData, index,  numMoved);  

如果想要快速定位
用hashmap会不会更快一点呢.

9 楼 hurricane1026 2009-01-10  

/*
 * @(#)ArrayList.java	1.56 06/04/21
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util;

/**
 * Resizable-array implementation of the <tt>List</tt> interface.  Implements
 * all optional list operations, and permits all elements, including
 * <tt>null</tt>.  In addition to implementing the <tt>List</tt> interface,
 * this class provides methods to manipulate the size of the array that is
 * used internally to store the list.  (This class is roughly equivalent to
 * <tt>Vector</tt>, except that it is unsynchronized.)<p>
 *
 * The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>,
 * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant
 * time.  The <tt>add</tt> operation runs in <i>amortized constant time</i>,
 * that is, adding n elements requires O(n) time.  All of the other operations
 * run in linear time (roughly speaking).  The constant factor is low compared
 * to that for the <tt>LinkedList</tt> implementation.<p>
 *
 * Each <tt>ArrayList</tt> instance has a <i>capacity</i>.  The capacity is
 * the size of the array used to store the elements in the list.  It is always
 * at least as large as the list size.  As elements are added to an ArrayList,
 * its capacity grows automatically.  The details of the growth policy are not
 * specified beyond the fact that adding an element has constant amortized
 * time cost.<p>
 *
 * An application can increase the capacity of an <tt>ArrayList</tt> instance
 * before adding a large number of elements using the <tt>ensureCapacity</tt>
 * operation.  This may reduce the amount of incremental reallocation.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access an <tt>ArrayList</tt> instance concurrently,
 * and at least one of the threads modifies the list structurally, it
 * <i>must</i> be synchronized externally.  (A structural modification is
 * any operation that adds or deletes one or more elements, or explicitly
 * resizes the backing array; merely setting the value of an element is not
 * a structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the list:<pre>
 *   List list = Collections.synchronizedList(new ArrayList(...));</pre>
 *
 * <p>The iterators returned by this class's <tt>iterator</tt> and
 * <tt>listIterator</tt> methods are <i>fail-fast</i>: if the list is
 * structurally modified at any time after the iterator is created, in any way
 * except through the iterator's own <tt>remove</tt> or <tt>add</tt> methods,
 * the iterator will throw a {@link ConcurrentModificationException}.  Thus, in
 * the face of concurrent modification, the iterator fails quickly and cleanly,
 * rather than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.<p>
 *
 * Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i><p>
 *
 * This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @version 1.56, 04/21/06
 * @see	    Collection
 * @see	    List
 * @see	    LinkedList
 * @see	    Vector
 * @since   1.2
 */

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer.
     */
    private transient Object[] elementData;

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param   initialCapacity   the initial capacity of the list
     * @exception IllegalArgumentException if the specified initial capacity
     *            is negative
     */
    public ArrayList(int initialCapacity) {
	super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
	this.elementData = new Object[initialCapacity];
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
	this(10);
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    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);
    }

    /**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
     */
    public void trimToSize() {
	modCount++;
	int oldCapacity = elementData.length;
	if (size < oldCapacity) {
            elementData = Arrays.copyOf(elementData, size);
	}
    }

    /**
     * Increases the capacity of this <tt>ArrayList</tt> instance, if
     * necessary, to ensure that it can hold at least the number of elements
     * specified by the minimum capacity argument.
     *
     * @param   minCapacity   the desired minimum capacity
     */
    public void ensureCapacity(int minCapacity) {
	modCount++;
	int oldCapacity = elementData.length;
	if (minCapacity > oldCapacity) {
	    Object oldData[] = elementData;
	    int newCapacity = (oldCapacity * 3)/2 + 1;
    	    if (newCapacity < minCapacity)
		newCapacity = minCapacity;
            // minCapacity is usually close to size, so this is a win:
            elementData = Arrays.copyOf(elementData, newCapacity);
	}
    }

    /**
     * Returns the number of elements in this list.
     *
     * @return the number of elements in this list
     */
    public int size() {
	return size;
    }

    /**
     * Returns <tt>true</tt> if this list contains no elements.
     *
     * @return <tt>true</tt> if this list contains no elements
     */
    public boolean isEmpty() {
	return size == 0;
    }

    /**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested
     * @return <tt>true</tt> if this list contains the specified element
     */
    public boolean contains(Object o) {
	return indexOf(o) >= 0;
    }

