女朋友跟我吐槽Java中ArrayList遍历时删除元素的各种姿势

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"简介"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们在项目开发过程中，经常会有需求需要删除ArrayList中的某个元素，而使用不正确的删除方式，就有可能抛出异常。以及在面试中，经常会遇到面试官询问ArrayList遍历时如何正确删除元素。所以在本篇文章中，我们会对几种删除元素的方式进行测试，并对原理进行研究，希望可以帮助到大家！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"ArrayList遍历时删除元素的几种姿势"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先结论如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第1种方法 - 普通for循环正序删除（结果：会漏掉元素判断）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第2种方法 - 普通for循环倒序删除（结果：正确删除）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第3种方法 - for-each循环删除（结果：抛出异常）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第4种方法 - Iterator遍历，使用ArrayList.remove()删除元素（结果：抛出异常）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第5种方法 - Iterator遍历，使用Iterator的remove删除元素（结果：正确删除）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面让我们来详细探究一下原因吧！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先初始化一个数组arrayList，假设我们要删除等于3的元素。"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" public static void main(String[] args) {\n ArrayList arrayList = new ArrayList();\n arrayList.add(1);\n arrayList.add(2);\n arrayList.add(3);\n arrayList.add(3);\n arrayList.add(4);\n arrayList.add(5);\n removeWayOne(arrayList);\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"第1种方法 - 普通for循环正序删除（结果：会漏掉元素判断）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"for (int i = 0; i < arrayList.size(); i++) {\n\tif (arrayList.get(i) == 3) {//3是要删除的元素\n\t\tarrayList.remove(i);\n\t\t//解决方案: 加一行代码i = i - 1; 删除元素后，下标减1\n\t}\n System.out.println(\"当前arrayList是\"+arrayList.toString());\n}\n//原ArrayList是[1, 2, 3, 3, 4, 5]\n//删除后是[1, 2, 3, 4, 5]"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"输出结果："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 4, 5]\n当前arrayList是[1, 2, 3, 4, 5]\n当前arrayList是[1, 2, 3, 4, 5]"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到少删除了一个3，"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"原因在于调用remove删除元素时，remove方法调用System.arraycopy()方法将后面的元素移动到前面的位置，也就是第二个3会移动到数组下标为2的位置，而在下一次循环时，i+1之后，i会为3，不会对数组下标为2这个位置进行判断，所以这种写法，在删除元素时，被删除元素a的后一个元素b会移动a的位置，而i已经加1，会忽略对元素b的判断，所以如果是连续的重复元素，会导致少删除。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"解决方案"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以在删除元素后，执行i=i-1，使得下次循环时再次对该数组下标进行判断。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"第2种方法 - 普通for循环倒序删除（结果：正确删除）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" for (int i = arrayList.size() -1 ; i>=0; i--) {\n if (arrayList.get(i).equals(3)) {\n arrayList.remove(i);\n }\n System.out.println(\"当前arrayList是\"+arrayList.toString());\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"输出结果："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 4, 5]\n当前arrayList是[1, 2, 4, 5]\n当前arrayList是[1, 2, 4, 5]\n当前arrayList是[1, 2, 4, 5]"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这种方法可以正确删除元素，因为调用remove删除元素时，remove方法调用System.arraycopy()将被删除元素a后面的元素向前移动，而不会影响元素a之前的元素，所以倒序遍历可以正常删除元素。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"第3种方法 - for-each循环删除（结果：抛出异常）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public static void removeWayThree(ArrayList arrayList) {\n for (Integer value : arrayList) {\n if (value.equals(3)) {//3是要删除的元素\n arrayList.remove(value);\n }\n System.out.println(\"当前arrayList是\"+arrayList.toString());\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"输出结果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 4, 5]\nException in thread \"main\" java.util.ConcurrentModificationException\n\tat java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)\n\tat java.util.ArrayList$Itr.next(ArrayList.java:851)\n\tat com.test.ArrayListTest1.removeWayThree(ArrayListTest1.java:50)\n\tat