我们常说的ArrayList是指java.util.ArrayList
Arrays.asList()获取到的ArrayList不是java.util包下的,而是java.util.Arrays.ArrayList;
它是Arrays类自己定义的一个静态内部类,这个内部类没有实现add()、remove()方法,而是直接使用它的父类AbstractList的相应方法。而AbstractList中的add()和remove()是直接抛出java.lang.UnsupportedOperationException异常
public static void main(String[] args) {
List<String> list = Arrays.asList("xin", "liu", "shijian");
// 遍历 ok
list.stream().forEach(System.out::println);
for (int i = 0; i < list.size(); i++) {
// 报异常:UnsupportedOperationException
list.add("haha");
// 报异常:UnsupportedOperationException
list.remove(i);
}
}
java.util.ArrayList.SubList 是ArrayList的内部类,可以add 和 remove
不建议对得到的子集合进行增、删操作
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
List<PersonDTO> subList = list.subList(0, 2);
int size = subList.size();
for (int i = 0; i < size; i++) {
PersonDTO personDTO = subList.get(i);
if (i == 0) {
// subList.remove(personDTO);
subList.add(personDTO);
break;
}
}
System.out.println(list);
}
遍历方式:普通for循环、增强for循环、forEach、stream forEach、迭代器
建议:遍历集合时删除元素用迭代器、新增元素可以新建一个集合
准备实验数据
private static List<PersonDTO> getDataList() {
List<PersonDTO> list = new ArrayList<>();
PersonDTO p1 = new PersonDTO();
p1.setPersonName("xiaohua1");
PersonDTO p2 = new PersonDTO();
p2.setPersonName("xiaohua2");
PersonDTO p3 = new PersonDTO();
p3.setPersonName("xiaohua3");
PersonDTO p4 = new PersonDTO();
p4.setPersonName("xiaohua4");
PersonDTO p5 = new PersonDTO();
p5.setPersonName("xiaohua5");
PersonDTO p6 = new PersonDTO();
p6.setPersonName("xiaohua6");
list.add(p1);
list.add(p2);
list.add(p3);
list.add(p4);
list.add(p5);
list.add(p6);
return list;
}
普通for循环,遍历元素时重新赋值失败
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
for (int i = 0; i < list.size(); i++) {
// 原因是这里personDTO是另一个栈变量,并不会对集合中的栈内容(对象在内存中的地址)进行改变
PersonDTO personDTO = list.get(i);
personDTO = null;
}
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=null), PersonDTO(bookEntityList=null, personName=xiaohua2, age=null), PersonDTO(bookEntityList=null, personName=xiaohua3, age=null), PersonDTO(bookEntityList=null, personName=xiaohua4, age=null), PersonDTO(bookEntityList=null, personName=xiaohua5, age=null), PersonDTO(bookEntityList=null, personName=xiaohua6, age=null)]
普通for循环,遍历元素时,对元素中的属性成功赋值
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
for (int i = 0; i < list.size(); i++) {
// 成功是因为它们虽然是不同的变量,但栈内容相同,都是同一个对象的内存地址,这里会更改到堆中对象的内容
PersonDTO personDTO = list.get(i);
personDTO.setAge(5);
}
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=5), PersonDTO(bookEntityList=null, personName=xiaohua2, age=5), PersonDTO(bookEntityList=null, personName=xiaohua3, age=5), PersonDTO(bookEntityList=null, personName=xiaohua4, age=5), PersonDTO(bookEntityList=null, personName=xiaohua5, age=5), PersonDTO(bookEntityList=null, personName=xiaohua6, age=5)]
普通for循环遍历时删除元素,ok
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
for (int i = 0; i < list.size(); i++) {
PersonDTO personDTO = list.get(i);
list.remove(personDTO);
}
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua2, age=null), PersonDTO(bookEntityList=null, personName=xiaohua4, age=null), PersonDTO(bookEntityList=null, personName=xiaohua6, age=null)]
普通for循环遍历时新增元素,size若不固定,报异常OutOfMemoryError
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
for (int i = 0; i < list.size(); i++) {
PersonDTO personDTO = list.get(i);
list.add(personDTO);
System.out.println("list.size(): " + list.size());
}
System.out.println(list);
}
打印结果:
Exception in thread “main” java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Arrays.java:3210)
at java.util.Arrays.copyOf(Arrays.java:3181)
at java.util.ArrayList.grow(ArrayList.java:267)
at java.util.ArrayList.ensureExplicitCapacity(ArrayList.java:241)
at java.util.ArrayList.ensureCapacityInternal(ArrayList.java:233)
at java.util.ArrayList.add(ArrayList.java:464)
后来又执行一次,到这个数据量还未结束
list.size(): 43014827
list.size(): 43014828
list.size(): 43014829