    /**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */



    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;
    }

    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */
    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;
    }

    /**
     * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The
     * elements themselves are not copied.)
     *
     * @return a clone of this <tt>ArrayList</tt> instance
     */
    public Object clone() {
	try {
	    ArrayList<E> v = (ArrayList<E>) 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();
	}
    }

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list in
     *         proper sequence
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * Returns an array containing all of the elements in this list in proper
     * sequence (from first to last element); the runtime type of the returned
     * array is that of the specified array.  If the list fits in the
     * specified array, it is returned therein.  Otherwise, a new array is
     * allocated with the runtime type of the specified array and the size of
     * this list.
     *
     * <p>If the list fits in the specified array with room to spare
     * (i.e., the array has more elements than the list), the element in
     * the array immediately following the end of the collection is set to
     * <tt>null</tt>.  (This is useful in determining the length of the
     * list <i>only</i> if the caller knows that the list does not contain
     * any null elements.)
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of the list
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this list
     * @throws NullPointerException if the specified array is null
     */
    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;
    }

    // Positional Access Operations

    /**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
	RangeCheck(index);

	return (E) elementData[index];
    }

    /**
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
	RangeCheck(index);

	E oldValue = (E) elementData[index];
	elementData[index] = element;
	return oldValue;
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
	ensureCapacity(size + 1);  // Increments modCount!!
	elementData[size++] = e;
	return true;
    }

    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
	if (index > size || index < 0)
	    throw new IndexOutOfBoundsException(
		"Index: "+index+", Size: "+size);

	ensureCapacity(size+1);  // Increments modCount!!
	System.arraycopy(elementData, index, elementData, index + 1,
			 size - index);
	elementData[index] = element;
	size++;
    }

    /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
	RangeCheck(index);

	modCount++;
	E oldValue = (E) elementData[index];

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

	return oldValue;
    }

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    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;
    }

    /*
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
    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; // Let gc do its work
    }

    /**
     * Removes all of the elements from this list.  The list will
     * be empty after this call returns.
     */
    public void clear() {
	modCount++;

	// Let gc do its work
	for (int i = 0; i < size; i++)
	    elementData[i] = null;

	size = 0;
    }

    /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the
     * specified collection's Iterator.  The behavior of this operation is
     * undefined if the specified collection is modified while the operation
     * is in progress.  (This implies that the behavior of this call is
     * undefined if the specified collection is this list, and this
     * list is nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
	Object[] a = c.toArray();
        int numNew = a.length;
	ensureCapacity(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
	return numNew != 0;
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
	if (index > size || index < 0)
	    throw new IndexOutOfBoundsException(
		"Index: " + index + ", Size: " + size);

	Object[] a = c.toArray();
	int numNew = a.length;
	ensureCapacity(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;
    }

    /**
     * Removes from this list all of the elements whose index is between
     * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.
     * Shifts any succeeding elements to the left (reduces their index).
     * This call shortens the list by <tt>(toIndex - fromIndex)</tt> elements.
     * (If <tt>toIndex==fromIndex</tt>, this operation has no effect.)
     *
     * @param fromIndex index of first element to be removed
     * @param toIndex index after last element to be removed
     * @throws IndexOutOfBoundsException if fromIndex or toIndex out of
     *              range (fromIndex &lt; 0 || fromIndex &gt;= size() || toIndex
     *              &gt; size() || toIndex &lt; fromIndex)
     */
    protected void removeRange(int fromIndex, int toIndex) {
	modCount++;
	int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

	// Let gc do its work
	int newSize = size - (toIndex-fromIndex);
	while (size != newSize)
	    elementData[--size] = null;
    }

    /**
     * Checks if the given index is in range.  If not, throws an appropriate
     * runtime exception.  This method does *not* check if the index is
     * negative: It is always used immediately prior to an array access,
     * which throws an ArrayIndexOutOfBoundsException if index is negative.
     */
    private void RangeCheck(int index) {
	if (index >= size)
	    throw new IndexOutOfBoundsException(
		"Index: "+index+", Size: "+size);
    }

    /**
     * Save the state of the <tt>ArrayList</tt> instance to a stream (that
     * is, serialize it).
     *
     * @serialData The length of the array backing the <tt>ArrayList</tt>
     *             instance is emitted (int), followed by all of its elements
     *             (each an <tt>Object</tt>) in the proper order.
     */
    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 array length
        s.writeInt(elementData.length);

