浅析ArrayList底层源码

这里只是对ArrayList的简要解析，不喜勿喷，多多点赞。下面直接看源码

public ArrayList extends AbstractList implements List{
    
  	
    //用于空实例的共享空数组实例。
    private static final Object[] EMPTY_ELEMENTDATA = {};
    
    //存储数据的容器
    private transient Object[] elementData;
    
    //数组的长度 -- 指针
    private int size;
    
    //默认容器长度
    private static final int DEFAULT_CAPACITY = 10;
    
    //数组容器最大的长度
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    
    public ArrayList(){
        super();
        //调用无参构造函数创建，直接将空数组EMPTY_ELEMENTDATA 的地址给它
        this.elementData = EMPTY_ELEMENTDATA;
    }
    
    public ArrayList(int initialCapacity) {
        super();
        //初始化长度小于0，就抛异常
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        //初始化数组
        this.elementData = new Object[initialCapacity];
    }
    
    public boolean add(E e) {
       	//判断是否扩容
        ensureCapacityInternal(size + 1);
        //添加元素
        elementData[size++] = e;
        return true;
    }
    
    private void ensureCapacityInternal(int minCapacity) {
        //使用无参构造创建对象时，第一次添加数据就会进入到此判断
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }
    
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        //准备扩容的长度 必须大于 现有数组的长度 
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    
    private void grow(int minCapacity) {
        
        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);
    }
    
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) 
            throw new OutOfMemoryError();
        
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
}

常见面试题

1. ArrayList list = new ArrayList();底层的数组容器就是个空数组-- {}
2. ArrayList list = new ArrayList(100);底层的数据容器就是 – new Object[100]
3. ArrayList默认初始化长度为10
4. 容器最大长度Integer.MAX_VALUE-8，因为一些vm在数组中保留一些头部信息，这个信息需要空间存储，所以减去8的空间就用来装头部信息
5. 扩容的长度是原来的1.5倍