Java集合移除某个元素

我会带着你远行 2022-09-06 01:35 210阅读 0赞

> **目录**
> 
> 一、fori移除
> 
> 二、加强for
> 
> 三、Iterator
> 
> 四、JDK8写法
> 
> --------------------
> 
> ### 一、fori移除 ###
> 
>     public static void main(String[] args) {
>             test1();
>         }
>     
>         private static void test1() {
>             List<String> list = new ArrayList<String>();
>             list.add("1");
>             list.add("2");
>             list.add("3");
>             list.add("1");
>             list.add("4");
>             list.add("5");
>             list.add("5");
>             list.add("6");
>             System.out.println("原始list元素：" + list.toString());
>             CopyOnWriteArrayList<String> copyList = new CopyOnWriteArrayList<>(list);
>     
>             //通过下表移除等于11的元素
>             for (int i = 0; i < list.size(); i++) {
>                 String item = list.get(i);
>                 if ("1".equals(item)) {
>                     list.remove(i);
>                 }
>             }
>             System.out.println("通过下表移除后的list元素：" + list.toString());
>     
>             //通过对象移除等于11的元素
>             for (int i = 0; i < copyList.size(); i++) {
>                 String item = copyList.get(i);
>                 if ("1".equals(item)) {
>                     copyList.remove(item);
>                 }
>             }
>             System.out.println("通过对象移除后的list元素：" + list.toString());
>         }
> 
> 运行结果：
> 
> ![20210819192045325.png][]
> 
> debug发现，移除元素后，集合大小也减一，但i却＋1了，会导致直接跳过原本第二个元素，操作第三个元素，如果被跳过的那个元素恰好满足移除条件，就会出现下面的情况，两个同时满足条件的元素会被漏掉第二个。
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYnVzaTAwMQ_size_16_color_FFFFFF_t_70][]
> 
>  缺点：相邻的满足条件删除不完全
> 
> ### 二、加强for ###
> 
>     private static void test2() {
>             List<String> list = new ArrayList<String>();
>             list.add("1");
>             list.add("2");
>             list.add("3");
>             list.add("1");
>             list.add("4");
>             list.add("5");
>             list.add("5");
>             list.add("6");
>             System.out.println("原始list元素："+ list.toString());
>     
>             //通过对象移除等于11的元素
>             for (String item : list) {
>                 if("1".equals(item)) {
>                     list.remove(item);
>                 }
>             }
>             System.out.println("通过对象移除后的list元素："+ list.toString());
>     
>         }
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYnVzaTAwMQ_size_16_color_FFFFFF_t_70 1][]
> 
> 运行代码循环第二次时报错，
> 
> 由于foreach是由迭代器实现的
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYnVzaTAwMQ_size_16_color_FFFFFF_t_70 2][]
> 
>  查找实现类
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYnVzaTAwMQ_size_16_color_FFFFFF_t_70 3][]
> 
>  ![20210819194033594.png][]
> 
> 追踪实现方法
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYnVzaTAwMQ_size_16_color_FFFFFF_t_70 4][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYnVzaTAwMQ_size_16_color_FFFFFF_t_70 5][]![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYnVzaTAwMQ_size_16_color_FFFFFF_t_70 6][]
> 
> 打两个打断点，判断移除第一个元素之前，
> 
> 会执行checkForComodification()方法ArrayList.this.size次
> 
> 移除第二个元素时会报错，此处校验值已经改变了
> 
> ![20210819195434173.png][]
> 
> 由于forEachRemaining的checkForComodification方法在remove之前执行，在 foreach 循环中执行 list.remove(item)，对 list 对象的 modCount 值进行了修改，而 list 对象的迭代器的 expectedModCount 值未进行修改，所以导致ConcurrentModificationException异常
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYnVzaTAwMQ_size_16_color_FFFFFF_t_70 7][]
> 
> ### 三、Iterator ###
> 
>     private static void test5() {
>             List<String> list = new ArrayList<String>();
>             list.add("1");
>             list.add("1");
>             list.add("2");
>             list.add("3");
>             list.add("1");
>             list.add("5");
>             list.add("1");
>             list.add("6");
>             System.out.println("原始list元素："+ list.toString());
>     
>             //通过迭代器移除等于11的元素
>             Iterator<String> iterator = list.iterator();
>             while(iterator.hasNext()) {
>                 String item = iterator.next();
>                 if("1".equals(item)) {
>                     iterator.remove();
>                 }
>             }
>             System.out.println("通过迭代器移除后的list元素："+ list.toString());
>         }
> 
> ![20210819201952621.png][]
> 
> ### 四、JDK8写法 ###
> 
>     private static void test5() {
>             List<String> list = new ArrayList<String>();
>             list.add("1");
>             list.add("1");
>             list.add("2");
>             list.add("3");
>             list.add("1");
>             list.add("5");
>             list.add("1");
>             list.add("6");
>             System.out.println("原始list元素："+ list.toString());
>     
>             //通过迭代器移除等于11的元素
>             list.removeIf("1"::equals);
>             System.out.println("通过迭代器移除后的list元素："+ list.toString());
>         }
> 
> ![20210819202149376.png][]

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[20210819195434173.png]: /images/20220829/22a453fc057a406da4abf068df828bde.png
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[20210819201952621.png]: /images/20220829/4cff54d7181a478ca0470ca2568661de.png
[20210819202149376.png]: /images/20220829/07361d7b54d74d77ac8c86f332b10e5a.png