2018年10月03日
目录
测试结论
测试例子
性能分析
1）数组Array：
2）列表ArrayList：
2.1 构造函数
2.2 成员变量
2.3 add 方法（队列末尾插入一个元素 / 队列特定位置插入一个元素）
2.4 remove方法（删除指定位置的元素 / 删除某个元素）
2.5 get 方法
测试结论
Java两个常用的数据结构进行性能的比较，发现ArrayList和array还是相差较大的，数组的遍历时间远远小于ArrayList。
测试例子
import java.util.ArrayList;
public class testCompareArrayAndList {
/**
* @param args
*/
public static void main(String[] args) {
ArrayList<Integer> arrayList = new ArrayList<Integer>();
//添加元素
for(int i =0;i< 11999999;i++){
arrayList.add(i);
}
//list => array
Integer[] array = arrayList.toArray(new Integer[arrayList.size()]);
long currentTimeMillis1 = System.currentTimeMillis();
for(Integer i :array){}
long currentTimeMillis2 = System.currentTimeMillis();
System.out.println("遍历数组耗费时间： "+(currentTimeMillis2 - currentTimeMillis1)+" ms");
currentTimeMillis1 = System.currentTimeMillis();
for(Integer i : arrayList){}
currentTimeMillis2 = System.currentTimeMillis();
System.out.println("遍历列表耗费时间： "+(currentTimeMillis2 - currentTimeMillis1)+" ms");
}
}
console：
遍历数组耗费时间： 2 ms
遍历列表耗费时间： 39 ms
性能分析
1）数组Array：
参考文章：《java数组详解》
2）列表ArrayList：
2.1 构造函数
一、初始化设置容量
/**
* 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];
}
二、无参构造函数
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
三、初始化元素
/**
* 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();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
2.2 成员变量
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 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 == EMPTY_ELEMENTDATA will be expanded to
* DEFAULT_CAPACITY when the first element is added.
*/
private transient Object[] elementData;
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;
1）elementData ： 内存实际保存元素是一个数组，即ArrayList是对数组的一个封装；
2）size：ArrayList的实际长度；
2.3 add 方法（队列末尾插入一个元素 / 队列特定位置插入一个元素）
/**
* 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;
}
/**
* Inserts the specified element at the specified position in this
* list. Shifts the element currently at that position (if any) and
* any subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
2.4 remove方法（删除指定位置的元素 / 删除某个元素）
/**
* 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;
}
/**
* 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;
}
2.5 get 方法
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
实际返回的是数组的某个元素。
以上是对源码的一些走读总结。
