本文介绍了Java中几个常用类及其方法。java.util.Arrays类是数组工具类，有二分查找、扩容缩容等方法；java.lang.Math类是数学类，包含取绝对值、取整等方法；java.lang.System类有获取系统时间、数组复制等系统函数，还给出了巩固例题。
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：
[370.5069491677524, 196.6812635069174, -360.5048683247735, -296.9707345890539, -248.18275801431668, 330.4816228586669, -83.76315619698659, 123.90888214798963, -288.4938711174634, 347.11337149526446, -237.63606767620138, 234.0763833448758, -423.2174433068593, 437.5319335124543, 477.39494908857966, -63.52554800479061, -189.15434718636624, -420.3981620197292, -356.635413884099, -118.68024437258975, -249.08160966211946, -164.64062114908205, 267.73263503815247, -244.01698981975795, 75.80884356006504, -161.0076696340871, 186.41421827809688, -201.74529345276937, -403.34287177804606, -424.7018129947752, -407.4372958785374, 439.8874670618925, 235.03475272891728, 451.7032900989036, -94.63135625309371, 93.7514920375761, -270.67989653240284, 226.49579129131337, 448.0221366776857, 360.63408120779377, -306.7179449434235, -428.25511855314755, -434.8824518357589, 350.6605431511523, 140.06152121597563, -78.5502205758545, -122.83652551630274, 389.50821223909304, 480.24820355724114, -216.21815809402455, 343.492615987632, 111.03136849447219, 304.50815037151335, -119.14606088041768, -315.2499951269141, -174.39057295279406, -30.77496151941648, 320.27070664221264, -497.7300739116909, 97.47672144505884, 117.87618615257918, 499.78473861773705, 124.03566531027604, -335.5127458556708, -208.62790871079818, -400.2969248122029, -194.45216537264133, 190.2127774328835, -292.6538311060517, 327.2592714625607, 418.8112178981787, -149.03121967343571, -378.6113204198135, 42.986234712738224, 173.78158362663407, 429.3069792635346, 422.4776976379379, -478.8039735767445, 233.99766587778856, 410.70952597237647, -239.6336454635926, 374.991995979983, 311.7558922775456, -438.3354136628692, -445.55510898513694, -389.19001872114865, 458.0682753336263, -395.001482516242, -308.39735963034764, -131.69462332131934, 298.7768215584704, -97.71464522752058, -415.12625464615985, -351.5274307617464, -89.41943095578017, -496.6507460368589, 35.157530483676965, 236.20433842160116, 379.3641021486826, -470.67865296241206] [-497.7300739116909, -496.6507460368589, -478.8039735767445, -470.67865296241206, -445.55510898513694, -438.3354136628692, -434.8824518357589, -428.25511855314755, -424.7018129947752, -423.2174433068593, -420.3981620197292, -415.12625464615985, -407.4372958785374, -403.34287177804606, -400.2969248122029, -395.001482516242, -389.19001872114865, -378.6113204198135, -360.5048683247735, -356.635413884099, -351.5274307617464, -335.5127458556708, -315.2499951269141, -308.39735963034764, -306.7179449434235, -296.9707345890539, -292.6538311060517, -288.4938711174634, -270.67989653240284, -249.08160966211946, -248.18275801431668, -244.01698981975795, -239.6336454635926, -237.63606767620138, -216.21815809402455, -208.62790871079818, -201.74529345276937, -194.45216537264133, -189.15434718636624, -174.39057295279406, -164.64062114908205, -161.0076696340871, -149.03121967343571, -131.69462332131934, -122.83652551630274, -119.14606088041768, -118.68024437258975, -97.71464522752058, -94.63135625309371, -89.41943095578017, -83.76315619698659, -78.5502205758545, -63.52554800479061, -30.77496151941648, 35.157530483676965, 42.986234712738224, 