Java中的遍历（遍历集合或数组的几种方式）

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本文主要总结了"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"中遍历集合或数组的几种方式，并介绍了各种遍历方式的实现原理，以及一些最佳实践。最后介绍了"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"集合类迭代器的快速失败（"},{"type":"codeinline","content":[{"type":"text","text":"fail-fast"}]},{"type":"text","text":"）机制。"}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"Java中的循环结构"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"遍历必然需要使用到循环结构，"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"中有以下几种循环结构："}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"while"}]},{"type":"text","text":"语句"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"do...while"}]},{"type":"text","text":"语句"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"基本"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"语句"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"语句"}]}]}]},{"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":"对于"},{"type":"codeinline","content":[{"type":"text","text":"do...while"}]},{"type":"text","text":"语句，其第一个循环体是必须会执行的，这对于空集合或者空数组是不适用的。所以我们一般不会使用"},{"type":"codeinline","content":[{"type":"text","text":"do...while"}]},{"type":"text","text":"语句来进行遍历。其余三种都是我们经常用来遍历的语法结构。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"语句和"},{"type":"codeinline","content":[{"type":"text","text":"while"}]},{"type":"text","text":"语句在一般情况下可以互相转化，下文我们并不将两种语句单独区分，会根据场景选择使用更加简单的方式。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"遍历数组"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"1. 使用下标遍历"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"使用循环遍历数组下标范围，在循环体中用下标访问数组元素："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"for (int i = 0; i < array.length; i++) {\n System.out.println(array[i]);\n}"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"2. 增强for循环"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"使用增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环语法结构来对数组进行遍历："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"for (int value : array) {\n System.out.println(value);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环 其实只是一种语法糖，使用 增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环 在遍历数组时，在编译过程会将其转化为 \"使用下标遍历\" 的方式，在字节码层面其实等价于第一种方式，效率上也没有太大差别。"}]},{"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":"关于增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环语法更详细的介绍，请移步："},{"type":"link","attrs":{"href":"https://docs.oracle.com/javase/specs/jls/se8/html/jls-14.html#jls-14.14.2","title":""},"content":[{"type":"text","text":"Java Language Specification - 14.14.2. The enhanced for statement"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"JAVA中的迭代器"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在 面向对象编程里，迭代器模式是一种设计模式，是一种最简单也最常见的设计模式。迭代器模式提供了一种方法顺序访问一个集合对象中的各个元素，而又不暴露其内部的表示。"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"中也提供了对迭代器模式的支持，主要是针对"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"的各种集合类进行遍历。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"接口是"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"中对迭代器的抽象接口定义，其定义如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public interface Iterator {\n // 是否还有下一个元素\n boolean hasNext();\n // 返回下一个元素\n E next();\n // 删除迭代过程中最近访问的一个元素\n // 也就是在next()之后调用remove()删除刚刚next()返回的元素\n default void remove() {\n throw new UnsupportedOperationException(\"remove\");\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口是在"},{"type":"codeinline","content":[{"type":"text","text":"Java 1.5"}]},{"type":"text","text":"中引入的，为了用来支持增强型"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环，只有实现了"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口的对象才可以使用增强型"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环。"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口定义如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public interface Iterable {\n // 返回迭代器\n Iterator iterator();\n // 使用函数式接口对增强型for循环进行包装，可以方便地使用lambda表达式来进行遍历\n default void forEach(Consumer super T> action) {\n Objects.requireNonNull(action);\n for (T t : this) {\n action.accept(t);\n }\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口提供了"},{"type":"codeinline","content":[{"type":"text","text":"forEach"}]},{"type":"text","text":"方法的默认实现，函数参数是一个函数式接口"},{"type":"codeinline","content":[{"type":"text","text":"action"}]},{"type":"text","text":"参数来表示对遍历到的每个元素的操作行为，实现逻辑是使用 增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环 遍历自身，循环中对每个元素都应用 参数"},{"type":"codeinline","content":[{"type":"text","text":"action"}]},{"type":"text","text":"所表示的操作行为。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"遍历List"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"List接口的定义"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"表示的是一个有序的元素集合。"