A, string class
1, string search
(1) str.indexOf (substr); // return substr in str first appears in the index
str: any string objects
substr: string to search for
(2) str.lastIndexOf (substr); // Returns the index of the last occurrence of substr in str years
str: any string
substr: string to search for
2, to obtain the specified index character
str.charAt (index); // Returns the index is the index of the character, index to int
str: any string objects
index: specified index
3, to get a substring
(1) str.substring (beginIndex); // return from beginIndex taken start until the end of the string substring, an int beginIndex
(2) str.substring (beginIndex, endIndex); // return from the beginIndex (inclusive) to intercept a endIndex (not included) ending substring
4, removal of spaces
str.trim () // return copy of the string, ignore leading spaces and trailing spaces
eg: String str = " ab cd ";
String str1 = str.trim();//str1 = "ab cd";
5, the replacement string
str.replace (oldStr, newStr); // replace () will replace all oldstr newStr
oldStr: To replace a character or string
newStr: string for replacing the original content of the new
6, the start and end of the string is determined
(1) str.startsWith (prefix); // str determines whether the prefix is a prefix, prefix of string type
(2) str.endsWith (suffix); // str determines whether the suffix is a suffix, suffix of string type
7, it is determined whether the same character string
(1) str1 == str2;
(2) str.equals (substr); // the same characters (case sensitive) and a length, returns true
(3) str.equalslgnoreCase (substr); // ignore case compare
8, by comparing two strings lexicographically
str.compareTo (substr); // str substr located before returning a positive integer, then returns a negative integer, is equal to (the equals returns true) returns 0
9, character case conversion
str.toUpperCase (); // uppercase letters to lowercase
str.toLowerCase (); // lowercase letters changed to uppercase
10, the string is divided
(1) str.split (sign); // The split given string sign, sign for the string type, regular expressions may be used
(2) str.split (sign, limit); // limit limits the number of divisions
11, the date and time format string
str.format(format, args);
format: format string (character conversion)
args: an identifier referenced by the format string description format
You must import java.util.Date class
(1) Date Format
eg：
Output: 2019
19
(2) Time Format
(3) the date and time format combination
(4) a conventional type format
And (1) much the same, just not the same format
12, Regular Expressions
str.matches (regex); // check whether the str format represented by regex
13, string builder
(1) str1 + str2; // generates a new string instance, increasing the overhead
(2) StringBuilder class greatly improves the efficiency increase in frequency of the string
StringBuilder builder = new StringBuilder ( ""); // creates a string builder
(3) builder.append (content); // append to the string builder content, content can be any data type or other objects
（4）builder.insert(index， arg);//向字符串生成器指定的位置index插入arg，arg可以是任何数据类型或其他对象
（5）builder.delete(start, end);//删除字符串从指定的索引start（包含）开始到end（不包含）的内容
二、数组
必须先导入java.util.Arrays类
1、填充替换数组元素
（1）Arrays.fillf(arr, value);//将arr数组全部初始化为value
（2）Arrays.fill(arr, start, end, value);//将arr数组从start（包含）到end（不包含）初始化为value
2、对数组进行排序
Arrays.sort(arr);//将arr数组中的元素按照字典序排序，数字优先于字母，大写字母优先于小写字母
3、复制数组
（1）Arrays.copyOf(arr, copylenth);//将arr从0开始复制，复制长度为copylenth，不够用0补
（2）Arrays.copyOfRange(arr, start, end);//将arr从start（包含）开始复制到end（不包含）停止，start必须在arr长度范围内，end可大于arr长度
4、数组查询
（1）Arrays.binarySearch(arr, key);//返回arr数组中key的索引值，否则返回“-1”或者“-插入点”。如果有多个则无法保证返回的是哪一个
//调用函数前必须对数组排序
（2）Arrays.binarySearch(arr, start, end, key);//在arr从start（包含）到end（不包含）的范围内查找key，返回索引