com.test.ArrayListTest1.main(ArrayListTest1.java:24)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"会抛出ConcurrentModificationException异常，主要在于for-each的底层实现是使用ArrayList.iterator的hasNext()方法和next()方法实现的，我们可以使用反编译进行验证，对包含上面的方法的类使用以下命令反编译验证"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"javac ArrayTest.java//生成ArrayTest.class文件\njavap -c ArrayListTest.class//对class文件反编译"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"得到removeWayThree方法的反编译代码如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" public static void removeWayThree(java.util.ArrayList);\n Code:\n 0: aload_0\n 1: invokevirtual #12 // Method java/util/ArrayList.iterator:()Ljava/util/Iterator;\n 4: astore_1\n 5: aload_1\n 6: invokeinterface #13, 1 // InterfaceMethod java/util/Iterator.hasNext:()Z 调用Iterator.hasNext()方法\n 11: ifeq 44\n 14: aload_1\n 15: invokeinterface #14, 1 // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;调用Iterator.next()方法\n 20: checkcast #9 // class java/lang/Integer\n 23: astore_2\n 24: aload_2\n 25: iconst_3\n 26: invokestatic #4 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;\n 29: invokevirtual #10 // Method java/lang/Integer.equals:(Ljava/lang/Object;)Z \n 32: ifeq 41\n 35: aload_0\n 36: aload_2\n 37: invokevirtual #15 // Method java/util/ArrayList.remove:(Ljava/lang/Object;)Z\n 40: pop\n 41: goto 5\n 44: return"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以很清楚得看到Iterator.hasNext()来判断是否还有下一个元素，和Iterator.next()方法来获取下一个元素。而因为在删除元素时，remove()方法会调用fastRemove()方法，其中会对modCount+1，代表对数组进行了修改，将修改次数+1。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" public boolean remove(Object o) {\n if (o == null) {\n for (int index = 0; index < size; index++)\n if (elementData[index] == null) {\n fastRemove(index);\n return true;\n }\n } else {\n for (int index = 0; index < size; index++)\n if (o.equals(elementData[index])) {\n fastRemove(index);\n return true;\n }\n }\n \t\treturn false;\n}\n\nprivate void fastRemove(int index) {\n modCount++;\n int numMoved = size - index - 1;\n if (numMoved > 0)\n \t\t\tSystem.arraycopy(elementData, index+1, elementData, index,numMoved);\n elementData[--size] = null; // clear to let GC do its work\n}\n\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"而当删除完元素后，进行下一次循环时，会调用下面源码中Itr.next()方法获取下一个元素，会调用checkForComodification()方法对ArrayList进行校验，判断在遍历ArrayList是否已经被修改，由于之前对modCount+1，而expectedModCount还是初始化时ArrayList.Itr对象时赋的值，所以会不相等，然后抛出ConcurrentModificationException异常。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"那么有什么办法可以让expectedModCount及时更新呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到下面Itr的源码中，在Itr.remove()方法中删除元素后会对 expectedModCount更新，所以我们在使用删除元素时使用Itr.remove()方法来删除元素就可以保证expectedModCount的更新了，具体看第5种方法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private class Itr implements Iterator {\n int cursor; // 游标\n int lastRet = -1; // index of last element returned; -1 if no such\n int expectedModCount = modCount;//期待的modCount值\n\n public boolean hasNext() {\n return cursor != size;\n }\n\n @SuppressWarnings(\"unchecked\")\n public E next() {\n checkForComodification();//判断expectedModCount与当前的modCount是否一致\n int i = cursor;\n if (i >= size)\n throw new NoSuchElementException();\n Object[] elementData = ArrayList.this.elementData;\n if (i >= elementData.length)\n throw new ConcurrentModificationException();\n cursor = i + 1;\n return (E) elementData[lastRet = i];\n }\n\n public void remove() {\n if (lastRet < 0)\n throw new IllegalStateException();\n checkForComodification();\n try {\n ArrayList.this.remove(lastRet);\n cursor = lastRet;\n lastRet = -1;\n expectedModCount = modCount;//更新expectedModCount\n } catch (IndexOutOfBoundsException ex) {\n throw new ConcurrentModificationException();\n }\n }\n\n final void checkForComodification() {\n if (modCount != expectedModCount)\n throw new ConcurrentModificationException();\n }\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"第4种方法 - Iterator遍历，使用ArrayList.remove()删除元素（结果：抛出异常）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"Iterator iterator = arrayList.iterator();\nwhile (iterator.hasNext()) {\n Integer value = iterator.next();\n if (value.equals(3)) {//3是要删除的元素\n \t\tarrayList.remove(value);\n }\n System.out.println(\"当前arrayList是\"+arrayList.toString());\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"输出结果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 4, 5]\nException in thread \"main\" java.util.ConcurrentModificationException\n\tat java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)\n\tat java.util.ArrayList$Itr.next(ArrayList.java:851)\n\tat com.test.ArrayListTest1.removeWayFour(ArrayListTest1.java:61)\n\tat com.test.ArrayListTest1.main(ArrayListTest1.java:25)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第3种方法在编译后的代码，其实是跟第4种是一样的，所以第四种写法也会抛出ConcurrentModificationException异常。这种需要注意的是，每次调用iterator的next()方法，会导致游标向右移动，从而达到遍历的目的。所以在单次循环中不能多次调用next()方法，不然会导致每次循环时跳过一些元素，我在一些博客里面看到了一些错误的写法，比如这一篇"},{"type":"link","attrs":{"href":"https://juejin.im/post/5b92844a6fb9a05d290ed46c","title":""},"content":[{"type":"text","text":"《在ArrayList的循环中删除元素，会不会出现问题？》"}]},{"type":"text","text":"文章中："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/a7/a7968c4cc1b12588e2a92d7fa45a96f6.png","alt":"image-20200101124822998","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"先调用iterator.next()获取元素，与elem进行比较，如果相等，再调用list.remove(iterator.next());来移除元素，这个时候的iterator.next()其实已经不是与elem相等的元素了，而是后一个元素了，我们可以写个demo来测试一下"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"ArrayList arrayList = new ArrayList();\narrayList.add(1);\narrayList.add(2);\narrayList.add(3);\narrayList.add(4);\narrayList.add(5);\narrayList.add(6);\narrayList.add(7);\n\nInteger elem = 3;\nIterator iterator = arrayList.iterator();\nwhile (iterator.hasNext()) {\n System.out.println(arrayList);\n if(iterator.next().equals(elem)) {\n \t\tarrayList.remove(iterator.next());\n }\n} "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"输出结果如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"[1, 2, 3, 4, 5, 6, 7]\n[1, 2, 3, 4, 5, 6, 7]\n[1, 2, 3, 4, 5, 6, 7]\n[1, 2, 3, 5, 6, 7]\nException in thread \"main\" java.util.ConcurrentModificationException\n\tat java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)\n\tat java.util.ArrayList$Itr.next(ArrayList.java:851)\n\tat com.test.ArrayListTest1.main(ArrayListTest1.java:29)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到移除的元素其实不是3，而是3之后的元素，因为调用了两次next()方法，导致游标多移动了。所以应该使用Integer value = iterator.next();将元素取出进行判断。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"第5种方法 - Iterator遍历，使用Iterator的remove删除元素（结果：正确删除）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"Iterator iterator = arrayList.iterator();\nwhile (iterator.hasNext()) {\n Integer value = iterator.next();\n if (value.equals(3)) {//3是需要删除的元素\n iterator.remove();\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"输出结果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 3, 4, 5]\n当前arrayList是[1, 2, 3, 4, 5]\n当前arrayList是[1, 2, 4, 5]\n当前arrayList是[1, 2, 4, 5]\n当前arrayList是[1, 2, 4, 5]"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以正确删除元素。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"跟第3种和第4种方法的区别在于是使用iterator.remove();来移除元素，而在remove()方法中会对iterator的expectedModCount变量进行更新，所以在下次循环调用iterator.next()方法时，expectedModCount与modCount相等，不会抛出异常。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"HashMap遍历时删除元素的几种姿势"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先结论如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第1种方法 - for-each遍历HashMap.entrySet，使用HashMap.remove()删除(结果：抛出异常)。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第2种方法-for-each遍历HashMap.keySet，使用HashMap.remove()删除(结果：抛出异常)。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第3种方法-使用HashMap.entrySet().iterator()遍历删除(结果：正确删除)。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面让我们来详细探究一下原因吧！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"HashMap的遍历删除方法与ArrayList的大同小异，只是api的调用方式不同。