ArrayList source code相关方法阅读

1、新增一个对象

/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacity(size + 1);  // Increments modCount!!
elementData[size++] = e;
return true;
}

/**
* Increases the capacity of this <tt>ArrayList</tt> instance, if
* necessary, to ensure that it can hold at least the number of elements
* specified by the minimum capacity argument.
*
* @param   minCapacity   the desired minimum capacity
*/
public void ensureCapacity(int minCapacity) {
modCount++;
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) {
Object oldData[] = elementData;
int newCapacity = (oldCapacity * 3)/2 + 1;
if (newCapacity < minCapacity)
newCapacity = minCapacity;
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
}

源码里面elementData对象是这样定义：private transient Object[] elementData;

在之前的开发中从来没用过transient 关键字，特意测试了一下，发现transient就是persistent的对立，当用到字节流、网络传输的时候一旦属性定义为transient将会出现获取值为null的情况。

详情可参考：What is transient keyword in Java?

还有一个需要了解知识点的地方就是Arrays.copyOf(elementData, newCapacity)方法，它是将elementData数组的数据按照newCapacity长度建立一个新的数组的同时，将原数组elementData数组里面的数据拷贝到新的数组中。

2、移除一个对象

/**
* Removes the first occurrence of the specified element from this list,
* if it is present.  If the list does not contain the element, it is
* unchanged.  More formally, removes the element with the lowest index
* <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists).  Returns <tt>true</tt> if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}

/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // Let gc do its work
}

这里面关键点就是fastRemove方法中的System.arraycopy(elementData, index+1, elementData, index, numMoved);

这个方法的含义是将elementData数组从index+1位置到numMoved位置的数据拷贝到数组elementData的index位置，依次往后填充

详情可参考：老紫竹

以上两点我们可以看出：list其实的存储其实就是数组；数组都是按位来存储，所以在插入或者是移除的时候都是依次顺序往后推移，所以我们的面试题经常会提到ArrayList查询效率要高于新增和删除的效率

这里暂时只列出两个方法，其他今后有时间研究再这里列出来，待续....