一、基本定义
Arrays类，全路径java.util.Arrays，主要功能为操作数组，Arrays类的所有方法均为静态方法，所以
调用方式全部为Arrays.方法名
二、常用方法
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1. <T> List<T> asList(T... a) |
可以将数组转化为相应的list集合，但是也只能转化为list，asList方法内部构建了一个内部静态类ArrayList，
这个ArrayList也继承自AbstractList，但并不是我们集合中常用的ArrayList，这两者是有区别的，需注意，
内部静态类AbstractList也实现了contains,forEach,replaceAll,sort,toArray等方法，但add,remove等方法则没有
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Integer[] array = new Integer[]{1,2,3}; int[] array2 = new int[]{1,2,3};List<Integer> list1 = Arrays.asList(1,2,3);List<Integer> list2 = Arrays.asList(array);//加入Java开发交流君样：593142328一起吹水聊天List<int[]> list3 = Arrays.asList(array2); |
2.void fill(int[] a, int val)、void fill(int[] a, int fromIndex, int toIndex, int val)、void fill(Object[] a, Object val)、void fill(Object[] a, int fromIndex, int toIndex, Object val)
fill方法有多个重载，分别对应几种基本数据类型以及引用类型（Object），
fill(int[] a, int val)会将整个数组的值全部覆盖为val
fill(int[] a, int fromIndex, int toIndex, int val)则提供了可选的开头和结尾（不包括）
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int[] array = new int[]{1,2,3};Arrays.fill(array, 1);Arrays.fill(array, 0, 2, 1);// {1,1,3}String[] str = {"123"};Arrays.fill(str, "1"); |
源码如下：
我们可以看到可选开头结尾的重载方法会先做数组越界的校验，防止非法输入
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/** * Assigns the specified double value to each element of the specified * range of the specified array of doubles. The range to be filled * extends from index <tt>fromIndex</tt>, inclusive, to index * <tt>toIndex</tt>, exclusive. (If <tt>fromIndex==toIndex</tt>, the * range to be filled is empty.) * * @param a the array to be filled * @param fromIndex the index of the first element (inclusive) to be * filled with the specified value * @param toIndex the index of the last element (exclusive) to be * filled with the specified value//加入Java开发交流君样：593142328一起吹水聊天 * @param val the value to be stored in all elements of the array * @throws IllegalArgumentException if <tt>fromIndex > toIndex</tt> * @throws ArrayIndexOutOfBoundsException if <tt>fromIndex < 0</tt> or * <tt>toIndex > a.length</tt> */ public static void fill(double[] a, int fromIndex, int toIndex,double val){ rangeCheck(a.length, fromIndex, toIndex); for ( int i = fromIndex; i < toIndex; i++) a[i] = val; } /** * Assigns the specified float value to each element of the specified array * of floats. * * @param a the array to be filled * @param val the value to be stored in all elements of the array */ public static void fill(float[] a, float val) { for (int i = 0, len = a.length; i < len; i++) a[i] = val; } /** * Checks that {@code fromIndex} and {@code toIndex} are in * the range and throws an exception if they aren't. */ private static void rangeCheck(int arrayLength, int fromIndex, int toIndex) { if (fromIndex > toIndex) { throw new IllegalArgumentException( "fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")"); } if (fromIndex < 0) { throw new ArrayIndexOutOfBoundsException(fromIndex); } if (toIndex > arrayLength) {//加入Java开发交流君样：593142328一起吹水聊天 throw new ArrayIndexOutOfBoundsException(toIndex); } } |
3.int[] copyOf(int[] original, int newLength)、int[] copyOfRange(int[] original, int from, int to)
存在多个重载方式，此处以int举例
从样例中我i们看到，copyOf复制后的数组长度可以大于复制前的数组，根据源码发现，超出的元素被填充为0，引用类型则填充为null
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int[] array = new int[]{1,2,3}; int[] array2 = Arrays.copyOf(array, 4); |
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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; } |
对于copyOfRange，可以选择复制的开头和结尾（不包括），且结尾下标可以大于原数组长度，超出的下标会被填充
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int[] array = new int[]{1,2,3,4,5,6,7,8,9}; int[] array2 = Arrays.copyOfRange(array, 3, 6); int[] array3 = Arrays.copyOfRange(array, 3, 10); |