Process finished with exit code 130
Java VisualVM监视图如下:
换种写法,固定size值,就运行ok了,普通for循环遍历时就可以新增元素了
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
int size = list.size();
for (int i = 0; i < size; i++) {
PersonDTO personDTO = list.get(i);
list.add(personDTO);
System.out.println("list.size(): " + list.size());
}
System.out.println(list);
}
打印结果:
list.size(): 7
list.size(): 8
list.size(): 9
list.size(): 10
list.size(): 11
list.size(): 12
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=null), PersonDTO(bookEntityList=null, personName=xiaohua2, age=null), PersonDTO(bookEntityList=null, personName=xiaohua3, age=null), PersonDTO(bookEntityList=null, personName=xiaohua4, age=null), PersonDTO(bookEntityList=null, personName=xiaohua5, age=null), PersonDTO(bookEntityList=null, personName=xiaohua6, age=null), PersonDTO(bookEntityList=null, personName=xiaohua1, age=null), PersonDTO(bookEntityList=null, personName=xiaohua2, age=null), PersonDTO(bookEntityList=null, personName=xiaohua3, age=null), PersonDTO(bookEntityList=null, personName=xiaohua4, age=null), PersonDTO(bookEntityList=null, personName=xiaohua5, age=null), PersonDTO(bookEntityList=null, personName=xiaohua6, age=null)]
增强for循环,遍历元素时重新赋值失败
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
for (PersonDTO dto : list) {
// 增强for循环内部实现是迭代器,我认为是调用了新方法,进行了值传递,dto是另一个变量了
dto = null;
}
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=null), PersonDTO(bookEntityList=null, personName=xiaohua2, age=null), PersonDTO(bookEntityList=null, personName=xiaohua3, age=null), PersonDTO(bookEntityList=null, personName=xiaohua4, age=null), PersonDTO(bookEntityList=null, personName=xiaohua5, age=null), PersonDTO(bookEntityList=null, personName=xiaohua6, age=null)]
增强for循环,遍历元素时,对元素中的属性成功赋值
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
for (PersonDTO dto : list) {
dto.setAge(7);
}
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=7), PersonDTO(bookEntityList=null, personName=xiaohua2, age=7), PersonDTO(bookEntityList=null, personName=xiaohua3, age=7), PersonDTO(bookEntityList=null, personName=xiaohua4, age=7), PersonDTO(bookEntityList=null, personName=xiaohua5, age=7), PersonDTO(bookEntityList=null, personName=xiaohua6, age=7)]
增强for循环遍历时删除元素,报异常ConcurrentModificationException
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
for (PersonDTO dto : list) {
list.remove(dto);
}
System.out.println(list);
}
打印结果:
Exception in thread “main” java.util.ConcurrentModificationException
at java.util.ArrayList I t r . c h e c k F o r C o m o d i f i c a t i o n ( A r r a y L i s t . j a v a : 911 ) a t j a v a . u t i l . A r r a y L i s t Itr.checkForComodification(ArrayList.java:911) at java.util.ArrayList Itr.checkForComodification(ArrayList.java:911)atjava.util.ArrayListItr.next(ArrayList.java:861)
增强for循环遍历时新增元素,报异常ConcurrentModificationException
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
for (PersonDTO dto : list) {
list.add(dto);
}
System.out.println(list);
}
打印结果:
Exception in thread “main” java.util.ConcurrentModificationException
at java.util.ArrayList I t r . c h e c k F o r C o m o d i f i c a t i o n ( A r r a y L i s t . j a v a : 911 ) a t j a v a . u t i l . A r r a y L i s t Itr.checkForComodification(ArrayList.java:911) at java.util.ArrayList Itr.checkForComodification(ArrayList.java:911)atjava.util.ArrayListItr.next(ArrayList.java:861)
forEach循环,遍历元素时重新赋值失败
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
// forEach循环内部实现是匿名内部类,调用了函数式接口的新方法,进行了值传递,dto是另一个变量了
list.forEach(dto -> dto = null);
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=null), PersonDTO(bookEntityList=null, personName=xiaohua2, age=null), PersonDTO(bookEntityList=null, personName=xiaohua3, age=null), PersonDTO(bookEntityList=null, personName=xiaohua4, age=null), PersonDTO(bookEntityList=null, personName=xiaohua5, age=null), PersonDTO(bookEntityList=null, personName=xiaohua6, age=null)]
