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
1 /** 2 * Appends the specified element to the end of this list. 3 * 4 * @param e element to be appended to this list 5 * @return <tt>true</tt> (as specified by {@link Collection#add}) 6 */ 7 public boolean add(E e) { 8 ensureCapacityInternal(size + 1); // 进行扩容校验 9 elementData[size++] = e; // 将值添加到数组后面,并将 size+1 10 return true; 11 } 12 13 14 15 /** 16 * The array buffer into which the elements of the ArrayList are stored. 17 * The capacity of the ArrayList is the length of this array buffer. Any 18 * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA 19 * will be expanded to DEFAULT_CAPACITY when the first element is added. 20 */ 21 transient Object[] elementData; // non-private to simplify nested class access 22 23 private void ensureCapacityInternal(int minCapacity) { 24 ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); // elementData 数组 25 } 26 27 28 29 /** 30 * Default initial capacity. 31 */ 32 private static final int DEFAULT_CAPACITY = 10; 33 34 /** 35 * Shared empty array instance used for default sized empty instances. We 36 * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when 37 * first element is added. 38 */ 39 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; 40 41 // 返回最大的 index 42 private static int calculateCapacity(Object[] elementData, int minCapacity) { 43 if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { // 与空数组实例对比 44 return Math.max(DEFAULT_CAPACITY, minCapacity); 45 } 46 return minCapacity; 47 } 48 49 50 51 private void ensureExplicitCapacity(int minCapacity) { 52 modCount++; 53 54 // overflow-conscious code 55 if (minCapacity - elementData.length > 0) 56 grow(minCapacity); 57 }
扩容调用方法,实际也就是数组复制的过程
1 /** 2 * Increases the capacity to ensure that it can hold at least the 3 * number of elements specified by the minimum capacity argument. 4 * 5 * @param minCapacity the desired minimum capacity 6 */ 7 private void grow(int minCapacity) { 8 // overflow-conscious code 9 int oldCapacity = elementData.length; 10 int newCapacity = oldCapacity + (oldCapacity >> 1); 11 if (newCapacity - minCapacity < 0) 12 newCapacity = minCapacity; 13 if (newCapacity - MAX_ARRAY_SIZE > 0) 14 newCapacity = hugeCapacity(minCapacity); 15 // minCapacity is usually close to size, so this is a win: 16 elementData = Arrays.copyOf(elementData, newCapacity); 17 }
add(int index, E element)
源码分析
1 /** 2 * Inserts the specified element at the specified position in this 3 * list. Shifts the element currently at that position (if any) and 4 * any subsequent elements to the right (adds one to their indices). 5 * 6 * @param index index at which the specified element is to be inserted 7 * @param element element to be inserted 8 * @throws IndexOutOfBoundsException {@inheritDoc} 9 */ 10 public void add(int index, E element) { 11 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 12 13 ensureCapacityInternal(size + 1); // Increments modCount!! 14 System.arraycopy(elementData, index, elementData, index + 1, 15 size - index); // 复制,向后移动 16 elementData[index] = element; 17 size++; 18 } 19 20 21 /** 22 * A version of rangeCheck used by add and addAll. 23 */ 24 private void rangeCheckForAdd(int index) { 25 if (index > size || index < 0) 26 throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); 27 }
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
在 github 上建了一个 repository ,Java Core Knowledge Tree,各位看官若是喜欢请给个star,以示鼓励,谢谢。
https://github.com/suifeng412/JCKTree
(以上是自己的一些见解,若有不足或者错误的地方请各位指出)
作者:那一叶随风 http://www.cnblogs.com/phpstudy2015-6/
原文地址: https://www.cnblogs.com/phpstudy2015-6/p/10618707.html
声明:本博客文章为原创,只代表本人在工作学习中某一时间内总结的观点或结论。转载时请在文章页面明显位置给出原文链接
原文地址:https://www.cnblogs.com/phpstudy2015-6/p/10618707.html
时间: 2024-11-09 09:25:30