51工具盒子哈喽，大家好，我是指北君。 日常开发中，我们会使用各种工具类，利用封装好的轮子，能让我们的开发事半功倍。但是在JDK中，有一个特别的工具类------java.lang.Arrays.class，其源码实现还是挺精湛，接下来让我们来揭开它神秘的面纱。
java.util.Arrays 类是 JDK 提供的一个工具类，用来处理数组的各种方法，而且每个方法基本上都是静态方法，能直接通过类名Arrays调用。
1、asList {#1aslist}
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| 1 2 3 | public static
作用是返回由指定数组支持的固定大小列表。
注意 ：这个方法返回的 ArrayList 不是我们常用的集合类 java.util.ArrayList。这里的 ArrayList 是 Arrays 的一个内部类 java.util.Arrays.ArrayList。这个内部类有如下属性和方法：
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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 | private static class ArrayList |
①、返回的 ArrayList 数组是一个定长列表，我们只能对其进行查看或者修改，但是不能进行添加或者删除操作
通过源码我们发现该类是没有add()或者remove() 这样的方法的，如果对其进行增加或者删除操作，都会调用其父类 AbstractList 对应的方法，而追溯父类的方法最终会抛出 UnsupportedOperationException 异常。如下：
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| 1 2 3 4 5 | String[] str = {"a","b","c"}; List
②、引用类型的数组和基本类型的数组区别
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| 1 2 3 4 5 6 7 | String[] str = {"a","b","c"}; List listStr = Arrays.asList(str); System.out.println(listStr.size());//3 int[] i = {1,2,3}; List listI = Arrays.asList(i); System.out.println(listI.size());//1 |
上面的结果第一个listStr.size()==3，而第二个 listI.size()==1。这是为什么呢？
我们看源码，在 Arrays.asList 中，方法声明为 List asList(T... a)。该方法接收一个可变参数，并且这个可变参数类型是作为泛型的参数。我们知道基本数据类型是不能作为泛型的参数的，但是数组是引用类型，所以数组是可以泛型化的，于是 int[] 作为了整个参数类型，而不是 int 作为参数类型。
所以将上面的方法泛型化补全应该是：
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| 1 2 3 4 5 6 7 8 9 10 11 | String[] str = {"a","b","c"}; Listin={1,2,3};ListlistIn=Arrays.asList(in);//这里参数为int的包装类Integer，所以集合长度为3System.out.println(listIn.size());//3 |
③、返回的列表ArrayList里面的元素都是引用，不是独立出来的对象
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| 1 2 3 4 5 6 7 | String[] str = {"a","b","c"}; List
这里的Arrays.toString()方法就是打印数组的内容，后面会介绍。我们看修改集合的内容，原数组的内容也变化了，所以这里传入的是引用类型。
④、已知数组数据，如何快速获取一个可进行增删改查的列表List？
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| 1 2 3 4 | String[] str = {"a","b","c"}; List
这里的ArrayList 集合类后面我们会详细讲解，大家目前只需要知道有这种用法即可。
⑤、Arrays.asList() 方法使用场景
Arrays工具类提供了一个方法asList, 使用该方法可以将一个变长参数或者数组转换成List 。但是，生成的List的长度是固定的；能够进行修改操作（比如，修改某个位置的元素）；不能执行影响长度的操作（如add、remove等操作），否则会抛出UnsupportedOperationException异常。
所以 Arrays.asList 比较适合那些已经有数组数据或者一些元素，而需要快速构建一个List，只用于读取操作，而不进行添加或删除操作的场景。
2、sort {#2sort}
该方法是用于数组排序，在 Arrays 类中有该方法的一系列重载方法，能对7种基本数据类型，包括 byte,char,double,float,int,long,short 等都能进行排序，还有 Object 类型（实现了Comparable接口），以及比较器 Comparator 。 
①、基本类型的数组
这里我们以 int[ ] 为例看看：
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| 1 2 3 | int[] num = {1,3,8,5,2,4,6,7}; Arrays.sort(num); System.out.println(Arrays.toString(num));//[1, 2, 3, 4, 5, 6, 7, 8] |
通过调用 sort(int[] a) 方法，将原数组按照升序的顺序排列。下面我们通过源码看看是如何实现排序的：
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| 1 2 3 | public static void sort(int[] a) { DualPivotQuicksort.sort(a, 0, a.length - 1, null, 0, 0); } |
在 Arrays.sort 方法内部调用 DualPivotQuicksort.sort 方法，这个方法的源码很长，分别对于数组的长度进行了各种算法的划分，包括快速排序，插入排序，冒泡排序都有使用。详细源码可以参考这篇博客。
②、对象类型数组
该类型的数组进行排序可以实现 Comparable 接口，重写 compareTo 方法进行排序。
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| 1 2 3 | String[] str = {"a","f","c","d"}; Arrays.sort(str); System.out.println(Arrays.toString(str));//[a, c, d, f] |
String 类型实现了 Comparable 接口，内部的 compareTo 方法是按照字典码进行比较的。
