ArrayList 源码分析

    // 默认情况下，数组的初始化大小
    private static final int DEFAULT_CAPACITY = 10;

    // 空数组
    private static final Object[] EMPTY_ELEMENTDATA = {};

    // 空数组
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    // 数据
    transient Object[] elementData; // non-private to simplify nested class access

    // 数据大小
    private int size;
    
    // 给数组指定初始化大小
    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);
        }
    }
    
    // 不指定大小的话默认给数组指定初始化大小为10
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    
    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);
    }
    
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        // 默认情况下扩充为原来的一半
        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:
        // 创建一个新数组并把原来数组里的内容拷贝到新数组中
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    
    public static <T> T[] copyOf(T[] original, int newLength) {
        return (T[]) copyOf(original, newLength, original.getClass());
    }
    
    public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
        @SuppressWarnings("unchecked")
        T[] copy = ((Object)newType == (Object)Object[].class)
            ? (T[]) new Object[newLength]
            : (T[]) Array.newInstance(newType.getComponentType(), newLength);
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }
    
    public E remove(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(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; // clear to let GC do its work

        return oldValue;
    }   
    
    public int size() {
        return size;
    }
    
    // 通过 native 层去拷贝代码
    // src ：原来的数组
    // srcPos：原来数组的开始位置
    // dest：新的数组
    // destPos：新数组的开始位置
    // length：拷贝多少个
    public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);

通过上面的代码来分析，ArrayList 其内部的实现方式其实就是数组，如果没指定数组的大小，那么在第一次添加数据的时候，数组的初始大小是 10 ，每次当不够用的时候默认会扩充原来数组的 1/2 ，每次扩充数组大小都会涉及到创建新数组和数据的拷贝复制。而数组的拷贝和逻动都是由我们的 native 层代码实现，接下来我们简单实现native层ArrayList逻辑。

实现 Native 层的 ArrayList

#include<malloc.h>
#include<memory.h>
#include <iostream>

using namespace std;

template<class E>
class ArrayList
{
public:
    //数组头指针
    E* elementData = NULL;
    //数组长度
    int index = 0;
    //数组容量大小
    int size = 0;

public:
    ArrayList();
    ArrayList(int size);
    ArrayList(const ArrayList& list);
    ~ArrayList();

public:
    bool add(E e);
    int len();
    int capacity();
    E get(int index);
    E remove(int index);
private:
    void ensureCapacityInternal(int minCapacity);
    void grow(int minCapacity);
};

template<class E>
ArrayList<E>::ArrayList()
{
    cout << "无参构造" << endl;
}

template<class E>
ArrayList<E>::ArrayList(int size)
{
    cout << "带参构造" << endl;
    if (size == 0) {
        return;
    }
    this->size = size;
    this->elementData = (E*)malloc(sizeof(E) * size);
}

template<class E>
ArrayList<E>::ArrayList(const ArrayList& list)
{
    cout << "拷贝构造" << endl;
    this->size = list.size;
    this->index = list.index;
    this->elementData = (E*)malloc(sizeof(E) * size);
    memcpy(this->elementData, list.elementData, sizeof(E) * size);
}

template<class E>
ArrayList<E>::~ArrayList()
{
    cout << "析构函数" << endl;
    if (this->elementData) {
        free(this->elementData);
        this->elementData = NULL;
    }
}

template<class E>
bool ArrayList<E>::add(E e)
{
    ensureCapacityInternal(index + 1);
    this->elementData[index++] = e;
    return true;
}

template<class E>
int ArrayList<E>::len()
{
    return this->index;
}

template<class E>
int ArrayList<E>::capacity()
{
    return this->size;
}

template<class E>
E ArrayList<E>::get(int index)
{
    return this->elementData[index];
}

template<class E>
E ArrayList<E>::remove(int index)
{
    E oldValue = this->elementData[index];
    int needMoved = this->index - index - 1;
    int i = 0;
    for (i; i < needMoved; i++)
    {
        this->elementData[index + i] = this->elementData[index + i + 1];
    }
    this->index -= 1;
    return oldValue;
}

template<class E>
void ArrayList<E>::ensureCapacityInternal(int minCapacity)
{
    if (this->elementData == NULL)
    {
        minCapacity = 10;
    }
    if (minCapacity - size > 0)
    {
        grow(minCapacity);
    }

}

template<class E>
void ArrayList<E>::grow(int minCapacity)
{
    int newCapacity = size + (size >> 1);
    if (newCapacity - minCapacity < 0)
    {
        newCapacity = minCapacity;
    }
    E* new_arr = (E*)malloc(sizeof(E) * newCapacity);

    if (this->elementData)
    {
        memcpy(new_arr, this->elementData, sizeof(E) * index);
        free(this->elementData);
    }

    this->elementData = new_arr;
    size = newCapacity;
}


int main() {
    //ArrayList<int> list = { 4 };
    //ArrayList<int> list(4);
    //list.add(1);
    //list.add(2);
    //list.add(3);
    //int i = 0;
    //for (i; i < list.index; i++)
    //{
    //  cout << "i:" << list.get(i) << endl;
    //}
    ArrayList<int>* list = new ArrayList<int>(5);
    list->add(1);
    list->add(2);
    list->add(3);

    cout << "remove 前 len:" << list->len() << " capacaity:" << list->capacity() << endl;
    int i = 0;
    for (i; i < list->index; i++)
    {
        cout << "i:" << list->get(i) << endl;
    }

    list->remove(1);
    cout << "remove 后 len:" << list->len() << " capacaity:" << list->capacity() << endl;

    i = 0;
    for (i; i < list->index; i++)
    {
        cout << "i:" << list->get(i) << endl;
    }
    getchar();
    return 0;
}

运行结果

带参构造
remove 前 len:3 capacaity:5
i:1
i:2
i:3
remove 后 len:2 capacaity:5
i:1
i:3