arraylist
arraylist 是通过一个数组来实现的，因此它是在连续的存储位置存放对象的引用，只不过它比 array 更智能，能够根据集合长度进行自动扩容。
假设让我们来实现一个简单的能够自动扩容的数组，我们最容易想到的点就是：
- add()的时候需要判断当前数组size+1是否等于此时定义的数组大小；
- 若小于直接添加即可；否则，需要先扩容再进行添加。
实际上，arraylist的内部实现原理也是这样子，我们可以来研究分析一下arraylist的源码
add(e e) 源码分析
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/**
* 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) {
ensurecapacityinternal(size + 1); // 进行扩容校验
elementdata[size++] = e; // 将值添加到数组后面，并将 size+1
return true;
}
/**
* 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
private void ensurecapacityinternal(int mincapacity) {
ensureexplicitcapacity(calculatecapacity(elementdata, mincapacity)); // elementdata 数组
}
/**
* default initial capacity.
*/
private static final int default_capacity = 10;
/**
* shared empty array instance used for default sized empty instances. we
* distinguish this from empty_elementdata to know how much to inflate when
* first element is added.
*/
private static final object[] defaultcapacity_empty_elementdata = {};
// 返回最大的 index
private static int calculatecapacity(object[] elementdata, int mincapacity) {
if (elementdata == defaultcapacity_empty_elementdata) { // 与空数组实例对比
return math.max(default_capacity, mincapacity);
}
return mincapacity;
}
private void ensureexplicitcapacity(int mincapacity) {
modcount++;
// overflow-conscious code
if (mincapacity - elementdata.length > 0)
grow(mincapacity);
}
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扩容调用方法，实际也就是数组复制的过程
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/**
* 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 void grow(int mincapacity) {
// overflow-conscious code
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);
}
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add(int index, e element) 源码分析
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/**
* inserts the specified element at the specified position in this
* list. shifts the element currently at that position (if any) and
* any subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws indexoutofboundsexception {@inheritdoc}
*/
public void add(int index, e element) {
rangecheckforadd(index); // 校验index是否超过当前定义的数组大小范围，超过则抛出 indexoutofboundsexception
ensurecapacityinternal(size + 1); // increments modcount!!
system.arraycopy(elementdata, index, elementdata, index + 1,
size - index); // 复制，向后移动
elementdata[index] = element;
size++;
}
/**
* a version of rangecheck used by add and addall.
*/
private void rangecheckforadd(int index) {
if (index > size || index < 0)
throw new indexoutofboundsexception(outofboundsmsg(index));
}
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从上面的源码分析可知，扩容和随机插入元素的消耗比较大，因此在实际开发中，应尽量指定arraylist大小，减少在随机插入操作。
优缺点
优点
- 封装了一个动态再分配的对象数组
- 使用索引进行随机访问效率高
缺陷
- 在数组中增删一个元素，所有元素都要往后往前移动，效率低下
知识脑图
以上所述是小编给大家介绍的java集合系列arraylist详解整合，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对服务器之家网站的支持！
原文链接：https://www.cnblogs.com/phpstudy2015-6/p/10618707.html