java study notes 13: regular expressions, System class, Date class, Calendar class

First, regular expressions
1, Overview: it is used to describe a match or series of matches a string syntax rules; in fact, a rule, have their own particular application.
2, is composed of the regular expression
(1), character

expression	On behalf of the meaning of
x	Character x. Example: 'a' represents a character
\	Backslash characters.
\n	New line (line feed) ( '\ u000A')
\r	Carriage return ( '\ u000D')
[abc]	a, b or C (Simple Profile)
[^abc]	Any characters, in addition to a, b or C (negative)
[A-zA-Z]	a to z or A to Z, including two letters (range)
[0-9]	Characters 0-9 are included
.	Any character. I is. Character itself, how it represent?.
\d	Digital: [0-9]
\w	Word characters: [a-zA-Z_0-9]

(2), the boundary matcher

expression	On behalf of the meaning of
^	Beginning of the line
$	End of the line
\b	Word boundary is not a word character place.

(3), Greedy quantifier

expression	On behalf of the meaning of
X? X	Or once there is no such "" empty string is not
X* X	Zero or more times greater than equal to 1 are counted multiple times
X+ X	One or more times
X{n} X	Exactly n times
X{n,} X	At least n times
X{n,m} X	At least n times, but not more than m times

(4), for example

  String regx="abc";//定义正则表达式
      regx="\\|";  // \\ 转意字符
      regx="[abcdef]"; // 是我这个中括号出现的某一个字符
      regx="[12345]";
      regx="[12344abcdef]";
      regx="[0-9]";
      regx="[a-z]";
      regx="[A-Z]";
      regx="[a-zA-Z0-9]";
      regx="[^abc]"; //^ 不是我这个列表里面的一个字符
      regx=".."; //匹配单个任意字符
      regx="\\.";// \\. 代表点本身
      regx="\\d"; //等价于 [0-9]
      regx="^\\w$";// 等价于 [a - z_A - Z0 - 9]
      //数量词
      regx="[a-z]*"; //* 0次或多次  空串 就是0次  1个也算多次
      regx="[0-9a-zA-Z]+"; // + 一次 或多次
      regx="[a-z]?";//一次 或一次也没有
      regx="[a-z]{5}"; //正好几次
      regx="[a-z]{5,}"; //至少几次
      regx="[a-z]{5,15}";//  大于等与5  小于等于 15
       regx = "[1-9][0-9]{4,14}";  //正则表达式 正则表达式的语法
   boolean b = "abcd".matches(regx);

3, the regular expression functions
(1), determining function

• public boolean matches(String regex)

 public class Test {
     public static void main(String[] args) {
       //需求：校验邮箱  org cn net com
       //[email protected]
       //定义邮箱的正则  6 ~18 个字符，可使用字母、数字、下划线，需以字母开头
       String emaliRegx="[a-zA-Z]\\w{5,17}@[1-9a-z]{2,16}\\.(com|net|cn|org)";
       boolean matches = "[email protected]".matches(emaliRegx);
       System.out.println(matches);
     }
  }

(2) intercept function

• public String[] split(String regex)；

public classTest {
    public static void main(String[] args) {
        String str="username=张三";
        String s = str.substring(0, str.indexOf("="));
        System.out.println(s);     //username
        String substring = str.substring(str.indexOf("=")+1);
        System.out.println(substring);   //张三
        String str2 = "username=|#==@张三=|||#==@password=123456";
        String[] strings = str2.split("[=\\|#@]+");
        System.out.println(strings[0]);  //username
        System.out.println(strings[1]);  //张三
        System.out.println(strings[2]);  //password
        System.out.println(strings[3]);  //123456
    }
}

(3) replacement function

• public String replaceAll(String regex,String replacement)

public class Test {
    public static void main(String[] args) {
        String s = "我爱学习编程".replace("编程", "java");
        System.out.println(s);    
        String str="我爱==#@@@=学习#===|||java";
        String s1 = str.replaceAll("[=#@\\|]+", "");//根据正则替换
        System.out.println(s1);
        //去掉中间空格
        String s2 = "我    爱   学       习   j ava";
        String s3 = s2.replaceAll(" +", "");
        System.out.println(s3);
    }
}

Two, System class
1 Overview: System class contains several useful class fields and methods. It can not be instantiated.
2, members of the method

• public static void gc () // call the garbage collector
• public static void exit (int status) // quit java virtual machine 0 0 non-normal exit is abnormal exit
• public static long currentTimeMillis () // Get the current time value in milliseconds

in standard "input stream. This stream is already open and ready to provide input data. Typically, this corresponds to the keyboard input stream

• Scanner sc = new Scanner(in);

out "standard" output stream. This stream is already open and ready to accept output data. Typically, this stream corresponding to the display output

• PrintStream out = System.out;

3, members of the class using the method System

public class Test {
    public static void main(String[] args) {
      //循环耗时
      //当前系统的毫秒值，从 1970 01 01 00:00:00 来参照的
        long start = System.currentTimeMillis();
        for (int i = 0; i < 1000; i++) {
            System.out.println(i);
        }
        long end = System.currentTimeMillis();
        System.out.println("循环耗时"+(end-start)+"毫秒");
        //获取指定的环境变量值。
        String java_home = System.getenv("JAVA_HOME");
        System.out.println(java_home);
        String path = System.getenv("Path");
        System.out.println(path);
    }
}

Three, the BigDecimal class
1 Overview: As at the time of operation, float type and double precision can easily be lost, therefore, in order to accurately represent, floating point calculation, Java provides BigDecimal. Immutable, arbitrary-precision signed decimal numbers.
2, the configuration method

• public BigDecimal(String val)

3, member methods

• public BigDecimal add(BigDecimal augend)//加
• public BigDecimal subtract(BigDecimal subtrahend)//减
• public BigDecimal multiply(BigDecimal multiplicand)//乘
• public BigDecimal divide(BigDecimal divisor)//除法
• public BigDecimal divide (BigDecimal divisor, int scale, int roundingMode) // scale back several decimal places, roundingMode trade-offs such as rounding mode

4, and member constructor method using a class BigDecimal

public class Test {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal("10");
        BigDecimal b = new BigDecimal("3");
        BigDecimal add = a.add(b); //加法
        BigDecimal subtract = a.subtract(b);//减法
        BigDecimal multiply = a.multiply(b);//乘法
        //如果不能整除，得指定这个保留的位数，以及取舍模式
        BigDecimal divide = a.divide(b,5,BigDecimal.ROUND_CEILING);//除法
        System.out.println(add.toString());//13
        System.out.println(subtract);//7
        System.out.println(multiply);//30
        System.out.println(divide);//3.33334
    }
}

Four, Date Class
1. Overview: class Date represents a specific moment, with millisecond precision.
2, the configuration method

• public Date()
• public Date (long date) // converts a type of milliseconds long as a date object

3, member methods

• public long getTime (): Get a subject object date values ​​ms
• public void setTime (long time): date object is set to a specified value in milliseconds

4, using for example

public classTest {
    public static void main(String[] args) {
        Date date = new Date();
        long time = date.getTime();//获取系统当前的毫秒值
        System.out.println(time);
        Date date2 = new Date();
        date2.setTime(1000 * 60 * 60 * 24 * 5); //在计算机元年上增加相应的毫秒值
        System.out.println(date2);
    }
}

Five, Calendar class
1, Overview: Calendar class is an abstract class that can not be directly new object, you can come get his target through his a static member method getInstance (), which is a particular moment with a group such as YEAR, MONTH, DAY_OF_MONTH transition between the calendar field HOUR provide some method, and provides some methods for manipulating the calendar field.
2, members of the method

• When public static Calendar getInstance () uses the default time zone and locale to get a calendar object
• public int get (int field) to obtain a given calendar field value corresponding to the field provided by the field to come and collect Calendar

3, add () and set () method

• The amount of time public void add (int field, int amount) according to the calendar rules, for a given calendar add or subtract a specified field
• public final void set (int year, int month, int date) set the calendar date time

4, using the method of the Calendar class

public class MyDemo2 {
    public static void main(String[] args) {
        Calendar instance = Calendar.getInstance();
        instance.add(Calendar.YEAR,-2);
        System.out.println(instance.get(Calendar.YEAR));
        Calendar instance1 = Calendar.getInstance();
        instance1.set(Calendar.YEAR,2016);
        System.out.println(instance1.get(Calendar.YEAR));
    }
}