	// 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();
        }

    }

    /**
     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
	// Read in size, and any hidden stuff
	s.defaultReadObject();

        // Read in array length and allocate array
        int arrayLength = s.readInt();
        Object[] a = elementData = new Object[arrayLength];

	// Read in all elements in the proper order.
	for (int i=0; i<size; i++)
            a[i] = s.readObject();
    }
}

就这么点代码，是个人都能看出来arraylist就是封装了数组在里面，你说封装个数组，还比数组快，可能么。
这也就罢了，您换个方式研究咱没意见，最后的结论竟然是为了性能请用数组。
要是这么喜欢性能，您用java干嘛阿。

PS.而且凡是读过书，读过代码，哪怕写过1k行代码的最菜的人，也知道大多数时候，维护一个定长数组需要付出多大的功夫，连cpp里面都漫天vector了。还有人发现了一块新大陆，你让别人能不吐么？
长得丑无所谓，但是出来吓人还是有所谓的。对于不懂的事情，别做总结性发言。

8 楼 DoubleEO 2009-01-09  

李逍遥 写道

楼主虽然是研究了一个已经约定俗成的做法
但毕竟是花了心思的
觉得无价值的可以什么都不说
觉得自己牛的可以别来这个版
但没必要冷嘲热讽吧
难道你们一生下来就什么都会啊

  说得好啊，其实你说出了最平凡的道理，但是能做到的人很少，咱们国家的程序员都是这样的，像古代的文人--自古相轻，你搞出了无论是不是好的东西，都会觉得你是个SB，一直是这样

7 楼 李逍遥 2009-01-09  

楼主虽然是研究了一个已经约定俗成的做法
但毕竟是花了心思的
觉得无价值的可以什么都不说
觉得自己牛的可以别来这个版
但没必要冷嘲热讽吧
难道你们一生下来就什么都会啊

6 楼 DoubleEO 2009-01-09  

hurricane1026 写道

DoubleEO 写道

抛出异常的爱 写道

如果为了快程序里都是[]这东西.....
让我出去吐吐回来再看.
能排序?能快速打印?能快速增加?

吐呗，可劲吐，只是一种思考，以前从来没想过这些性能问题，而且又没说永远不许用集合对象，
只有通过细致的性能测评和对系统的详尽分析，才能做出正确的选择！！

如果你考虑性能问题就从这些地方入手，还是把编程当做业余爱好吧。

外国一个编程的高手，Peter hagger--大名鼎鼎的practicial java的作者，也考虑了这个问题，我就是看他的文章，思考的。总之，谢谢你的建议。

5 楼 hurricane1026 2009-01-09  

DoubleEO 写道

抛出异常的爱 写道

如果为了快程序里都是[]这东西.....
让我出去吐吐回来再看.
能排序?能快速打印?能快速增加?

吐呗，可劲吐，只是一种思考，以前从来没想过这些性能问题，而且又没说永远不许用集合对象，
只有通过细致的性能测评和对系统的详尽分析，才能做出正确的选择！！

如果你考虑性能问题就从这些地方入手，还是把编程当做业余爱好吧。

4 楼 DoubleEO 2009-01-09  

hurricane1026 写道

抛出异常的爱 写道

如果为了快程序里都是[]这东西.....
让我出去吐吐回来再看.
能排序?能快速打印?能快速增加?

吐啊吐啊就习惯了。这个版面真让人不敢来啊。

呵呵，你是不是楼上的小弟啊~

3 楼 DoubleEO 2009-01-09  

抛出异常的爱 写道

如果为了快程序里都是[]这东西.....
让我出去吐吐回来再看.
能排序?能快速打印?能快速增加?

吐呗，可劲吐，只是一种思考，以前从来没想过这些性能问题，而且又没说永远不许用集合对象，
只有通过细致的性能测评和对系统的详尽分析，才能做出正确的选择！！

2 楼 hurricane1026 2009-01-09  

抛出异常的爱 写道

如果为了快程序里都是[]这东西.....
让我出去吐吐回来再看.
能排序?能快速打印?能快速增加?

吐啊吐啊就习惯了。这个版面真让人不敢来啊。

1 楼 抛出异常的爱 2009-01-09  

如果为了快程序里都是[]这东西.....
让我出去吐吐回来再看.
能排序?能快速打印?能快速增加?