标签:turned specified elements string dex number system sci not
ArrayList是我们经常使用的一个数据结构,我们通常把其用作一个可变长度的动态数组使用,大部分时候,可以替代数组的作用,我们不用事先设定ArrayList的长度,只需要往里不断添加元素即可,ArrayList会动态增加容量。ArrayList是作为List接口的一个实现。 那么ArrayList背后使用的数据结构是什么呢? ArrayList是如何保证动态增加容量,使得能够正确添加元素的呢?
要回答上面的问题,我们就需要对ArrayList的源码进行一番分析,深入了解其实现原理的话,我们就自然能够解答上述问题。
需要说明的是,本文所分析的源码引用自JDK 8版本
ArrayList使用的存储的数据结构 从源码中我们可以发现,ArrayList使用的存储的数据结构是Object的对象数组。 其实这也不能想象,我们知道ArrayList是支持随机存取的类似于数组,所以自然不可能是链表结构。
/** * 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
我想大家一定对这里出现的transient关键字很疑惑,我们都知道ArrayList对象是可序列化的,但这里为什么要用transient关键字修饰它呢?查看源码,我们发现ArrayList实现了自己的readObject和writeObject方法,所以这保证了ArrayList的可序列化。具体序列化的知识我们在此不过多赘述。有兴趣的读者可以参考笔者关于序列化的文章。
ArrayList提供了三个构造函数。下面我们依次来分析
public ArrayList(int initialCapacity);//当我们初始化的时候,给ArrayList指定一个初始化大小的时候,就会调用这个构造方法
List<String> myList = new ArrayList<String>(7);
源码中这个方法的实现如下
/** * 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); } }
这里的EMPTY_ELEMENTDATA 实际上就是一个共享的空的Object数组对象。
/** * Shared empty array instance used for empty instances. */ private static final Object[] EMPTY_ELEMENTDATA = {};
上述代码很容易理解,如果用户指定的初始化容量大于0,就new一个相应大小的数组,如果指定的大小为0,就复制为共享的那个空的Object数组对象。如果小于0,就直接抛出异常。
myList = new ArrayList();
源码中的实现是
/** * Constructs an empty list with an initial capacity of ten. */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } //其中DEFAULTCAPACITY_EMPTY_ELEMENTDATA 定义为 ** * 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 = {};
注释中解释的很清楚,就是说刚初始化的时候,会是一个共享的类变量,也就是一个Object空数组,当第一次add的时候,这个数组就会被初始化一个大小为10的数组。
Set<Integer> set = new HashSet<>(); set.add(1); set.add(2); set.add(3); set.add(4); ArrayList<Integer> list = new ArrayList<>(set);
源码中是这么实现的
/** * Constructs a list containing the elements of the specified * collection, in the order they are returned by the collection‘s * iterator. * * @param c the collection whose elements are to be placed into this list * @throws NullPointerException if the specified collection is null */ public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); if ((size = elementData.length) != 0) { // c.toArray might (incorrectly) not return Object[] (see 6260652) if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); } else { // replace with empty array. this.elementData = EMPTY_ELEMENTDATA; } }
首先调用给定的collection的toArray方法将其转换成一个Array。 然后根据这个array的大小进行判断,如果不为0,就调用Arrays的copyOf的方法,复制到Object数组中,完成初始化,如果为0,就直接初始化为空的Object数组。
/** * 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); // Increments modCount!! elementData[size++] = e; return true; }
ensureCapacityInternal的实现如下:
private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); }
DEFAULT_CAPACITY为:
private static final int DEFAULT_CAPACITY = 10;
这也就实现了当我们不指定初始化大小的时候,添加第一个元素的时候,数组会扩容为10.
private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); }
这个函数判断是否需要扩容,如果需要就调用grow方法扩容
/** * 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); }
我们可以看到grow方法将数组扩容为原数组的1.5倍,调用的是Arrays.copy 方法。 在jdk6及之前的版本中,采用的还不是右移的方法
我们移除元素的时候,有两种方法,一是指定下标,二是指定对象
list.remove(3);//index list.remove("aaa");//object
下面先来分析第一种,也就是
/** * Removes the element at the specified position in this list. * Shifts any subsequent elements to the left (subtracts one from their * indices). * * @param index the index of the element to be removed * @return the element that was removed from the list * @throws IndexOutOfBoundsException {@inheritDoc} */ public E remove(int index) { rangeCheck(index); modCount++; E oldV/** * Removes the element at the specified position in this list. * Shifts any subsequent elements to the left (subtracts one from their * indices). * * @param index the index of the element to be removed * @return the element that was removed from the list * @throws IndexOutOfBoundsException {@inheritDoc} */ public E remove(int index) { rangeCheck(index); modCount++; E oldValue = elementData(index); int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work return oldValue; }alue = elementData(index); int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work return oldValue; }
对于数组的元素删除算法我们应该很熟悉,删除一个数组元素,我们需要将这个元素后面的元素全部向前移动,并将size减1. 我们看到源码中,首先检查下标是否在可用范围内。然后调用System.arrayCopy方法将右边的数组向左移动,并且将size减一,并置为null。
public boolean remove(Object o)
源码中实现如下:
/** * 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; }
我们可以看到,这个remove方法会移除数组中第一个符合的给定对象,如果不存在就什么也不做,如果存在多个只移除第一个。 fastRemove方法如下
/* * 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; // clear to let GC do its work }
可以理解为简化版的remove(index)方法。
标签:turned specified elements string dex number system sci not
原文地址:https://www.cnblogs.com/lukelook/p/10853234.html