},{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"接口继承了"},{"type":"codeinline","content":[{"type":"text","text":"Collection"}]},{"type":"text","text":"接口，"},{"type":"codeinline","content":[{"type":"text","text":"Collection"}]},{"type":"text","text":"接口又继承了"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口，其定义如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public interface Iterable {\n // 返回Iterator迭代器\n Iterator iterator();\n}\npublic interface Collection extends Iterable {\n}\npublic interface List extends Collection {\n // 获取指定位置的元素\n E get(int index);\n // 获取ListIterator迭代器\n ListIterator listIterator();\n // 从指定位置获取ListIterator迭代器\n ListIterator listIterator(int index);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到"},{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"除了可以通过"},{"type":"codeinline","content":[{"type":"text","text":"iterator()"}]},{"type":"text","text":"方法获得"},{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"迭代器之外，还可以通过"},{"type":"codeinline","content":[{"type":"text","text":"listIterator()"}]},{"type":"text","text":"方法获得"},{"type":"codeinline","content":[{"type":"text","text":"ListIterator"}]},{"type":"text","text":"迭代器。"},{"type":"codeinline","content":[{"type":"text","text":"ListIterator"}]},{"type":"text","text":"迭代器相比于"},{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"迭代器之外，访问和操作元素的方法更加丰富："}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"只能向后迭代，而"},{"type":"codeinline","content":[{"type":"text","text":"ListIterator"}]},{"type":"text","text":"可以向两个方向迭代"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"只能在迭代过程中删除元素，而"},{"type":"codeinline","content":[{"type":"text","text":"ListIterator"}]},{"type":"text","text":"可以添加元素、删除元素、修改元素。"}]}]}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public interface ListIterator extends Iterator {\n // Query Operations\n // 是否还有后一个元素\n boolean hasNext();\n // 访问后一个元素\n E next();\n // 是否还有前一个元素\n boolean hasPrevious();\n // 访问前一个元素\n E previous();\n // 后一个元素的下标\n int nextIndex();\n // 前一个元素的下标\n int previousIndex();\n\n // Modification Operations\n // 删除元素\n void remove();\n // 修改元素\n void set(E e);\n // 添加元素 \n void add(E e);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"List的遍历方法"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"1. 使用下标遍历"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"接口提供了"},{"type":"codeinline","content":[{"type":"text","text":"get"}]},{"type":"text","text":"方法来访问指定位置的元素，所以与遍历数组一样，"},{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"也可以通过遍历"},{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"下标并使用"},{"type":"codeinline","content":[{"type":"text","text":"get"}]},{"type":"text","text":"方法访问元素来遍历"},{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"for (int i = 0; i < list.size(); i++) {\n System.out.println(list.get(i));\n}"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2. 使用Iterator迭代器"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"可以使用继承自"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口的"},{"type":"codeinline","content":[{"type":"text","text":"iterator()"}]},{"type":"text","text":"方法来获得"},{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"迭代器，使用"},{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"迭代器来遍历"},{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"Iterator iterator = list.iterator();\nwhile (iterator.hasNext()){\n System.out.println(iterator.next());\n}"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3. 使用ListIterator迭代器"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"还可以使用"},{"type":"codeinline","content":[{"type":"text","text":"listIterator()"}]},{"type":"text","text":"方法来获得"},{"type":"codeinline","content":[{"type":"text","text":"ListIterator"}]},{"type":"text","text":"迭代器，使用"},{"type":"codeinline","content":[{"type":"text","text":"ListIterator"}]},{"type":"text","text":"迭代器来遍历"},{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"ListIterator listIterator = list.listIterator();\nwhile (listIterator.hasNext()){\n System.out.println(listIterator.next());\n}\n// ListIterator 可以向前遍历\nlistIterator = list.listIterator(list.size() - 1);\nwhile (listIterator.hasPrevious()){\n System.out.println(listIterator.previous());\n}"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"4. 增强for循环"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们前面说到，实现了"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口的对象可以使用增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环遍历。"},{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"接口继承自"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口，所以可以使用增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环来遍历"},{"type":"codeinline","content":[{"type":"text","text":"List"}]},{"type":"text","text":"："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"for (Integer value : list) {\n System.out.println(value);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"增强性"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环是一种语法糖，但是与遍历数组不一样的是，使用增强型"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环在遍历实现了"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口的对象时，会在编译过程中将其转化为使用"},{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"迭代器进行遍历的方式。