手撕ArrayList底层源码

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
    //外部操作数
    protected transient int modCount = 0;//2
}

public class ArrayList<E> extends AbstractList<E> implements List<E>{
    //默认容量
    private static final int DEFAULT_CAPACITY = 10;
    //空内容的数组(长度为0的数组)
    private static final Object[] EMPTY_ELEMENTDATA = {};
	//默认容量空内容的数字(长度为0的数组)
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    //元素容器
    transient Object[] elementData;//new Object[10]{张三,李四,null,null,null,...}
    //元素个数
    private int size;//2
    //最大的数组长度
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    
    //initialCapacity - 100
    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);
        }
    }
    
    //e - 李四
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    
    //minCapacity - 2
    private void ensureCapacityInternal(int minCapacity) {
        //使用无参构造创建对象，第一次添加元素时进入的判断
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            //minCapacity - 10
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }
    
    //minCapacity - 2
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    
    //minCapacity - 10
    private void grow(int minCapacity) {
        //oldCapacity - 0
        int oldCapacity = elementData.length;
        //newCapacity - 0
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            //newCapacity - 10
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        
        //数组的扩容
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
}

ArrayList<String> list = new ArrayList<>();
//ArrayList<String> list = new ArrayList<>(10000);

list.add("张三");
list.add("李四");
list.add("王五");
list.add("赵六");