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ArrayList定义
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable
ArrayList继承于AbstractList，实现了List, RandomAccess, Cloneable, java.io.Serializable这些接口。
ArrayList 实现了RandomAccess接口，即提供了随机访问功能。RandomAccess是java中用来被List实现，为List提供快速访问功能的。
ArrayList 实现了Cloneable接口，即覆盖了函数clone()，能被克隆。
ArrayList 实现java.io.Serializable接口，这意味着ArrayList支持序列化，能通过序列化去传输。
ArrayList中的操作不是线程安全的！//TODO
ArrayList的API
// Collection中定义的API
boolean add(E object)
boolean addAll(Collection<? extends E> collection)
void clear()
boolean contains(Object object)
boolean containsAll(Collection<?> collection)
boolean equals(Object object)
int hashCode()
boolean isEmpty()
Iterator<E> iterator()
boolean remove(Object object)
boolean removeAll(Collection<?> collection)
boolean retainAll(Collection<?> collection)
int size()
<T> T[] toArray(T[] array)
Object[] toArray()
// List新增的API
void add(int location, E object)
boolean addAll(int location, Collection<? extends E> collection)
E get(int location)
int indexOf(Object object)
int lastIndexOf(Object object)
ListIterator<E> listIterator(int location)
ListIterator<E> listIterator()
E remove(int location)
E set(int location, E object)
List<E> subList(int start, int end)
// ArrayList新增的API
Object clone()
void ensureCapacity(int minimumCapacity)
void trimToSize()
void removeRange(int fromIndex, int toIndex)
ArrayList包含了两个重要的对象：elementData 和 size。
(01) elementData 是"Object[]类型的数组"，它保存了添加到ArrayList中的元素。实际上，elementData是个动态数组，我们能通过构造函数 ArrayList(int initialCapacity)来执行它的初始容量为initialCapacity；如果通过不含参数的构造函数ArrayList()来创建ArrayList，则elementData的容量默认是10。elementData数组的大小会根据ArrayList容量的增长而动态的增长，具体的增长方式，请参考源码分析中的ensureCapacity()函数。elementData用transient修饰，不被序列化。
(02) size 则是动态数组的实际大小。
ArrayList的三种构造函数
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);
}
}
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
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;
}
}
ArrayList的三种遍历方式
/**
* FileName: ArrayListIterator
* Author: Sandy
* Date: 2018/12/2 16:49
* Description: ArrayList遍历方式及其比较
* Version: v1.0.0
*/
package ArrayListDemo;
import com.sun.xml.internal.bind.v2.schemagen.xmlschema.List;
import java.io.ObjectInputStream;
import java.util.ArrayList;
import java.util.Iterator;
public class ArrayListIterator {
public static void iteratorArrayListByIterator(ArrayList arrayList){
long startTime;
long endTime;
Iterator iterator = arrayList.iterator();
startTime = System.currentTimeMillis();
while(iterator.hasNext()){
iterator.next();
}
endTime = System.currentTimeMillis();
long interval = endTime - startTime;
System.out.println("通过迭代器遍历ArrayList效率为：" + interval);
}
public static void iteratorArrayListByRondomAccess(ArrayList arrayList){
long startTime;
long endTime;
Object value;
startTime = System.currentTimeMillis();
for (int i = 0; i < arrayList.size(); i++){
value = arrayList.get(i);
}
endTime = System.currentTimeMillis();
long interval = endTime - startTime;
System.out.println("通过随机访问遍历ArrayList效率为：" + interval);
}
public static void iteratorArrayListByFor(ArrayList arrayList){
long startTime;
long endTime;
Object value;
startTime = System.currentTimeMillis();
for(Object object : arrayList){
value = object;
}
endTime = System.currentTimeMillis();
long interval = endTime - startTime;
System.out.println("通过For循环遍历ArrayList效率为：" + interval);
}
public static void main(String[] args) {
ArrayList arrayList = new ArrayList();
for(int i = 0; i < 10000; i++){
arrayList.add(i);
}
//效率对比
iteratorArrayListByIterator(arrayList);
iteratorArrayListByRondomAccess(arrayList);
iteratorArrayListByFor(arrayList);
}
}
输出：
通过迭代器遍历ArrayList效率为：9
通过索引值遍历ArrayList效率为：8796
通过For循环遍历ArrayList效率为：7
ArrayList应用
package ArrayListDemo;
import java.util.ArrayList;
import java.util.Arrays;
/**
* @ClassName: ArrayListDemo
* @Author sandy.n.hao
* @Date: 2018/12/10
* @Version v1.0.0
* @Description: //TODO
*/
public class ArrayListDemo {
public static void main(String[] args) {
String [] data = new String[] {"abc","def","ghi"};
ArrayList arrayList1 = new ArrayList() {
{
//调用ArrayList的add()方法，add方法检查是否需要扩容，并且修改modCount+1，增加元素
add("abc");
add(123);
add("def");
add(456);
}
};
ArrayList arrayList2 = new ArrayList(Arrays.asList(data));
//List的并集，不排除重复
arrayList2.addAll(arrayList1);
System.out.println(arrayList2);
//List的交集，不排除重复
// arrayList2.retainAll(arrayList1);
// System.out.println(arrayList2);
//List的差集，不排除重复
// arrayList2.removeAll(arrayList1);
// System.out.println(arrayList2);
}
}
输出：
并集：[abc, def, ghi, abc, 123, def, 456]
交集：[abc, def]
差集：[ghi]
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