Java ArrayList在遍历时删除元素

2023-12-28 12:03:20

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 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);
    }
}

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


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)]


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:

在这里插入图片描述

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 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)

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)]


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)

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)]


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)

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)]


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)

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)) {
            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

文章来源:https://blog.csdn.net/weixin_43024834/article/details/135257336
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