API (common class 2)

1.String class

• byte[] getBytes()
• char[] toCharArray()
• static String valueOf(char[] chs)
• String toLowerCase()
• String toUpperCase()
• String concat(String str)
• Stirng[] split( Separator );
• String replace(char old,char new)
• String replace(String old,String new)
• replaceAll(String regex, String replacement)
• replaceFirst(String regex, String replacement)
• String trim()

 String a=" China ";
        byte[] b=a.getBytes("utf-8");// Convert the string to Byte Array 
        String c=new String();// take Byte Array to string 
        System.out.println(Arrays.toString(b));// The array needs Arrays.toString Output 
        System.out.println(c);// Non array direct output 
        String a1=" China ";
        char[] c1=a1.toCharArray();// Output the string directly as an array 
        System.out.println(Arrays.toString(c1));

int i=Integer.parseInt("10");//  Convert string to basic type 
        Integer ii=new Integer("20");// Convert string to wrapper type 
        String c= "null";
        String s=String.valueOf(c);
        //public static String valueOf(Object obj) {
//        return (obj == null) ? "null" : obj.toString();
//         It is recommended to convert other types to String when , Use valueOf()
        System.out.println(i);//10
        System.out.println(ii);//20
        System.out.println(s);//null
        String a="  abC1deab23fG  ";
        System.out.println(a.toLowerCase(Locale.forLanguageTag(a)));// Convert all characters to lowercase 
        System.out.println(a.toUpperCase(Locale.forLanguageTag(a)));// Capitalize all characters 
        String b="abc";
        String d=b.concat("efg");// Connection string 
        System.out.println(d);
        String a1="a,b,c,d";
        String[]a2=a1.split(",");// Split returns array values 
        System.out.println(Arrays.toString(a2));
        String a3=a.replace("ab","AA");// Replace 
        System.out.println(a3);
        String a4=a.replaceAll("ab","BB");// Replace , Regular expressions 
        System.out.println(a4);
        String a5=a.replaceAll("\\d","");
        System.out.println(a5);
        int a6=a.trim().length();// Remove the space at both ends 
        System.out.println(a.length());
        System.out.println(a6);

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2.StringBuffer class

summary ： If we splice strings , Every splicing , Will build a new String object , Time consuming , It's a waste of space . and StringBuffer And then we can solve this problem Thread safe variable character sequence .

String、StringBuilder and Stringbuffer The difference between ：

• StringBuffer It is a multi-threaded operation and the safe methods are synchronized Keyword modification .
• StringBuilder A single thread , High security .
• String Immutable value , A few splicing operations can , A large number of operations are not recommended .

The same thing ： The underlying implementation is completely consistent , Inside the class, there is a char Array , No, final modification , After that, the increase and decrease of object characters are direct operations on the underlying array .

Reverse function ：

public StringBuffer reverse()

Interception function ：

public String substring(int start)

public String substring(int start,int end)

Add functionality ：

public StringBuffer append(String str)

public StringBuffer insert(int offset,String str)

Delete function ：

public StringBuffer deleteCharAt(int index)

public StringBuffer delete(int start,int end)

Replacement function ：

public StringBuffer replace(int start,int end,String str)

// StringBuffer(): Variable string with buffer , If you need a lot of character splicing , It is recommended to use ~;
        StringBuffer s=new StringBuffer("abcdefg");
        s.append("AA");// Add... To the end by default 
        s.append("BB");
        s.append("CC");
        s.append("DD");
        s.append("FF");
        s.insert(0,"P");// Insert... At specified location 
        s.delete(0,4);// Delete the specified interval 
        s.deleteCharAt(3);// Delete... In the specified location 
        s.replace(0,2,"zp");// Specify interval substitution 
        s.reverse();// Reverse string 
        String s2=s.substring(0,5);// Intercept an interval and reassign it to a new object 
        StringBuilder s1=new StringBuilder("drtg");// The underlying method is similar to StringBuffer equally 
        System.out.println(s);
        System.out.println(s2);