首先初始化一个HashMap，我们要删除key包含\"3\"字符串的键值对。"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"HashMap hashMap = new HashMap();\nhashMap.put(\"key1\",1);\nhashMap.put(\"key2\",2);\nhashMap.put(\"key3\",3);\nhashMap.put(\"key4\",4);\nhashMap.put(\"key5\",5);\nhashMap.put(\"key6\",6);"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"第1种方法 - for-each遍历HashMap.entrySet，使用HashMap.remove()删除(结果：抛出异常)"}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"for (Map.Entry entry: hashMap.entrySet()) {\n String key = entry.getKey();\n if(key.contains(\"3\")){\n hashMap.remove(entry.getKey());\n }\n System.out.println(\"当前HashMap是\"+hashMap+\" 当前entry是\"+entry);\n\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"输出结果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"当前HashMap是{key1=1, key2=2, key5=5, key6=6, key3=3, key4=4} 当前entry是key1=1\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key3=3, key4=4} 当前entry是key2=2\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key3=3, key4=4} 当前entry是key5=5\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key3=3, key4=4} 当前entry是key6=6\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key4=4} 当前entry是key3=3\nException in thread \"main\" java.util.ConcurrentModificationException\n\tat java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)\n\tat java.util.HashMap$EntryIterator.next(HashMap.java:1463)\n\tat java.util.HashMap$EntryIterator.next(HashMap.java:1461)\n\tat com.test.HashMapTest.removeWayOne(HashMapTest.java:29)\n\tat com.test.HashMapTest.main(HashMapTest.java:22)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"第2种方法-for-each遍历HashMap.keySet，使用HashMap.remove()删除(结果：抛出异常)"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"Set keySet = hashMap.keySet();\nfor(String key : keySet){\n if(key.contains(\"3\")){\n keySet.remove(key);\n }\n System.out.println(\"当前HashMap是\"+hashMap+\" 当前key是\"+key);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"输出结果如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"当前HashMap是{key1=1, key2=2, key5=5, key6=6, key3=3, key4=4} 当前key是key1\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key3=3, key4=4} 当前key是key2\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key3=3, key4=4} 当前key是key5\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key3=3, key4=4} 当前key是key6\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key4=4} 当前key是key3\nException in thread \"main\" java.util.ConcurrentModificationException\n\tat java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)\n\tat java.util.HashMap$KeyIterator.next(HashMap.java:1453)\n\tat com.test.HashMapTest.removeWayTwo(HashMapTest.java:40)\n\tat com.test.HashMapTest.main(HashMapTest.java:23)"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"第3种方法-使用HashMap.entrySet().iterator()遍历删除(结果：正确删除)"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"Iterator> iterator = hashMap.entrySet().iterator();\nwhile (iterator.hasNext()) {\n Map.Entry entry = iterator.next();\n if(entry.getKey().contains(\"3\")){\n iterator.remove();\n }\n System.out.println(\"当前HashMap是\"+hashMap+\" 当前entry是\"+entry);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"输出结果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"当前HashMap是{key1=1, key2=2, key5=5, key6=6, key4=4, deletekey=3} 当前entry是key1=1\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key4=4, deletekey=3} 当前entry是key2=2\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key4=4, deletekey=3} 当前entry是key5=5\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key4=4, deletekey=3} 当前entry是key6=6\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key4=4, deletekey=3} 当前entry是key4=4\n当前HashMap是{key1=1, key2=2, key5=5, key6=6, key4=4} 当前entry是deletekey=3"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第1种方法和第2种方法抛出ConcurrentModificationException异常与上面ArrayList错误遍历-删除方法的原因一致，HashIterator也有一个expectedModCount，在遍历时获取下一个元素时，会调用next()方法，然后对"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"expectedModCount和modCount进行判断，不一致就抛出ConcurrentModificationException异常。"