以下代码片都是 jdk1.8 ArrayList中的官方代码
/**
* 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();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
解读：ArrayList的构造方法，用得比较少，至少我用得比较少。参数是Collection的实现类都行，
由此我想到了一个好玩的东西，如果要将两个集合想加，那么可以试试这个方法，虽然官方提供了
public boolean addAll(int index, Collection<? extends E> c) 和public boolean addAll(Collection<? extends E> c) 这两个方法。
/**
* 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++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
解读：这是个好东西。
ArrayList所说没有用的值并不是null，而是ArrayList每次容量不够用时申请的存储空间会稍稍多一些，1.5倍+1，
这样就会出现当size() = 1000的时候，ArrayList已经申请了1200空间的情况 此时trimToSize
的作用只是去掉预留元素位置，就是删除多余的200,官方指导的是可以用来优化存储
/**
* 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) {
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
// any size if not default element table
? 0
// larger than default for default empty table. It's already
// supposed to be at default size.
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
解读：这更是个好东西。因为官方都建议使用了，
* <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.
简单来说就是业务情况预先设置集合的容量，这样能够大大提高初始化速度。可能有人说直接指定容量呢，实际情况中你哪会知道容量会多大，就算知道，那效率更是不敢看
下面是测试代码，运行看看就知道了
package sourceCode.ArrayList;
import java.util.ArrayList;
/**
* ArrayList.ensureCapacity(N)性能测试
*
*/
public class entureCapacityTest {
@SuppressWarnings({ "unchecked", "rawtypes" })
public static void main(String[] args) {
final int N = 100000000;
Object obj = new Object();
// 1.没用调用ensureCapacity()方法初始化ArrayList对象
ArrayList list = new ArrayList();
long startTime = System.currentTimeMillis();
for (int i = 0; i <= N; i++) {
list.add(obj);
}
long endTime = System.currentTimeMillis();
System.out.println("没有调用ensureCapacity()方法所用时间：" + (endTime - startTime) + "ms");
// 2.调用ensureCapacity()方法初始化ArrayList对象
list = new ArrayList();
startTime = System.currentTimeMillis();
// 预先设置list的大小
list.ensureCapacity(N);
for (int i = 0; i <= N; i++) {
list.add(obj);
}
endTime = System.currentTimeMillis();
System.out.println("调用ensureCapacity()方法所用时间：" + (endTime - startTime) + "ms");
// 3.直接指定容量
list = new ArrayList(N);
startTime = System.currentTimeMillis();
for (int i = 0; i <= N; i++) {
list.add(obj);
}
endTime = System.currentTimeMillis();
System.out.println("直接设置容量所用时间：" + (endTime - startTime) + "ms");
}
}
/**
* Retains only the elements in this list that are contained in the
* specified collection. In other words, removes from this list all
* of its elements that are not contained in the specified collection.
*
* @param c collection containing elements to be retained in this list
* @return {@code true} if this list changed as a result of the call
* @throws ClassCastException if the class of an element of this list
* is incompatible with the specified collection
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if this list contains a null element and the
* specified collection does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>),
* or if the specified collection is null
* @see Collection#contains(Object)
*/
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
解读：取两个集合的交集