Scanner
Create input stream object
//System.in代表从键盘输入
Scanner sc = new Scanner(System.in);
Get the string entered by the keyboard
String str = sc.next();
Get a number entered by the keyboard
int num = sc.nextInt();
Close input stream
sc.close();
Random
Create an object
Random r = new Random();
Get random int numbers
int num1 = r.nextInt();
Get a random int number with range (left closed and right open)
int num2 = r.nextInt(10); //随机取0~9数字
Calendar
Methods to get or set various calendar fields
Create an object
Calendar c = Calendar.getInstance();
Get the current local time
c.getTime().toLocaleString();
Get the current time with Date
Date time = new Date(System.currentTimeMillis());
new SimpleDateFormat("YYYY-MM-DD hh:mm:ss").format(time);
Array
Array creation
int[] a = new int[10];
Convert array to string
String s = String.valueOf(c); //c为数组
String s = Arrays.toString(c); //以集合的形式转换为字符串
Turn the array into a collection
String[] s = { "a", "b", "c", "d", "e" };
ArrayList<String> arrayList = new ArrayList<String>(Arrays.asList(stringArray));
Get the length of the array
c1.length //返回一个整数，注意没有()
Arrays
Arrays is an array tool class.
Convert array to string
String s = Arrays.toString(c); //以集合的形式转换为字符串
Sort the array in ascending order
Arrays.sort(a); //a是数组
如果是数值，sort默认按照升序从小到大
如果是字符串，sort默认按照字母的升序
如果是自定义的类型，那么这个自定义的类需要有Comparable或者Comparator接口的支持
Determine whether two arrays are equal
Arrays.equals(a, b) //需要元素内容和元素顺序都相同才相等
ArrayList
The length of the Array array cannot be changed, and the length of the ArrayList collection can be changed
Create an ArrayList collection
//创建字符串集合
ArrayList<String> list = new ArrayList<>();
//创建整数集合
ArrayList<Integer> list = new ArrayList<>(); //泛型在尖括号内填写，只能写引用类型，不能是基本类型
Output collection
System.out.println(list);
Add string to collection
list.add("要添加进集合的字符串"); //返回一个布尔值提示是否添加成功
Read the string in the collection
list.get(0); //括号内填第几个元素下标，从0开始 ，返回对应元素
Delete the string in the collection
list.remove(0); //括号内填第几个元素下标，从0开始，返回被删除元素
Get the collection length
list.size(); //返回集合长度
Iterate over the collection
一：
for (int i = 0; i < list.size(); i++) {
System.out.println(list.get(i));
}
二：
for (String str:list) {
System.out.println(str);
}
Math
Math tools
Absolute value
Math.abs(-2.5); //2.5
Arrangement
Math.ceil(6.9); //向上取整，7.0
Math.floor(6.9); //向下取整，6.0
rounding
Math.round(6.9); //7
Power operation, find the m power of n
Math.pow(3,2); //9.0
square root
Math.sqrt(9); //3.0
Random number (0~1)
Math.random()
String
The string effect is the character array chae[ ], but the bottom layer is the byte array byte[]
You can directly click the "Hello" method
The content of the string cannot be changed, and all operations on the string must be returned with a new string.
Create String
//空参构造
String s = new String();
//字符数组创建
char[] c = {'a', 'b', 'c'};
String s = new String(c);
System.out.println(s); //abc
//字节数组创建
byte[] b = {97,98,99};
String s = new String(b);
System.out.println(s); //abc
Replacement character
s.replace('o', '*') //前者是旧字符，后者是新字符
Compare strings
s.equals(s1); //比较s和s1内容是否相同，一般常量放前面："abc".equals(s);
s==s1; //比较s和s1地址值是否相同，可用于比较两个对象是否引用同一实例
s.equalsIgnoreCase(s1) //和equals一样，但是忽略大小写差别
Intercept string
s.substring(4,10) //截取字符串函数，包含第4位，不含第10位
s.charAt(0) //截取下标为0的字符
====================================================================================================
//从字符串下标为4的字符开始到下标为9的字符结束截取字符串到字符数组,第四个是目标数组偏移量
Str1.getChars(4, 10, Str2, 0);
String Str1 = new String("abcdefghijklmn");
char[] Str2 = new char[6];
Str1.getChars(4, 10, Str2, 0);
System.out.println(Str2); //efghij
====================================================================================================
String[] s1 = s.split("@") //根据分隔符@截取字符串，用字符串数组接收,（）括号中参数为正则表达式
. $ ) ( * + [ ? \ ^ { | 以上十二个特殊字符不能直接使用，要在前面添加\\
String s1 = new String("aa.bb.cc");
String[] s2 = s1.split(".");
for (String s:s2)
System.out.println(s); //没有输出
String[] s3 = s1.split("\\.");
for (String s:s3)
System.out.println(s); //输出aa换行bb换行cc
Conversion of strings and character arrays
char[] c = s.toCharArray(); //字符串转换为字符数组
s.valueOf(c) //字符数组转换为字符串，c是字符数组
Concatenate strings at the end to form a new string
s3 = s1.concat(s2)
s3 = s1+s2; //结果和上面一样
Remove the leading and trailing spaces of the string
s.trim()
String case conversion
s.toLowerCase() //转换为小写
s.toUpperCase() //转换为大写
Get string length
s.length()
Find the position where a character or substring appears
indexOf('a') //查找字符或者子串第一次出现的地方，返回对应下标，没有返回-1
lastIndexOf("abc") //查找字符或者子串是后一次出现的地方，返回对应下标，没有返回-1
Format string
String.format("a:%c",'a').sout; //输出a:a，类似C语言的输出方法
StringBuffer
StringBuffer constructor:
StringBuffer()
StringBuffer(int size)
StringBuffer(String str)
StringBuffer(CharSequence chars)
Convert to string
sb.toString();
Get the length
sb.length();
Reverse the characters in the StringBuffer object
StringBuffer reverse()
Intercept substring
String substring(int startIndex)
String substring(int startIndex,int endIndex)
Get a character
char charAt(int where)
Replace a character
void setCharAt(int where,char ch)
Intercept a certain string to a character array
void getChars(int sourceStart,int sourceEnd,char target[],int targetStart)
sb.getChars(0,3,a,0); //截取下标为0-2的字符串到字符数组a中，数组偏移量为0
Additional character skewer
append() //追加字符串到stringbuff对象末尾
Insert string
insert()
StringBuffer insert(int index,String str)
StringBuffer insert(int index,char ch)
StringBuffer insert(int index,Object obj) //index指定将字符串插入到StringBuffer对象中的位置的下标。
Delete character segment
StringBuffer delete(0,4) //删除下标为0-3的字符
StringBuffer deleteCharAt(5) //删除下标为5的字符
replace
StringBuffer replace(0,2,"abc") //将下标为0-2的字符串替换为abc