3.Math class ：

• abs The absolute value
• sqrt square root
• pow(double a, double b) a Of b The next power
• max(double a, double b)
• min(double a, double b)
• random() return 0.0 To 1.0 The random number
• long round(double a) double Data of type a Convert to long type （ rounding ）

System.out.println(Math.floor(9.1));
        System.out.println(Math.ceil(9.5));
        System.out.println(Math.round(4.4));
        System.out.println(Math.round(9.5));
        System.out.println(Math.random());
        Random random= new Random();
        System.out.println(random.nextBoolean());
        System.out.println(random.nextLong());
        System.out.println(random.nextInt());
        byte []bytes=new byte[5];
        random.nextBytes(bytes);// no return value 
        System.out.println(Arrays.toString(bytes));

3.Random class ：

• random.nextLong()  Return random number
• random.nextBoolean() Random return true and false
• random.nextint() from 0（ contain ） To the specified number （ It doesn't contain ）
• random.nextBytes()

 4.Date class ：

 Date date=new Date();// Create a date object , This object contains program meteorites   The time of that moment 
        System.out.println(date.getTime());// since 1970 1-1 0：0：0~ Milliseconds up to now 
        // Method with strikethrough , Deemed expired method ,api There is a new way to replace , have access to 
        System.out.println(date.getDate());
        System.out.println(date.getClass());
        System.out.println(date.getDay());
        System.out.println(date.getHours());
        System.out.println(date.getMinutes());
        System.out.println(date.getMonth());
        System.out. println(date.getSeconds());
        System.out.println(date.getYear());
        System.out.println(date.getTimezoneOffset());
        System.out.println(date);

5.Calendar

Generalization ：Calendar Class is an abstract class , The object that implements a specific subclass in actual use , establish The process of object is transparent to programmers , Just use getInstance Method creation that will do .

 // Contains richer calendar information 
        Calendar calender=new GregorianCalendar();
        Calendar calendar1=new GregorianCalendar();
        System.out.println(calendar1.getTime());
        System.out.println(calendar1.get(Calendar.YEAR));
        System.out.println(calendar1.get(Calendar.PM));

6.SimpleDateFormat class

SimpleDateFormat Date formatting class

● Construction method

SimpleDateFormat( Format ); // yyyy-MM-dd

● Date to string

Date now=new Date();

myFmt.format(now);

● String to date

myFmt.parse(“2018-02-10”);

String date format and The specified format must be consistent

for example :String s = “2018-03-15”;

new SimpleDateFormat(“yyyy-MM-dd”);

Date date=new Date();
        SimpleDateFormat a=new SimpleDateFormat("yyyy-MM-dd ");
        String c=a.format(date);//date Type to string type 
        System.out.println(c);
        String b="2020-1-1";
        SimpleDateFormat d=new SimpleDateFormat("yyyy-MM-dd");
        System.out.println(b);
        try{
            Date date1=d.parse(b);// String to date type 
            System.out.println(date1);
        }
        catch(ParseException e){
            e.printStackTrace();
        }

7.BigInteger class

  stay Java in , There are many digital processing classes , such as Integer class , however Integer Class has certain limitations .

● We all know Integer yes Int The wrapper class ,int The maximum value of is 2^31-1. If you wish to describe Larger integer data , Use Integer Data types cannot be implemented , therefore Java Provided in BigInteger class .

● BigInteger The number range of type is Integer,Long The number range of types is much larger , Other branches An integer of arbitrary precision , In other words, in the operation BigInteger Type can accurately represent any The integer value of the size without losing any information .

8.BigDecimal class

● In computers, whether it's float still double All floating point numbers , And computers are binary , Floating point numbers lose

Certain accuracy .

● The root cause is : Decimal values usually do not have exactly the same binary representation ; The binary representation of decimal numbers can be

Can it be inaccurate . Can only be infinitely close to that value .

double a = 1.0-0.9;

double b = 0.8-0.7;

System.out.println(a==b); 

 System.out.println(11-10.9);// The result is 0.0999999999964
        BigDecimal bigDecimal=new BigDecimal("11");
        BigDecimal bigDecimal1=new BigDecimal("10.9");
        System.out.println(bigDecimal.subtract(bigDecimal1));// The result is 0.1