Commonly used classes

Packaging ： A wrapper class encapsulates a basic type into a class that contains properties and methods , Convenient object operation .（ The packaging class is located in Java.lang In bag ）

Packaging class and data class, idiomatic class 1. The basic data type is converted to a wrapper class 2. Convert wrapper class to base type 3. Automatic conversion of basic data and packaging class （ Wrapper classes are not intended to replace basic types ）

Automatic packing and unpacking （auto-baxing&unboxing）

Automatic boxing ： The basic data type is automatically encapsulated in a wrapper of the same type , Such as ：Integer i= 100;

It's essentially time for us to add the compiler ：Inter i = Integer.valueOf(100);

Automatic dismantling ： Wrapper class objects are automatically converted to basic data types , Such as ：int a = new Integer(100);

Essentially, , When compiling, the compiler adds ：int a = new Intrger(100).intValue();

{1. Boxing and UnBoxing , Packing ： basic --> class new Interger(int)---->new Integer.valueOf(int i)

Unpacking ： class ---> basic intValue()

2. Method     And string conversion method    

（a） character string --->Integer  Interger(String s)   Integer.parsenInt(Strng s )   Integer.valueOf(String s) 

  (b)Integer---> character string      toString()      String.valueOf(Object  obj)   Interger---->int +""}

String( Immutable character sequence )  Java String is Unicode Character sequence , for example Java There are four Unicode character string J,a,v,a form     Java Symbols are allowed "+" Connect two strings .

String Common methods of class

        char  charAt(int indx) Returns the second... In the string index Characters

        boolean equals（String other） If the string and other equal （ Ignore case ）, Then return to true

Practice on the computer

use String The use of the class , Definition and use of methods with parameters    – Realize member registration , The length of user name is required to be no less than 3, The password length is not less than 6, Twice during registration The password entered must be the same

public class Register {
    public static void main(String[] args) {
        Scanner cz  = new Scanner(System.in);
        System.out.println(" Welcome to the member registration system ：");
        boolean flag  = true;
        while(flag){
            System.out.println(" Please enter a user name ");
            String name = cz.next();
            System.out.println(" Please input a password ");
            String pwd = cz.next();
            System.out.println(" Please input the password again ");
            String pwd1 = cz.next();
            if(name.length()<3||pwd.length()<6){
                System.out.println(" The user name length is less than 3 Or the password length is less than 6");
            }else if(!pwd.equals(pwd1)){
                System.out.println(" The two passwords are different ");
            }else if(name.length()>=3&&pwd.length()>=6&&pwd.equals(pwd1)){
                System.out.println(" Registered successfully ！ Please remember the user name and password ");
                break;
            }


        }
    }
}