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"abstract class HashIterator {\n Node next; // next entry to return\n Node current; // current entry\n int expectedModCount; // for fast-fail\n int index; // current slot\n\n HashIterator() {\n expectedModCount = modCount;\n Node[] t = table;\n current = next = null;\n index = 0;\n if (t != null && size > 0) { // advance to first entry\n do {} while (index < t.length && (next = t[index++]) == null);\n }\n }\n\n public final boolean hasNext() {\n return next != null;\n }\n\n final Node nextNode() {\n Node[] t;\n Node e = next;\n if (modCount != expectedModCount)\n throw new ConcurrentModificationException();\n if (e == null)\n throw new NoSuchElementException();\n if ((next = (current = e).next) == null && (t = table) != null) {\n do {} while (index < t.length && (next = t[index++]) == null);\n }\n return e;\n }\n\n public final void remove() {\n Node p = current;\n if (p == null)\n throw new IllegalStateException();\n if (modCount != expectedModCount)\n throw new ConcurrentModificationException();\n current = null;\n K key = p.key;\n removeNode(hash(key), key, null, false, false);\n expectedModCount = modCount;\n }\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"PS：ConcurrentModificationException是什么？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"根据ConcurrentModificationException的文档介绍，一些对象不允许并发修改，当这些修改行为被检测到时，就会抛出这个异常。（例如一些集合不允许一个线程一边遍历时，另一个线程去修改这个集合）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一些集合（例如Collection, Vector, ArrayList，LinkedList, HashSet, Hashtable, TreeMap, AbstractList, Serialized Form）的Iterator实现中，如果提供这种并发修改异常检测，那么这些Iterator可以称为是\"fail-fast Iterator\"，意思是快速失败迭代器，就是检测到并发修改时，直接抛出异常，而不是继续执行，等到获取到一些错误值时在抛出异常。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"异常检测主要是通过modCount和expectedModCount两个变量来实现的，"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"modCount "}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"集合被修改的次数，一般是被集合(ArrayList之类的)持有，每次调用add()，remove()方法会导致modCount+1"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"expectedModCount "}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"期待的modCount，一般是被Iterator(ArrayList.iterator()方法返回的iterator对象)持有，一般在Iterator初始化时会赋初始值，在调用Iterator的remove()方法时会对expectedModCount进行更新。（可以看看上面的ArrayList.Itr源码）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"然后在Iterator调用next()遍历元素时，会调用checkForComodification()方法比较modCount和expectedModCount，不一致就抛出ConcurrentModificationException。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"单线程操作Iterator不当时也会抛出ConcurrentModificationException异常。（上面的例子就是）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"WechatIMG4995.jpeg"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/36/3664bc238eaa6e0fe36e65ee5a3ac62e.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"总结"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因为ArrayList和HashMap的Iterator都是上面所说的“fail-fast Iterator”，Iterator在获取下一个元素，删除元素时，都会比较expectedModCount和modCount，不一致就会抛出异常。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以当使用Iterator遍历元素(for-each遍历底层实现也是Iterator)时，需要删除元素，一定需要使用 "},{"type":"text","marks":[{"type":"strong"}],"text":"Iterator的remove()方法"},{"type":"text","text":" 来删除，而不是直接调用ArrayList或HashMap自身的remove()方法,否则会导致Iterator中的expectedModCount没有及时更新，之后获取下一个元素或者删除元素时，expectedModCount和modCount不一致，然后抛出ConcurrentModificationException异常。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"最后"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"PS:本文已收录到1.2K+ Star数开源学习指南——《大厂面试指北》，如果想要了解更多，"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"请访问"},{"type":"link","attrs":{"href":"http://notfound9.github.io/interviewGuide/","title":""},"content":[{"type":"text","text":"http://notfound9.github.io/interviewGuide/"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}