看一看ArrayList的源码？

public class ArrayList<E>

/**
     * 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;

    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);
        }
    }
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

ArrayList<Exception> elist = new ArrayList<Exception>();
list.add(new Exception("11"));
list.add(new Exception("22"));
ArrayList<Throwable>  tlist = new ArrayList<>(elist);

ArrayList< User> list = new ArrayList< User>();      这个是用的空构造函数，尖括号中填的是泛型类型
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 public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
 private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

  private static final int DEFAULT_CAPACITY = 10;
  private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        //如果list是空集合的话，返回默认的初始容量10作为所需长度
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        //否则，返回长度加一作为所需长度
        return minCapacity;
    }
  private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // 当所需的长度大于当前数组的长度，那么执行grow方法进行扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

  private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
          //如果扩容后的长度小于当前长度加一，那么取当前长度加一作为扩容后的长度
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
        //如果扩容后的长度大于数组的最大长度，返回一个
        //Integer.MAX_VALUE 或 Integer.MAX_VALUE-8长度作为扩容后长度
            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) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }