java.util.Arrays，java.lang.Math，java.lang.System 类的常用方法汇总

java.util.Arrays类是数组的工具类，一般数组常用的方法包括

二分查找：public static int binarySearch(array[],int key)，返回key的下标index

扩容缩容：public static int[] copyOf(array[]，newLength)，返回新数组

取部分：public static int[] copyOfRange(array[],fromindex,toindex) ，注意[from,to)是开区间，返回新数组

数组从小到大快速排序：public static void Arrays.sort(array[])，

整体赋值：public static void fill(array[],value),

输出为字符串：public static String toString(array[])，返回字符串

检查是否相等：public static boolean equals(array1[],array2[])，返回布尔值

java.lang.Math类是常用的数学类，常用的方法有：

取绝对值：public static int abs(int value)，返回绝对值

向上取整：public static int ceil(double value)，

向下取整：public static int floor(double value);

四舍五入：public static long round(double value);

[0,1)de随机数：public static double random();

java.lang.System类有很多常用系统函数：

取当前系统时间距离1970年1月1日0点的毫秒数：public static long currentTimeMillis()

数组复制粘贴：public static void arrayCopy(Object src, int srcPos, Object dest, int destPos, int length):

　　* src：源数组

　　* srcPos：源数组的起始下标

　　* dest：目标数组

　　* destPos：目标数组的起始下标

* src[srcPos]元素放到dest[destPos]

* 依次存放

* length：一共要复制几个

*
　　* 如果src和dest是同一个数组时，当[srcPos]>[destPos]时，从后往前移动，一般用于元素的删除
　　* 如果src和dest是同一个数组时，当[srcPos]<[destPos]时，从前往后移动，

提供一个自己加深巩固印象的例题：

 1 import java.lang.reflect.Array;
 2 import java.util.Arrays;
 3 import java.util.Scanner;
 4 
 5 public class ArraysUtil {
 6 
 7     public static void main(String[] args) {
 8         double[] array=new double[100];
 9         // TODO Auto-generated method stub
10         for(int i=0;i<=Array.getLength(array)-1;i++) {
11             double num=Math.random()*1000-500;
12             array[i]=num;
13         }
14         System.out.print(Arrays.toString(array));//将数组转换为字符串并输出
15         double[] array1=new double[array.length];
16         array1=Arrays.copyOf(array, array.length);//复制整个数组长度给array1
17         Arrays.sort(array);//对数组进行快排
18         System.out.println();
19         System.out.print(Arrays.toString(array));//输出快排后的数组
20         System.out.println();
21         System.out.println(Arrays.equals(array, array1));//判断快排后的数组和快排前的数组是否相等
22         System.out.println(Arrays.toString(Arrays.copyOfRange(array, 0, 10)));//复制从下标0开始的10个元素返回数组并转换为字符串输出
23         System.out.println(Arrays.toString(Arrays.copyOfRange(array, array.length-10, array.length)));
24         double num=Math.random()*1000-500;
25         Arrays.fill(array1, num);//将数组全部元素赋值为num
26         System.out.println(Arrays.toString(array1));
27         
28         for(int i=0;i<=array.length-1;i++) {
29             array[i]=Math.abs(array[i]);//数组元素取绝对值
30         }
31         System.out.println(Arrays.toString(array));
32         System.out.println(System.currentTimeMillis());//输出当前系统时间，距离1970.1.1 0:0:0的毫秒数
33         
34         for(int i=0;i<=array.length-1;i++) {
35             System.out.print(Math.ceil(array[i])+" ");//向上取整
36         }
37         System.out.println();
38         System.out.println(System.currentTimeMillis());
39         
40         for(int i=0;i<=array.length-1;i++) {
41             System.out.print(Math.floor(array[i])+" ");//向下取整
42         }
43         System.out.println();
44         System.out.println(System.currentTimeMillis());
45         
46         for(int i=0;i<=array.length-1;i++) {
47             array[i]=Math.round(array[i]);//四舍五入
48         }
49         Arrays.sort(array);
50         System.out.println(Arrays.toString(array));
51         System.out.println(System.currentTimeMillis());
52         
53         System.out.println("input");
54         Scanner input= new Scanner(System.in);
55         int key=input.nextInt();
56         int index=Arrays.binarySearch(array, key);//二分查找值为key的下标
57         System.out.println(index);
58         System.arraycopy(array, index+1, array, index, array.length-index-1);//覆盖掉key
59         array=Arrays.copyOf(array, array.length-1);//数组缩容
60         System.out.println(Arrays.toString(array));    
61 
62         //在key1处增加一个随机元素
63         System.out.println("add");
64         int key1=input.nextInt();
65         int num1=(int)(Math.random()*1000);
66         double[] array2=Arrays.copyOfRange(array, key1, array.length);//将数组array从下标key1复制到最后一个下标，注意是开区间所以要加一个
67         array=Arrays.copyOf(array, array.length+1);
68         System.arraycopy(array2, 0, array, key1+1, array2.length);
69         array[key1]=num1;
70         System.out.println(Arrays.toString(array));
71     }
72 }

结果不唯一：

java.util.Arrays，java.lang.Math，java.lang.System 类的常用方法汇总

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false
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1562830503071
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