forEach循环,遍历元素时,对元素中的属性成功赋值
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
list.forEach(dto -> dto.setAge(6));
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=6), PersonDTO(bookEntityList=null, personName=xiaohua2, age=6), PersonDTO(bookEntityList=null, personName=xiaohua3, age=6), PersonDTO(bookEntityList=null, personName=xiaohua4, age=6), PersonDTO(bookEntityList=null, personName=xiaohua5, age=6), PersonDTO(bookEntityList=null, personName=xiaohua6, age=6)]
forEach循环遍历时删除元素,报异常ConcurrentModificationException
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
list.forEach(dto -> list.remove(dto));
System.out.println(list);
}
打印结果:
Exception in thread “main” java.util.ConcurrentModificationException
at java.util.ArrayList.forEach(ArrayList.java:1262)
forEach循环遍历时新增元素,报异常ConcurrentModificationException
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
list.forEach(dto -> list.add(new PersonDTO()));
System.out.println(list);
}
打印结果:
Exception in thread “main” java.util.ConcurrentModificationException
at java.util.ArrayList.forEach(ArrayList.java:1262)
stream forEach循环,遍历元素时重新赋值失败
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
// stream forEach循环内部实现是匿名内部类,调用了函数式接口的新方法,进行了值传递,dto是另一个变量了
list.stream().forEach(dto -> dto = null);
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=null), PersonDTO(bookEntityList=null, personName=xiaohua2, age=null), PersonDTO(bookEntityList=null, personName=xiaohua3, age=null), PersonDTO(bookEntityList=null, personName=xiaohua4, age=null), PersonDTO(bookEntityList=null, personName=xiaohua5, age=null), PersonDTO(bookEntityList=null, personName=xiaohua6, age=null)]
stream forEach循环,遍历元素时,对元素中的属性成功赋值
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
list.stream().forEach(dto -> dto.setAge(8));
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=8), PersonDTO(bookEntityList=null, personName=xiaohua2, age=8), PersonDTO(bookEntityList=null, personName=xiaohua3, age=8), PersonDTO(bookEntityList=null, personName=xiaohua4, age=8), PersonDTO(bookEntityList=null, personName=xiaohua5, age=8), PersonDTO(bookEntityList=null, personName=xiaohua6, age=8)]
stream forEach循环遍历时删除元素,报异常ConcurrentModificationException
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
list.stream().forEach(dto -> list.remove(dto));
System.out.println(list);
}
打印结果:
Exception in thread “main” java.util.ConcurrentModificationException
at java.util.ArrayList.forEach(ArrayList.java:1262)
stream forEach循环遍历时新增元素,报异常ConcurrentModificationException
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
list.stream().forEach(dto -> list.add(new PersonDTO()));
System.out.println(list);
}
打印结果:
Exception in thread “main” java.util.ConcurrentModificationException
at java.util.ArrayList A r r a y L i s t S p l i t e r a t o r . f o r E a c h R e m a i n i n g ( A r r a y L i s t . j a v a : 1390 ) a t j a v a . u t i l . s t r e a m . R e f e r e n c e P i p e l i n e ArrayListSpliterator.forEachRemaining(ArrayList.java:1390) at java.util.stream.ReferencePipeline ArrayListSpliterator.forEachRemaining(ArrayList.java:1390)atjava.util.stream.ReferencePipelineHead.forEach(ReferencePipeline.java:580)
迭代器循环,遍历元素时重新赋值失败
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
Iterator<PersonDTO> iterator = list.iterator();
while (iterator.hasNext()) {
// 原因是这里dto是另一个栈变量,并不会对集合中的栈内容(对象在内存中的地址)进行改变
PersonDTO dto = iterator.next();
dto = null;
}
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=null), PersonDTO(bookEntityList=null, personName=xiaohua2, age=null), PersonDTO(bookEntityList=null, personName=xiaohua3, age=null), PersonDTO(bookEntityList=null, personName=xiaohua4, age=null), PersonDTO(bookEntityList=null, personName=xiaohua5, age=null), PersonDTO(bookEntityList=null, personName=xiaohua6, age=null)]
迭代器循环,遍历元素时,对元素中的属性成功赋值
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
