Java wrapper class (note 17)

Java wrapper class

The concept of packaging

The Java language is an object-oriented language, but the basic data types in Java are not object-oriented, which brings a lot of inconvenience in actual use. A corresponding class is represented, so the eight classes corresponding to the basic data types are collectively referred to as wrapper classes, and some places are also translated as outer classes or data type classes.

2. Basic data types correspond to wrapper classes

Basic data types (native classes) - wrapper classes

byte - Byte

short - Short

int   - Integer

long  - Long

float - Float

double- Double

char    - Character

boolean -Boolean

3. Some practical applications of basic data types in related packaging classes

//The example takes the maximum value of the long type

System.out.println(Long.MAX_VALUE);

  

//The example takes the uppercase form of a character

System.out.println(Character.toUpperCase(c));

  

//Example to judge whether a character is a number or not

System.out.println( Character.isDigit('9')); true

  

//Example convert number to string

int x=90; 

x+""; //This works

Integer.toString(x); //This is also possible

  

//Example convert String to numeric type

String a="4.14";

String b="90";

System.out.println(  Float.parseFloat(a) + Integer.parseInt(b) );  =>94.14

Boolean.parseBoolean("true")  => true

  

//example 

Integer x=new Integer("90");   

int a= x.intValue(); Convert a wrapper class object to a basic data type

  

//The example is related to the base

binary

int x=65;

System.out.println(Integer.toBinaryString(x)); // 0B1000001

Octal

System.out.println(Integer.toOctalString(x)); // 0101

hex

System.out.println(Integer.toHexString(x)); // 0x41

//Example: How to generate four digits like 0011,1111,0110 (job)    

int x=new Random().nextInt(16);

System.out.println(Integer.toBinaryString(x)); //Find a way to add 0 to the front

  

====== About Unboxing and Boxing ======

Boxing => turn primitive data type into object type 9 => Integer(9)

Unboxing => turn the object type into a basic data type Integer(9) => 9

  

Integer x=new Integer("90"); //Yes

Integer y=new Integer(90); //Yes

System.out.println(x==y);  //false;

System.out.println(x.equals(y));  //true

      

From jdk1.5 onwards 

Integer x= 5; //can be autoboxed 

x=x+3; //Yes, there is an automatic unboxing operation in the middle, after the operation, it will be automatically boxed

Integer x= null; //Yes

x=x+3; //Error when running, a null pointer exception

Integer a=100;

Integer b=100; 

System.out.println(a==b); //true

But the following writing

Integer a=128;

Integer b=128; 

System.out.println(a==b);  //false