所以这种方式本质上与上一种方式是一样的。"}]},{"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":"关于增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环语法更详细的介绍，请移步："},{"type":"link","attrs":{"href":"https://docs.oracle.com/javase/specs/jls/se8/html/jls-14.html#jls-14.14.2","title":""},"content":[{"type":"text","text":"Java Language Specification - 14.14.2. The enhanced for statement"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"5. Iterable接口的forEach方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"List接口实现了"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口，"},{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口中提供了"},{"type":"codeinline","content":[{"type":"text","text":"forEach"}]},{"type":"text","text":"方法来更加方便的遍历集合。其参数是一个函数式接口"},{"type":"codeinline","content":[{"type":"text","text":"action"}]},{"type":"text","text":"，来表示对遍历到的每个元素的操作行为。并且因为参数是一个函数式接口，所以我们可以使用"},{"type":"codeinline","content":[{"type":"text","text":"lamdba"}]},{"type":"text","text":"表达式更简洁的表达遍历过程。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Iterable"}]},{"type":"text","text":"接口的"},{"type":"codeinline","content":[{"type":"text","text":"forEach"}]},{"type":"text","text":"方法的默认实现是使用增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环来遍历自身。所以如果没有重写"},{"type":"codeinline","content":[{"type":"text","text":"forEach"}]},{"type":"text","text":"方法的话，这种方式本质上与上一种方式是一样的。"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"list.forEach(value -> System.out.println(value));\nlist.forEach(System.out::println);"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"最佳实践"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面几种方式其实本质上来讲只有两种方式："}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"使用循环遍历集合的下标范围，配合"},{"type":"codeinline","content":[{"type":"text","text":"get"}]},{"type":"text","text":"方法获取集合元素 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Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the 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"},{"type":"codeinline","content":[{"type":"text","text":"foreach"}]},{"type":"text","text":" 循环里进行元素的 "},{"type":"codeinline","content":[{"type":"text","text":"remove"}]},{"type":"text","text":"/"},{"type":"codeinline","content":[{"type":"text","text":"add"}]},{"type":"text","text":" 操作。"},{"type":"codeinline","content":[{"type":"text","text":"remove"}]},{"type":"text","text":" 元素请使用 "},{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"方式，如果并发操作，需要对 "},{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":" 对象加锁。"}]}]},{"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":"上面说的"},{"type":"codeinline","content":[{"type":"text","text":"foreach"}]},{"type":"text","text":"循环指的上面我们提到的使用增强"},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"循环进行遍历的方式。这种方式本质上使用的是迭代器方式。所以更明确一点，这个规范其实就是在说："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在使用"},{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"迭代器遍历集合过程中，不要通过集合本身的"},{"type":"codeinline","content":[{"type":"text","text":"remove"}]},{"type":"text","text":"/"},{"type":"codeinline","content":[{"type":"text","text":"add"}]},{"type":"text","text":"方法来进行元素的 "},{"type":"codeinline","content":[{"type":"text","text":"remove"}]},{"type":"text","text":"/"},{"type":"codeinline","content":[{"type":"text","text":"add"}]},{"type":"text","text":" 操作，"},{"type":"codeinline","content":[{"type":"text","text":"remove"}]},{"type":"text","text":"请使用"},{"type":"codeinline","content":[{"type":"text","text":"Iterator"}]},{"type":"text","text":"提供的"},{"type":"codeinline","content":[{"type":"text","text":"remove"}]},{"type":"text","text":"方法。"}]}]},{"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","marks":[{"type":"strong"}],"text":"正确"},{"type":"text","text":"方式："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"List list = new ArrayList<>();\nlist.add(1);\nlist.add(2);\nlist.add(3);\nIterator iterator = list.iterator();\nwhile (iterator.hasNext()) {\n Integer item = iterator.next();\n if (item == 1) {\n iterator.remove();\n }\n}\nlist.forEach(System.out::println); // 输出 2 3 "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"错误"},{"type":"text","text":"方式："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"List list = new ArrayList<>();\nlist.add(1);\nlist.add(2);\nlist.add(3);\nfor (Integer item : list) { // 会在这里抛出异常\n if (item == 1) {\n list.remove(item); // 使用 list.remove 删除\n }\n}\nlist.forEach(System.out::println);"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上述代码会在第五行抛出"},{"type":"codeinline","content":[{"type":"text","text":"java.util.ConcurrentModificationException"}]},{"type":"text","text":"异常。