首先看这两类都实现List接口,而List接口一共有三个实现类,分别是ArrayList、Vector和LinkedList。List用于存放多个元素,能够维护元素的次序,并且允许元素的重复。3个具体实现类的相关区别如下:
- ArrayList是最常用的List实现类,内部是通过数组实现的,它允许对元素进行快速随机访问。数组的缺点是每个元素之间不能有间隔,当数组大小不满足时需要增加存储能力,就要讲已经有数组的数据复制到新的存储空间中。当从ArrayList的中间位置插入或者删除元素时,需要对数组进行复制、移动、代价比较高。因此,它适合随机查找和遍历,不适合插入和删除。
- Vector与ArrayList一样,也是通过数组实现的,不同的是它支持线程的同步,即某一时刻只有一个线程能够写Vector,避免多线程同时写而引起的不一致性,但实现同步需要很高的花费,因此,访问它比访问ArrayList慢。
- LinkedList是用链表结构存储数据的,很适合数据的动态插入和删除,随机访问和遍历速度比较慢。另外,他还提供了List接口中没有定义的方法,专门用于操作表头和表尾元素,可以当作堆栈、队列和双向队列使用。
关于ArrayList和Vector区别如下:
- ArrayList在容量不够时默认是扩展50% ,Vector是默认扩展1倍。
- Vector提供indexOf(obj, start)接口,ArrayList没有。
- Vector属于线程安全级别的,但是大多数情况下不使用Vector,因为线程安全需要更大的系统开销。
查看Java源代码,发现当数组的大小不够的时候,需要重新建立数组,然后将元素拷贝到新的数组内,ArrayList和Vector的扩展数组的大小不同。
ArrayList: Jdk 1.8
import java.util.Arrays; /** * Created by sheting on 10/22/2017 */ public class ReadSrcCodeArrayList<E> { private int size; private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; transient Object[] elementData; private static final int DEFAULT_CAPACITY = 10; transient int modCount = 0; private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; public ReadSrcCodeArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } public boolean add(E e) { ensureCapacityInternal(size + 1); elementData[size++] = e; return true; } private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); //默认ArrayList的容量为10 } ensureExplicitCapacity(minCapacity); } private void ensureExplicitCapacity(int minCapacity) { modCount++; if (minCapacity - elementData.length > 0) grow(minCapacity); } private void grow(int minCapacity) { int oldCapacity = elementData.length; // >> 向右移动一位 相当于除以2, 因此ArrayList每次在原有的基础上增加一半 int newCapacity = oldCapacity + (oldCapacity >> 1); if (newCapacity - minCapacity < 0) newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); elementData = Arrays.copyOf(elementData, newCapacity); } private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } }
Vector: Jdk 1.8
import java.util.Ar rays; /** * Created by sheting on 10/22/2017 */ public class ReadSrcCodeVector<E> { protected Object[] elementData; protected int capacityIncrement; private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; protected transient int modCount = 0; protected int elementCount; public ReadSrcCodeVector() { this(10); //默认容量是10 } public ReadSrcCodeVector(int initialCapacity) { this(initialCapacity, 0); } public ReadSrcCodeVector(int initialCapacity, int capacityIncrement) { if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: " + initialCapacity); this.elementData = new Object[initialCapacity]; this.capacityIncrement = capacityIncrement; } public synchronized boolean add(E e) { modCount++; ensureCapacityHelper(elementCount + 1); elementData[elementCount++] = e; return true; } private void ensureCapacityHelper(int minCapacity) { if (minCapacity - elementData.length > 0) grow(minCapacity); } private void grow(int minCapacity) { int oldCapacity = elementData.length; // 原来的基础上扩大一倍 int newCapacity = oldCapacity + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity); if (newCapacity - minCapacity < 0) newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); elementData = Arrays.copyOf(elementData, newCapacity); } private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } }