标签：最大的 zed 快速 数据 least style share storage math
先看
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
继承自AbstractList，实现了4个接口，List,RandomAccess,Cloneable,java.io.Serializable.
List:这是一个序列，可以有重复元素。
RandomAccess：可以随机快速访问的标记。
Cloneable: 它实现了clone方法，可以合法地调用object.clone()方法。
java.io.Serializable：支持序列化。
1 /** 2 * Default initial capacity. 3 */ 4 private static final int DEFAULT_CAPACITY = 10;
默认初始大小为10。
1 /** 2 * Shared empty array instance used for empty instances. 3 */ 4 private static final Object[] EMPTY_ELEMENTDATA = {};
为空的实例所共享的空数组实例
1 /** 2 * Shared empty array instance used for default sized empty instances. We 3 * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when 4 * first element is added. 5 */ 6 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
用于默认大小的空实例的共享空数组实例。 将此与EMPTY_ELEMENTDATA区分开来，以了解添加第一个元素时要膨胀多少。
1 /** 2 * The array buffer into which the elements of the ArrayList are stored. 3 * The capacity of the ArrayList is the length of this array buffer. Any 4 * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA 5 * will be expanded to DEFAULT_CAPACITY when the first element is added. 6 */ 7 transient Object[] elementData; // non-private to simplify nested class access
可见ArrayList内部是用数组实现的。trasient 修饰符，这个属性不会被序列化。当第一个元素添加到空的ArrayList来的时候，这个数组会扩展到默认大小，也就是10.
1 /** 2 * The size of the ArrayList (the number of elements it contains). 3 * 4 * @serial 5 */ 6 private int size;
ArrayList的大小。也就是这个容器中元素的个数。
/** * Constructs an empty list with the specified initial capacity. * * @param initialCapacity the initial capacity of the list * @throws IllegalArgumentException if the specified initial capacity * is negative */ 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); } }
这是ArrayList的一个带参构造方法，可以看到，初始容量不能为负，否则会主动抛出异常。这里，如果初始容量大于0，新建了一个Object数组并赋给了elementData。如果初始容量等于0，直接将EMPTY_ELEMENTDATA赋值给elementData。换句话说，在jdk1.8中，所有以带初始容量入参的构造方法创建的且传入的初始容量为0的ArrayList变量，指向的是同一个空的对象数组常量。
1 /** 2 * Constructs an empty list with an initial capacity of ten. 3 */ 4 public ArrayList() { 5 this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; 6 }
无参构造方法。会将DEFAULTCAPACITY_EMPTY_ELEMENTDATA赋值给elementData.
1 /** 2 * Constructs a list containing the elements of the specified 3 * collection, in the order they are returned by the collection‘s 4 * iterator. 5 * 6 * @param c the collection whose elements are to be placed into this list 7 * @throws NullPointerException if the specified collection is null 8 */ 9 public ArrayList(Collection<? extends E> c) { 10 elementData = c.toArray(); 11 if ((size = elementData.length) != 0) { 12 // c.toArray might (incorrectly) not return Object[] (see 6260652) 13 if (elementData.getClass() != Object[].class) 14 elementData = Arrays.copyOf(elementData, size, Object[].class); 15 } else { 16 // replace with empty array. 17 this.elementData = EMPTY_ELEMENTDATA; 18 } 19 }
一个带参的构造方法，可以传入E的子类组成的集合。elementData直接被赋值c.toArray()。如果这时它长度为0，
会被替换成内部的EMPTY_ELEMENTDATA；如果长度不为0，进一步判断它的class是否为Object[].class，
如果不是，则会用Arrays.copyOf()方法对其按照Object[]的class进行一次拷贝。
PS:see 6260652指的是以前这个地方有bug,具体可以看：https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6260652
1 /** 2 * Trims the capacity of this <tt>ArrayList</tt> instance to be the 3 * list‘s current size. An application can use this operation to minimize 4 * the storage of an <tt>ArrayList</tt> instance. 5 */ 6 public void trimToSize() { 7 modCount++; 8 if (size < elementData.length) { 9 elementData = (size == 0) 10 ? EMPTY_ELEMENTDATA 11 : Arrays.copyOf(elementData, size); 12 } 13 }
调整ArrayList的容量为当前列表的大小。这个方法可以用来最小化ArrayList实例所占用的空间。
1 /** 2 * Increases the capacity of this <tt>ArrayList</tt> instance, if 3 * necessary, to ensure that it can hold at least the number of elements 4 * specified by the minimum capacity argument. 5 * 6 * @param minCapacity the desired minimum capacity 7 */ 8 public void ensureCapacity(int minCapacity) { 9 int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) 10 // any size if not default element table 11 ? 0 12 // larger than default for default empty table. It‘s already 13 // supposed to be at default size. 14 : DEFAULT_CAPACITY; 15 16 if (minCapacity > minExpand) { 17 ensureExplicitCapacity(minCapacity); 18 } 19 }
增加ArrayList 实例的容量，如有必要，保证它至少可以容纳minCapacity(最小容量)个元素。minExpand(最小扩展)在elementData为空数据的时候为默认容量10，否则为0。当入参最小容量大于这个最小扩展时，会调用ensureExplicitCapacity()方法，并传入minCapacity。
1 private static int calculateCapacity(Object[] elementData, int minCapacity) { 2 if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { 3 return Math.max(DEFAULT_CAPACITY, minCapacity); 4 } 5 return minCapacity; 6 }
计算容量，如果elementData为空数据，返回初始容量10和minCapacity中的大者，否则直接返回minCapacity。
1 private void ensureCapacityInternal(int minCapacity) { 2 ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); 3 }
内部的保证容量的方法，先调calculateCapacity()计算容量，再调ensureExplicitCapacity()。
1 private void ensureExplicitCapacity(int minCapacity) { 2 modCount++; 3 4 // overflow-conscious code 5 if (minCapacity - elementData.length > 0) 6 grow(minCapacity); 7 }
保证精确的容量，成员变量modCount自增一次，当最小容量大于elementData的长度时，调用grow()，并传入minCapacity。
/** * The maximum size of array to allocate. * Some VMs reserve some header words in an array. * Attempts to allocate larger arrays may result in * OutOfMemoryError: Requested array size exceeds VM limit */ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
这个私有常量表明数组最大的长度可以为Integer的最大值减8，如果尝试分配更大的数组会引发内存溢出。
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 }
grow()方法上面ensureExplicitCapacity()方法有调用。旧容量就是elementData的长度，新容量是老容量加老容量左移1位，就是1.5倍的样子。再看如果新容量小于最小容量，就取最小容量。如果新容量大于最大数组大小，就调hugeCapacity()计算一下。最后，将elementData按照上面得出的最小容量，进行一次拷贝。
1 private static int hugeCapacity(int minCapacity) { 2 if (minCapacity < 0) // overflow 3 throw new OutOfMemoryError(); 4 return (minCapacity > MAX_ARRAY_SIZE) ? 5 Integer.MAX_VALUE : 6 MAX_ARRAY_SIZE; 7 }
大容量计算，如果minCapacity小于0，直接抛内存溢出error；再比较它与最大数组大小，小于则取整型最大值，超过了则取最大数组大小。
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标签：最大的 zed 快速 数据 least style share storage math
原文地址：https://www.cnblogs.com/julymaple/p/4211614.html