③、没有实现Comparable接口的，可以通过Comparator实现排序
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| 1 2 3 4 5 6 7 8 9 10 11 | Person[] p = new Person[]{new Person("zhangsan",22),new Person("wangwu",11),new Person("lisi",33)}; Arrays.sort(p,new Comparator
3、binarySearch {#3binarysearch}
用二分法查找数组中的某个元素。该方法和 sort 方法一样，适用于各种基本数据类型以及对象。
注意：二分法是对以及有序的数组进行查找（比如先用Arrays.sort()进行排序，然后调用此方法进行查找）。找到元素返回下标，没有则返回 -1
实例：
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| 1 2 3 4 | int[] num = {1,3,8,5,2,4,6,7}; Arrays.sort(num); System.out.println(Arrays.toString(num));//[1, 2, 3, 4, 5, 6, 7, 8] System.out.println(Arrays.binarySearch(num, 2));//返回元素的下标 1 |
具体源码实现：
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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 | public static int binarySearch(int[] a, int key) { return binarySearch0(a, 0, a.length, key); } private static int binarySearch0(int[] a, int fromIndex, int toIndex,int key) { int low = fromIndex; int high = toIndex - 1; while (low <= high) { int mid = (low + high) >>> 1;//取中间值下标 int midVal = a[mid];//取中间值 if (midVal < key){ low = mid + 1; else if (midVal > key) high = mid - 1; } else{ return mid; } } return -(low + 1); } |
4、copyOf {#4copyof}
拷贝数组元素。底层采用 System.arraycopy() 实现，这是一个native方法。
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| 1 2 3 | public static native void arraycopy(Object src, int srcPos, Object dest, int destPos, int length); |
src:源数组
srcPos:源数组要复制的起始位置
dest:目的数组
destPos:目的数组放置的起始位置
length:复制的长度
注意：src 和 dest都必须是同类型或者可以进行转换类型的数组。
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| 1 2 3 4 | int[] num1 = {1,2,3}; int[] num2 = new int[3]; System.arraycopy(num1, 0, num2, 0, num1.length); System.out.println(Arrays.toString(num2));//[1, 2, 3] |
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| 1 2 3 4 5 6 7 8 9 10 11 | /** * @param original 源数组 * @param newLength //返回新数组的长度 * @return */ public static int[] copyOf(int[] original, int newLength) { int[] copy = new int[newLength]; System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength)); return copy; } |
5、fill {#5fill}
该系列方法用于给数组赋值，并能指定某个范围赋值。
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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 | //给a数组所有元素赋值 val public static void fill(int[] a, int val) { for (int i = 0, len = a.length; i < len; i++){ a[i] = val; } } //给从 fromIndex 开始的下标，toIndex-1结尾的下标都赋值 val,左闭右开 public static void fill(int[] a, int fromIndex, int toIndex, int val) { rangeCheck(a.length, fromIndex, toIndex);//判断范围是否合理 for (int i = fromIndex; i < toIndex; i++){ a[i] = val; } } |
6、toString 和 deepToString {#6tostring-和-deeptostring}
toString 用来打印一维数组的元素，而 deepToString 用来打印多层次嵌套的数组元素。
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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | public static String toString(int[] a) { if (a == null){ return "null"; } int iMax = a.length - 1; if (iMax == -1){ return "[]"; } StringBuilder b = new StringBuilder(); b.append('['); for (int i = 0; ; i++) { b.append(a[i]); if (i == iMax){ return b.append(']').toString(); } b.append(", "); } } |
7、小结 {#7小结}
好了，这就是JDK中java.lang.Arrays 类的源码解析。
指北君后续的文章会给大家介绍JDK的各种源码，让大家吃透JDK，另外还有各种工作趣闻，面试宝典。
我是指北君，操千曲而后晓声，观千剑而后识器。感谢各位人才的：点赞、收藏和评论，我们下期更精彩！
