@zero1036
2017-03-14T17:11:52.000000Z
字数 2315
阅读 1330
Java-Base
ArrayList 是一个数组队列,相当于动态数组。与Java中的数组相比,它的容量能动态增长。它继承于AbstractList,实现了List, RandomAccess, Cloneable, java.io.Serializable这些接口。
继承关系如下:
java.lang.Object
↳ java.util.AbstractCollection<E>
↳ java.util.AbstractList<E>
↳ java.util.ArrayList<E>
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable {}
public boolean add(E e) {
// 确定ArrayList的容量大小
ensureCapacityInternal(size + 1); // Increments modCount!!
// 添加e到ArrayList中
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
//minCapacity:最小容量 = size + 1(当执行add方法时)
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);
}
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
//新容量 = 旧容量 + 旧容量二进制位数右移1位 (例:15 = 10 + 5),即,每次扩容都增加原始容量的二分一,减少执行Arrays.copyOf()
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);
}
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
@SuppressWarnings("unchecked")
//验证参数是否Object[]数组,是:new Object[];否:调用Arrays的native方法newArray()
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
//最后执行方法System.arraycopy(),调用native方法arraycopy();参数24:源数组与目标数组的起始复制位,默认为0、0
System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength));
return copy;
}
Collections排序采用Timsort排序实现,参考:Timsort排序算法
使用方法:
List list = new ArrayList<Integer>();
...
Collections.sort(list, new Comparator<Integer>() {
/**
* 升序:Comparator.compare(右元素, 左元素) >= 0;
* 降序:Comparator.compare(右元素, 左元素) < 0
*/
public int compare(Integer o1, Integer o2) {
if (o2 > o1) {
return 1;
} else {
return -1;
}
}
});
java.util.ArrayList并非java API的线程安全类型,而java.util.Vector(矢量队列)则可以理解是ArrayList的线程安全版本。但以上两者都不是绝对的线程安全。参考:Vector源码分析