Collection ArrayList
一、 源码分析
1. ArrayList 类组成
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable // 1. 实现了List接口，List继承Collection接口，Collection接口继承Iterable接口
{
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10; // 2. 默认数组容量大小(最小为10)
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; // 3. 共享空数组，不用赋值
/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
*/
transient Object[] elementData; // 4. ！！！存储数据数组(transient防止序列化，用自定义序列化方法，防止浪费多余空间)
/**
* 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
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; // 5. 构造函数
}
2. add(E)方法
* 1. 自动判断是否需要增加存储空间，如果需要，增加数组长度的一半(oldSize + oldSize>>1)；
* 2. mouCount++,如果在迭代器遍历是数据结构发生变化，报异常；
* 3. Arrays.copyOf(data,newSize) 生成新数组；
* 4. 赋值&Size++;
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); } private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } /** * Increases the capacity to ensure that it can hold at least the * number of elements specified by the minimum capacity argument. * * @param minCapacity the desired minimum capacity */ private void grow(int minCapacity) { // overflow-conscious code int oldCapacity = elementData.length; int newCapacity = oldCapacity + (oldCapacity >> 1); if (newCapacity - minCapacity < 0) newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); // minCapacity is usually close to size, so this is a win: elementData = Arrays.copyOf(elementData, newCapacity); }
3. add(index,element)&remove(index)方法
耗时操作
System.arraycopy(Object src,
int srcPos,
Object dest,
int destPost,
int length);
/** * 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) { rangeCheckForAdd(index); ensureCapacityInternal(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 = elementData(index); int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work return oldValue; }
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n. 问题分析
1. arraylist 存储大小如何自动增长，增长多少？
容量检查方法判断，如果容量不够，增加size/2(默认是10);
2. ArrayList&LinkedList区别？
多数情况下，当你遇到访问元素比插入或者是删除元素更加频繁的时候，你应该使用ArrayList。
另外一方面，当你在某个特别的索引中，插入或者是删除元素更加频繁，或者你压根就不需要访问元素的时候，你会选择LinkedList。
这里的主要原因是，在ArrayList中访问元素的最糟糕的时间复杂度是”1″，而在LinkedList中可能就是”n”了。在ArrayList中增加或者删除某个元素，通常会调用System.arraycopy方法，这是一种极为消耗资源的操作，因此，在频繁的插入或者是删除元素的情况下，LinkedList的性能会更加好一点。
3. 当传递ArrayList到某个方法中，或者某个方法返回ArrayList，什么时候要考虑安全隐患？如何修复安全违规这个问题呢？
当array被当做参数传递到某个方法中，如果array在没有被复制的情况下直接被分配给了成员变量，那么就可能发生这种情况，即当原始的数组被调用的方法改变的时候，传递到这个方法中的数组也会改变。下面的这段代码展示的就是安全违规以及如何修复这个问题。
ArrayList被直接赋给成员变量——安全隐患：
修复这个安全隐患：
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