ArrayList的扩容机制-编程知识

ArrayList的扩容机制

ArrayList 的底层操作机制源码分析

ArrayList中维护一个Object类型的数组elementData
transient Object[] elementData; //transient表示瞬间短暂的，表示该属性不会被序列化
当创建ArrayList对象时，如果使用的是无参构造器，则初始elementData容量为0，第一次添加，则扩容elementData 为10，如需再次扩容，则扩容elementData为1.5倍
如果使用的是指定大小的构造器，则初始elementData容量为指定大小，如需再次扩容，则直接扩容elementData为1.5倍

public class ArrayListSource_ {public static void main(String[] args) {ArrayList list = new ArrayList(8);//  ArrayList list = new ArrayList();for (int i = 1; i <= 10; i++) {list.add(i);}for (int i = 11; i <= 15; i++) {list.add(i);}list.add(200);list.add(200);list.add(null);}
}

 public ArrayList() {this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;}

 public boolean add(E e) {ensureCapacityInternal(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}

  private static int calculateCapacity(Object[] elementData, int minCapacity) {if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {return Math.max(DEFAULT_CAPACITY, minCapacity);}return minCapacity;}

    private void ensureCapacityInternal(int minCapacity) {ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));}

    private void grow(int minCapacity) {// overflow-conscious codeint 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);}