75.80884356006504, 93.7514920375761, 97.47672144505884, 111.03136849447219, 117.87618615257918, 123.90888214798963, 124.03566531027604, 140.06152121597563, 173.78158362663407, 186.41421827809688, 190.2127774328835, 196.6812635069174, 226.49579129131337, 233.99766587778856, 234.0763833448758, 235.03475272891728, 236.20433842160116, 267.73263503815247, 298.7768215584704, 304.50815037151335, 311.7558922775456, 320.27070664221264, 327.2592714625607, 330.4816228586669, 343.492615987632, 347.11337149526446, 350.6605431511523, 360.63408120779377, 370.5069491677524, 374.991995979983, 379.3641021486826, 389.50821223909304, 410.70952597237647, 418.8112178981787, 422.4776976379379, 429.3069792635346, 437.5319335124543, 439.8874670618925, 448.0221366776857, 451.7032900989036, 458.0682753336263, 477.39494908857966, 480.24820355724114, 499.78473861773705] false [-497.7300739116909, -496.6507460368589, -478.8039735767445, -470.67865296241206, -445.55510898513694, -438.3354136628692, -434.8824518357589, -428.25511855314755, -424.7018129947752, -423.2174433068593] [422.4776976379379, 429.3069792635346, 437.5319335124543, 439.8874670618925, 448.0221366776857, 451.7032900989036, 458.0682753336263, 477.39494908857966, 480.24820355724114, 499.78473861773705] [15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188, 15.062960922778188] [497.7300739116909, 496.6507460368589, 478.8039735767445, 470.67865296241206, 445.55510898513694, 438.3354136628692, 434.8824518357589, 428.25511855314755, 424.7018129947752, 423.2174433068593, 420.3981620197292, 415.12625464615985, 407.4372958785374, 403.34287177804606, 400.2969248122029, 395.001482516242, 389.19001872114865, 378.6113204198135, 360.5048683247735, 356.635413884099, 351.5274307617464, 335.5127458556708, 315.2499951269141, 308.39735963034764, 306.7179449434235, 296.9707345890539, 292.6538311060517, 288.4938711174634, 270.67989653240284, 249.08160966211946, 248.18275801431668, 244.01698981975795, 239.6336454635926, 237.63606767620138, 216.21815809402455, 208.62790871079818, 201.74529345276937, 194.45216537264133, 189.15434718636624, 174.39057295279406, 164.64062114908205, 161.0076696340871, 149.03121967343571, 131.69462332131934, 122.83652551630274, 119.14606088041768, 118.68024437258975, 97.71464522752058, 94.63135625309371, 89.41943095578017, 83.76315619698659, 78.5502205758545, 63.52554800479061, 30.77496151941648, 35.157530483676965, 42.986234712738224, 75.80884356006504, 93.7514920375761, 97.47672144505884, 111.03136849447219, 117.87618615257918, 123.90888214798963, 124.03566531027604, 140.06152121597563, 173.78158362663407, 186.41421827809688, 190.2127774328835, 196.6812635069174, 226.49579129131337, 233.99766587778856, 234.0763833448758, 235.03475272891728, 236.20433842160116, 267.73263503815247, 298.7768215584704, 304.50815037151335, 311.7558922775456, 320.27070664221264, 327.2592714625607, 330.4816228586669, 343.492615987632, 347.11337149526446, 350.6605431511523, 360.63408120779377, 370.5069491677524, 374.991995979983, 379.3641021486826, 389.50821223909304, 410.70952597237647, 418.8112178981787, 422.4776976379379, 429.3069792635346, 437.5319335124543, 439.8874670618925, 448.0221366776857, 451.7032900989036, 458.0682753336263, 477.39494908857966, 480.24820355724114, 499.78473861773705] 1562830503065 498.0 497.0 479.0 471.0 