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/** * Copies the specified range of the specified array into a new array. * The initial index of the range (<tt>from</tt>) must lie between zero * and <tt>original.length</tt>, inclusive. The value at * <tt>original[from]</tt> is placed into the initial element of the copy * (unless <tt>from == original.length</tt> or <tt>from == to</tt>). * Values from subsequent elements in the original array are placed into * subsequent elements in the copy. The final index of the range * (<tt>to</tt>), which must be greater than or equal to <tt>from</tt>, * may be greater than <tt>original.length</tt>, in which case * <tt>0</tt> is placed in all elements of the copy whose index is * greater than or equal to <tt>original.length - from</tt>. The length * of the returned array will be <tt>to - from</tt>. * * @param original the array from which a range is to be copied * @param from the initial index of the range to be copied, inclusive * @param to the final index of the range to be copied, exclusive. * (This index may lie outside the array.) * @return a new array containing the specified range from the original array, * truncated or padded with zeros to obtain the required length * @throws ArrayIndexOutOfBoundsException if {@code from < 0} * or {@code from > original.length} * @throws IllegalArgumentException if <tt>from > to</tt> * @throws NullPointerException if <tt>original</tt> is null * @since 1.6 *///加入Java开发交流君样：593142328一起吹水聊天 public static int[] copyOfRange(int[] original, int from, int to) { int newLength = to - from; if (newLength < 0) throw new IllegalArgumentException(from + " > " + to); int[] copy = new int[newLength]; System.arraycopy(original, from, copy, 0, Math.min(original.length - from, newLength)); return copy; } |
4.boolean equals(int[] a, int[] a2)、boolean equals(Object[] a, Object[] a2)
比较2个数组是否相等，基本类型的元素会依次进行==判断，引用类型则会在判空后使用equals【白嫖资料】
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public static boolean equals(int[] a, int[] a2) { if (a==a2) return true; if (a==null || a2==null) return false; int length = a.length; if (a2.length != length) return false; for (int i=0; i<length; i++) if (a[i] != a2[i]) return false; return true; } public static boolean equals(Object[] a, Object[] a2) { if (a==a2) return true; if (a==null || a2==null) return false; int length = a.length; if (a2.length != length) return false; for (int i=0; i<length; i++) { Object o1 = a[i]; Object o2 = a2[i]; if (!(o1==null ? o2==null : o1.equals(o2))) return false; } return true; } |
5.String toString(int[] a)
假设我们想输出一个数组的全部元素，一种方法是利用循环遍历所有元素后挨个输出
但Arrays提供了一个方案可以直接调用，toString内部实现其实也是通过遍历来实现，
利用可变字符串StringBuilder来构建
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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(", "); } } |
6.int binarySearch(int[] a, int key)
Arrays内置的二分查找方法，使用条件为参数数组a是有序的，如无序
会导致返回结果错误
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1 public static int binarySearch(int[] a, int fromIndex, int toIndex, 2 int key) { 3 rangeCheck(a.length, fromIndex, toIndex); 4 return binarySearch0(a, fromIndex, toIndex, key); 5 } 6 7 // Like public version, but without range checks. 8 private static int binarySearch0(int[] a, int fromIndex, int toIndex, 9 int key) { 10 int low = fromIndex; 11 int high = toIndex - 1; 1213 while (low <= high) { 14 int mid = (low + high) >>> 1; 15 int midVal = a[mid]; 1617 if (midVal < key) 18 low = mid + 1; 19 else if (midVal > key) 20 high = mid - 1; 21 else22 return mid; // key found23 } 24 return -(low + 1); // key not found.25 } |
总结
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原文链接：https://blog.csdn.net/wj1314250/article/details/118615724