Iterator<PersonDTO> iterator = list.iterator();
while (iterator.hasNext()) {
PersonDTO dto = iterator.next();
dto.setAge(9);
}
System.out.println(list);
}
打印结果:
[PersonDTO(bookEntityList=null, personName=xiaohua1, age=9), PersonDTO(bookEntityList=null, personName=xiaohua2, age=9), PersonDTO(bookEntityList=null, personName=xiaohua3, age=9), PersonDTO(bookEntityList=null, personName=xiaohua4, age=9), PersonDTO(bookEntityList=null, personName=xiaohua5, age=9), PersonDTO(bookEntityList=null, personName=xiaohua6, age=9)]
迭代器遍历时删除元素,可以成功
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
Iterator<PersonDTO> iterator = list.iterator();
while (iterator.hasNext()) {
PersonDTO dto = iterator.next();
iterator.remove();
}
System.out.println(list);
}
打印结果:
[]
迭代器遍历时新增元素,报异常:ConcurrentModificationException
public static void main(String[] args) {
List<PersonDTO> list = getDataList();
Iterator<PersonDTO> iterator = list.iterator();
while (iterator.hasNext()) {
PersonDTO dto = iterator.next();
list.add(new PersonDTO());
}
System.out.println(list);
}
打印结果:
Exception in thread “main” java.util.ConcurrentModificationException
at java.util.ArrayList I t r . c h e c k F o r C o m o d i f i c a t i o n ( A r r a y L i s t . j a v a : 911 ) a t j a v a . u t i l . A r r a y L i s t Itr.checkForComodification(ArrayList.java:911) at java.util.ArrayList Itr.checkForComodification(ArrayList.java:911)atjava.util.ArrayListItr.next(ArrayList.java:861)
场景:集合中对象比较多,可能造成OOM,集合中的一部分元素使用完成后立即删除
import com.google.common.collect.Lists;
import org.apache.commons.collections4.CollectionUtils;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Objects;
public class ArrayListDemo {
private static final int SUBLIST_SIZE = 2;
public static void main(String[] args) {
// 不适合list中有元素减少的场景
// 1.得到总list,这个时候的list接下来还可能会继续扩展
List<PersonDTO> list = getDataList();
System.out.println("刚开始, list.size = " + list.size());
// 2.业务逻辑代码:list还可能会扩展
// 3.处理完子集合,就删除它的元素
deleteSubList(list);
System.out.println("处理一波后, list.size = " + list.size());
// 4.list不再扩展,删除剩下元素,也是一个一个子集合的删除元素
deleteSubList(list, false);
System.out.println("最后处理后, list.size = " + list.size());
}
private static void deleteSubList(List<PersonDTO> list) {
// isNotEnd 代表list集合可能还会增加元素
deleteSubList(list, true);
}
private static void deleteSubList(List<PersonDTO> list, boolean isNotEnd) {
if (Objects.isNull(isNotEnd)) {
isNotEnd = false;
}
if (CollectionUtils.isNotEmpty(list)) {
int size = list.size() / SUBLIST_SIZE;
if ((list.size() % SUBLIST_SIZE) > 0) {
size++;
}
for (int i = 0; i < size; i++) {
if (CollectionUtils.isNotEmpty(list)) {
List<PersonDTO> subList = Lists.partition(list, SUBLIST_SIZE).get(0);
// 不再继续处理业务逻辑: list中的数据量小于SUBLIST_SIZE && list中还可能增加元素
if ((list.size() < SUBLIST_SIZE) && isNotEnd) {
break;
}
// 使用subList处理业务逻辑
// 删出list中的subList
Iterator<PersonDTO> iterator = list.iterator();
int j = 0;
while (iterator.hasNext()) {
iterator.next();
j++;
if (j <= SUBLIST_SIZE) {
iterator.remove();
}
if (j > SUBLIST_SIZE) {
break;
}
}
System.out.println("删除subList后, list.size = " + list.size());
}
}
}
}
private static List<PersonDTO> getDataList() {
List<PersonDTO> list = new ArrayList<>();
PersonDTO p1 = new PersonDTO();
p1.setPersonName("xiaohua1");
PersonDTO p2 = new PersonDTO();
p2.setPersonName("xiaohua2");
PersonDTO p3 = new PersonDTO();
p3.setPersonName("xiaohua3");
PersonDTO p4 = new PersonDTO();
p4.setPersonName("xiaohua4");
PersonDTO p5 = new PersonDTO();
p5.setPersonName("xiaohua5");
PersonDTO p6 = new PersonDTO();
p6.setPersonName("xiaohua6");
PersonDTO p7 = new PersonDTO();
p7.setPersonName("xiaohua7");
list.add(p1);
list.add(p2);
list.add(p3);
list.add(p4);
list.add(p5);
list.add(p6);
list.add(p7);
return list;
}
}
打印结果:
刚开始, list.size = 7
删除subList后, list.size = 5
删除subList后, list.size = 3
删除subList后, list.size = 1
处理一波后, list.size = 1
删除subList后, list.size = 0
最后处理后, list.size = 0