446.0 439.0 435.0 429.0 425.0 424.0 421.0 416.0 408.0 404.0 401.0 396.0 390.0 379.0 361.0 357.0 352.0 336.0 316.0 309.0 307.0 297.0 293.0 289.0 271.0 250.0 249.0 245.0 240.0 238.0 217.0 209.0 202.0 195.0 190.0 175.0 165.0 162.0 150.0 132.0 123.0 120.0 119.0 98.0 95.0 90.0 84.0 79.0 64.0 31.0 36.0 43.0 76.0 94.0 98.0 112.0 118.0 124.0 125.0 141.0 174.0 187.0 191.0 197.0 227.0 234.0 235.0 236.0 237.0 268.0 299.0 305.0 312.0 321.0 328.0 331.0 344.0 348.0 351.0 361.0 371.0 375.0 380.0 390.0 411.0 419.0 423.0 430.0 438.0 440.0 449.0 452.0 459.0 478.0 481.0 500.0 1562830503068 497.0 496.0 478.0 470.0 445.0 438.0 434.0 428.0 424.0 423.0 420.0 415.0 407.0 403.0 400.0 395.0 389.0 378.0 360.0 356.0 351.0 335.0 315.0 308.0 306.0 296.0 292.0 288.0 270.0 249.0 248.0 244.0 239.0 237.0 216.0 208.0 201.0 194.0 189.0 174.0 164.0 161.0 149.0 131.0 122.0 119.0 118.0 97.0 94.0 89.0 83.0 78.0 63.0 30.0 35.0 42.0 75.0 93.0 97.0 111.0 117.0 123.0 124.0 140.0 173.0 186.0 190.0 196.0 226.0 233.0 234.0 235.0 236.0 267.0 298.0 304.0 311.0 320.0 327.0 330.0 343.0 347.0 350.0 360.0 370.0 374.0 379.0 389.0 410.0 418.0 422.0 429.0 437.0 439.0 448.0 451.0 458.0 477.0 480.0 499.0 1562830503071 [31.0, 35.0, 43.0, 64.0, 76.0, 79.0, 84.0, 89.0, 94.0, 95.0, 97.0, 98.0, 111.0, 118.0, 119.0, 119.0, 123.0, 124.0, 124.0, 132.0, 140.0, 149.0, 161.0, 165.0, 174.0, 174.0, 186.0, 189.0, 190.0, 194.0, 197.0, 202.0, 209.0, 216.0, 226.0, 234.0, 234.0, 235.0, 236.0, 238.0, 240.0, 244.0, 248.0, 249.0, 268.0, 271.0, 288.0, 293.0, 297.0, 299.0, 305.0, 307.0, 308.0, 312.0, 315.0, 320.0, 327.0, 330.0, 336.0, 343.0, 347.0, 351.0, 352.0, 357.0, 361.0, 361.0, 371.0, 375.0, 379.0, 379.0, 389.0, 390.0, 395.0, 400.0, 403.0, 407.0, 411.0, 415.0, 419.0, 420.0, 422.0, 423.0, 425.0, 428.0, 429.0, 435.0, 438.0, 438.0, 440.0, 446.0, 448.0, 452.0, 458.0, 471.0, 477.0, 479.0, 480.0, 497.0, 498.0, 500.0] 1562830503071 input 95 9 [31.0, 35.0, 43.0, 64.0, 76.0, 79.0, 84.0, 89.0, 94.0, 97.0, 98.0, 111.0, 118.0, 119.0, 119.0, 123.0, 124.0, 124.0, 132.0, 140.0, 149.0, 161.0, 165.0, 174.0, 174.0, 186.0, 189.0, 190.0, 194.0, 197.0, 202.0, 209.0, 216.0, 226.0, 234.0, 234.0, 235.0, 236.0, 238.0, 240.0, 244.0, 248.0, 249.0, 268.0, 271.0, 288.0, 293.0, 297.0, 299.0, 305.0, 307.0, 308.0, 312.0, 315.0, 320.0, 327.0, 330.0, 336.0, 343.0, 347.0, 351.0, 352.0, 357.0, 361.0, 361.0, 371.0, 375.0, 379.0, 379.0, 389.0, 390.0, 395.0, 400.0, 403.0, 407.0, 411.0, 415.0, 419.0, 420.0, 422.0, 423.0, 425.0, 428.0, 429.0, 435.0, 438.0, 438.0, 440.0, 446.0, 448.0, 452.0, 458.0, 471.0, 477.0, 479.0, 480.0, 497.0, 498.0, 500.0] add 10 [31.0, 35.0, 43.0, 64.0, 76.0, 79.0, 84.0, 89.0, 94.0, 97.0, 583.0, 98.0, 111.0, 118.0, 119.0, 119.0, 123.0, 124.0, 124.0, 132.0, 140.0, 149.0, 161.0, 165.0, 174.0, 174.0, 186.0, 189.0, 190.0, 194.0, 197.0, 202.0, 209.0, 216.0, 226.0, 234.0, 234.0, 235.0, 236.0, 238.0, 240.0, 244.0, 248.0, 249.0, 268.0, 271.0, 288.0, 293.0, 297.0, 299.0, 305.0, 307.0, 308.0, 312.0, 315.0, 320.0, 327.0, 330.0, 336.0, 343.0, 347.0, 351.0, 352.0, 357.0, 361.0, 361.0, 371.0, 375.0, 379.0, 379.0, 389.0, 390.0, 395.0, 400.0, 403.0, 407.0, 411.0, 415.0, 419.0, 420.0, 422.0, 423.0, 425.0, 428.0, 429.0, 435.0, 438.0, 438.0, 440.0, 446.0, 448.0, 452.0, 458.0, 471.0, 477.0, 479.0, 480.0, 497.0, 498.0, 500.0]
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