线性表—— 顺序存储结构
线性表 顺序存储结构，指的是用一段地址连续的存储单元依次存储线性表的数据元素。
Java ArrayList
对象的定义
/**
* Resizable-array implementation of the List interface
*/
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 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 = {};
/**
* 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; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;
本质是一个 Object[] 数组。
CRUD
Construct
/**
*
* Params: 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;
}
Add
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
public void add(int index, E element) {
//检查 index 合法性
rangeCheckForAdd(index);
//数组有足够的空间来进行数组 copy
ensureCapacityInternal(size + 1); // Increments modCount!!
//数组 copy
System.arraycopy(elementData, index, elementData, index + 1, size - index);
//进行 add 操作
elementData[index] = element;
//增加 size
size++;
}
/**
* src – the source array.
* srcPos – starting position in the source array.
* dest – the destination array.
* destPos – starting position in the destination data.
* length – the number of array elements to be copied.
*/
public static native void arraycopy(Object src, int srcPos, Object dest, int destPos, int length);
Remove
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; // clear to 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++;
// clear to let GC do its work
for (int i = 0; i < size; i++) elementData[i] = null;
size = 0;
}
Get
public E get(int index) {
//判断数组越界
rangeCheck(index);
return elementData(index);
}
public E set(int index, E element) {
//判断数据越界
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
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;
}
数组长度和线性表长度区别
数组长度
数组长度是存放线性表的存储空间的长度，存储分配后这个量是一般是不会变的。一般像 Java 这种高级语言，用编程手段实现动态分配数组，但是存在一定性能消耗。
Java ArrayList 添加元素时，保证数组长度够用。
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return 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;
// 扩容 1.5倍
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);
}
线性表长度
线性袤的长度是线性表中数据元素的个数，随着线性表插入和删除操作的进行，这个量是变化的。
线性表顺序存储结构的优缺点
优点
- 无须为表示表中元素之间的逻辑关系而增加额外的存储空间。
- 可以快速获取表中任一位置元素。
缺点
- 插入或者删除操作需要移动大量的元素。
- 当线性表长度变化较大时，难以确定存储空间容量，进行扩容存在性能损耗。
- 造成存储空间的碎片。
ArrayList 总结
- ArrayList 底层实现为 Object[]
- 指定位置的 add 与 remove 核心逻辑均为 System.arraycopy