因为其本质还是使用迭代器遍历，所以为了方便理解其原因，我们将上述方式改写为："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"List list = new ArrayList<>();\nlist.add(1);\nlist.add(2);\nlist.add(3);\nIterator iterator = list.iterator();\nwhile (iterator.hasNext()) {\n Integer item = iterator.next(); // 会在这里抛出异常\n if (item == 1) {\n list.remove(item); // 使用 list.remove 删除\n }\n}\nlist.forEach(System.out::println);"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这段代码会在第"},{"type":"codeinline","content":[{"type":"text","text":"7"}]},{"type":"text","text":"行抛出"},{"type":"codeinline","content":[{"type":"text","text":"java.util.ConcurrentModificationException"}]},{"type":"text","text":"异常。与第一种方式的差别仅仅在于使用了"},{"type":"codeinline","content":[{"type":"text","text":"List.remove"}]},{"type":"text","text":"方法而不是"},{"type":"codeinline","content":[{"type":"text","text":"Iterator.remove"}]},{"type":"text","text":"方法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"快速失败的实现原理"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以"},{"type":"codeinline","content":[{"type":"text","text":"ArrayList"}]},{"type":"text","text":"为例来说明快速失败机制的实现原理，其他类的实现方式也是大致相同的。"}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"ArrayList"}]},{"type":"text","text":"中有一个属性"},{"type":"codeinline","content":[{"type":"text","text":"modCount"}]},{"type":"text","text":"，是一个记录"},{"type":"codeinline","content":[{"type":"text","text":"ArrayList"}]},{"type":"text","text":"修改次数的计数器。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"ArrayList"}]},{"type":"text","text":"的"},{"type":"codeinline","content":[{"type":"text","text":"iterator"}]},{"type":"text","text":"/"},{"type":"codeinline","content":[{"type":"text","text":"listIterator"}]},{"type":"text","text":"方法被调用时，会创建出"},{"type":"codeinline","content":[{"type":"text","text":"Iteartor"}]},{"type":"text","text":"/"},{"type":"codeinline","content":[{"type":"text","text":"ListIterator"}]},{"type":"text","text":"的实现类 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xt":"remove"}]},{"type":"text","text":"方法删除倒数第二个元素，程序能够正确删除，并不会像我们上面所说的抛出"},{"type":"codeinline","content":[{"type":"text","text":"ConcurrentModificationException"}]},{"type":"text","text":"异常。"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"List list = new ArrayList<>();\nlist.add(1);\nlist.add(2);\nlist.add(3);\nIterator iterator = list.iterator();\nwhile (iterator.hasNext()) {\n Integer item = iterator.next();\n if (item == 2) {\n list.remove(item);\n }\n}\nlist.forEach(System.out::println); // 输出 1 3"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个\""},{"type":"codeinline","content":[{"type":"text","text":"bug"}]},{"type":"text","text":"\"发生的原因在于，在第二次执行"},{"type":"codeinline","content":[{"type":"text","text":"iterator.next()"}]},{"type":"text","text":"后，迭代器记录的下一次将要访问的下标应该是"},{"type":"codeinline","content":[{"type":"text","text":"2"}]},{"type":"text","text":"，而在执行"},{"type":"codeinline","content":[{"type":"text","text":"list.remove()"}]},{"type":"text","text":"删除元素后，"},{"type":"codeinline","content":[{"type":"text","text":"list"}]},{"type":"text","text":"的"},{"type":"codeinline","content":[{"type":"text","text":"size"}]},{"type":"text","text":"变为了"},{"type":"codeinline","content":[{"type":"text","text":"2"}]},{"type":"text","text":"，所以在下次执行"},{"type":"codeinline","content":[{"type":"text","text":"iterator.hasNext()"}]},{"type":"text","text":"时认为已经没有元素要继续迭代了，返回"},{"type":"codeinline","content":[{"type":"text","text":"false"}]},{"type":"text","text":"，结束循环。"}]},{"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"},{"type":"codeinline","content":[{"type":"text","text":"fail-fast"}]},{"type":"text","text":"机制并不能够完全保证所有的并发修改的情况都抛出"},{"type":"codeinline","content":[{"type":"text","text":"ConcurrentModificationException"}]},{"type":"text","text":"异常，在程序中也不应该依赖于这个异常信息。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在"},{"type":"link","attrs":{"href":"https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html","title":""},"content":[{"type":"text","text":"ArrayList (Java Platform SE 8 )"}]},{"type":"text","text":"中也指出了"},{"type":"codeinline","content":[{"type":"text","text":"fail-fast"}]},{"type":"text","text":"这一性质："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs."}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"参考"}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://docs.oracle.com/javase/specs/jls/se8/html/jls-14.html#jls-14.14.2","title":""},"content":[{"type":"text","text":"The Java Language Specification, Java SE 8 Edition"}]}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://blog.csdn.net/HJF_HUANGJINFU/article/details/51220253#t23","title":""},"content":[{"type":"text","text":"Java遍历集合的几种方法分析（实现原理、算法性能、适用场合）"}]}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html","title":""},"content":[{"type":"text","text":"ArrayList (Java Platform SE 8 )"}]}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}