java基础之ArrayList和Vector的主要区别;
List接口下一共实现了三个类:ArrayList,Vector,LinkedList。
LinkedList主要保持数据的插入顺序的时候使用,采用链表结构。
ArrayList,Vector主要区别为以下几点:
(1):Vector是线程安全的,源码中有很多的synchronized可以看出,而ArrayList不是。导致Vector效率无法和ArrayList相比;
(2):ArrayList和Vector都采用线性连续存储空间,当存储空间不足的时候,ArrayList默认增加为原来的50%,Vector默认增加为原来的一倍;
(3):Vector可以设置capacityIncrement,而ArrayList不可以,从字面理解就是capacity容量,Increment增加,容量增长的参数。
源码分析:
首先看看构造器:
ArrayList:三个
/** * Constructs an empty list with an initial capacity of ten. * 构造一个默认初始容量为10的list */ public ArrayList() { super(); this.elementData = EMPTY_ELEMENTDATA; } /** * 构造一个指定默认长度的list initialCapacity 不能小于0; * 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) { super(); if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); this.elementData = new Object[initialCapacity]; } /** 构造一个包含collection 元素的list * 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) { ... }
Vector:四个
//构造一个指定默认长度的list public Vector(int initialCapacity) { this(initialCapacity, 0); } //构造一个默认初始容量为10的list public Vector() { this(10); } //构造一个包含collection 元素的list public Vector(Collection<? extends E> c) { ... } //区别在于可以设置capacityIncrement public Vector(int initialCapacity, int capacityIncrement) { super(); ... }
vector多了一个public Vector(int initialCapacity, int capacityIncrement)构造器,可以设置容量增长,arraylist是没有的。
主要添加源码分析
ArrayList类:
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } private void ensureCapacityInternal(int minCapacity) { modCount++; // overflow-conscious code //如果添加一个元素之后,新容器的大小大于容器的容量,那么就无法存值了,需要扩充空间 if (minCapacity - elementData.length > 0) grow(minCapacity); } private void grow(int minCapacity) { // overflow-conscious code int oldCapacity = elementData.length; int newCapacity = oldCapacity + (oldCapacity >> 1); //扩充的空间增加原来的50%(即是原来的1.5倍) if (newCapacity - minCapacity < 0) //如果容器扩容之后还是不够,那么干脆直接将minCapacity设为容器的大小 newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0) //如果扩充的容器太大了的话,那么就执行hugeCapacity newCapacity = hugeCapacity(minCapacity); // minCapacity is usually close to size, so this is a win: elementData = Arrays.copyOf(elementData, newCapacity); }
Vector类:
public synchronized boolean add(E e) { modCount++; ensureCapacityHelper(elementCount + 1); elementData[elementCount++] = e; return true; } private void ensureCapacityHelper(int minCapacity) { // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } private void grow(int minCapacity) { // overflow-conscious code int oldCapacity = elementData.length; int newCapacity = oldCapacity + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity); /** 这个扩容需要做个判断:如果容量增量初始化的不是0,即使用的public Vector(int initialCapacity,int capacityIncrement) 构造方法进行的初始化,那么扩容的容量是(oldCapacity+capacityIncrement),就是原来的容量加上容量增量的值; 如果没有设置容量增量,那么扩容后的容量就是(oldCapacity+oldCapacity),就是原来容量的二倍。 **/ if (newCapacity - minCapacity < 0) newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); elementData = Arrays.copyOf(elementData, newCapacity); }
注:转载自:blog.csdn.net/ldxlz224/article/details/52574821