我们常说的ArrayList是指java.util.ArrayList

1. Arrays.asList()获取到的ArrayList只能遍历，不能增加或删除元素

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 oklist.stream().forEach(System.out::println);for (int i = 0; i < list.size(); i++) {// 报异常：UnsupportedOperationExceptionlist.add("haha");// 报异常：UnsupportedOperationExceptionlist.remove(i);}
}

2. java.util.ArrayList.SubList有实现add()、remove()方法

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);}

3. 遍历集合时对元素重新赋值、对元素中的属性赋值、删除元素、新增元素

遍历方式：普通for循环、增强for循环、forEach、stream forEach、迭代器

• 对元素重新赋值：各种遍历方式都做不到
• 对元素中的属性赋值：各种遍历方式都能做到
• 对集合新增元素：普通for循环可以做到，其他遍历方式都做不到
• 对集合删除元素：普通for循环和迭代器可以做到，其他方式都做不到

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

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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;}

3.1 普通for循环

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)]

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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)]

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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)]

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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)]

3.2 增强for循环

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$Itr.checkForComodification(ArrayList.java:911)atjava.util.ArrayList$Itr.next(ArrayList.java:861)

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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$Itr.checkForComodification(ArrayList.java:911)atjava.util.ArrayList$Itr.next(ArrayList.java:861)

3.3 forEach循环

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)]

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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)]

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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)

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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)

3.4 stream forEach循环

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)]

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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)

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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$ArrayListSpliterator.forEachRemaining(ArrayList.java:1390)atjava.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580)

3.5 迭代器

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)]

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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)]

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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：
[]

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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$Itr.checkForComodification(ArrayList.java:911)atjava.util.ArrayList$Itr.next(ArrayList.java:861)

4. 对大集合进行分组，遍历分组后的大集合，各个子集合使用完成后立即将元素删除

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) && ((list.size() > SUBLIST_SIZE) || !isNotEnd)) {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subListIterator<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