Integer case study of packaging

introduce

First of all, I wrote the title of learning Integer As a case study , Because I know a packaging class , Others can learn in simple terms ~

1. What is packaging ：

Previously defined variables , Basic data types are often used ,
For basic data types , It's just a number , Add point attribute , Adding point method , Add a point constructor ,
The basic data types are encapsulated correspondingly , A new class is generated ,—》 Packaging .
int,byte…—> Basic data type
Packaging —> Reference data type

2. Corresponding relation ：

 Basic data type            The corresponding packaging class             Inheritance relationships 
byte                 Byte                  ---》Number---》Object
short                Short                 ---》Number---》Object
int                  Integer               ---》Number---》Object
long                 Long                  ---》Number---》Object
float                Float                 ---》Number---》Object
double               Double                ---》Number---》Object
char                 Character             Object
boolean              Boolean               Object

3. There are already basic data types , Why package as a wrapper class ？

（1）java Language Object oriented language , I am best at operating various classes .
（2） I used to learn to load data —》 Array ,int[] String[] double[] Student[]
After learning, you can install data —》 aggregate , There is a feature , Only data of reference data type can be loaded

Integer Packaging

1. Direct use without guide package

2. Class inheritance ：

3. Implementation interface ：

4. This class is final modification , Then this class cannot have subclasses , uninheritable ：

5. Wrapper class is the encapsulation of basic data types ： Yes int Type encapsulation produces Integer

6. attribute

// attribute ：
        System.out.println(Integer.MAX_VALUE);
        System.out.println(Integer.MIN_VALUE);
        //“ Too much ” principle ：
        System.out.println(Integer.MAX_VALUE+1);
        System.out.println(Integer.MIN_VALUE-1);
 Running results ：
2147483647
-2147483648
-2147483648
2147483647

7. Constructors （ Found no empty parameter constructor ）

（1）int Type as a constructor parameter ：

Integer i1 = new Integer(12);

（2）String Type as a constructor parameter ：

Integer i2 = new Integer("12");
Integer i3 = new Integer("abcdef");

8. Packaging specific mechanism ： Automatic boxing Automatic dismantling

// Automatic boxing ：int--->Integer
    Integer i = 12;
    System.out.println(i);
    // Automatic dismantling ：Integer--->int
    Integer i2 = new Integer(12);
    int num = i2;
    System.out.println(num)

（1） Automatic boxing Automatic dismantling It's from JDK1.5 New features in the future
（2） Automatic boxing Automatic dismantling ： Fast type conversion between basic data types and wrapper classes .

verification ：

You can customize the break point to test whether you enter valueOf In the method ：

9. Common methods ：

valueOf The bottom layer of the method ：

Code ：

public class Test04 {
    
    // This is a main Method , It's the entrance to the program ：
    public static void main(String[] args) {
    
        //compareTo： Only three values are returned ： Or 0,-1,1
        Integer i1 = new Integer(6);
        Integer i2 = new Integer(12);
        System.out.println(i1.compareTo(i2));// return (x < y) ? -1 : ((x == y) ? 0 : 1);
        //equals:Integer Yes Object Medium equals Method is overridden , The comparison is the one encapsulated at the bottom value Value .
        //Integer The object is through new Keyword created objects ：
        Integer i3 = new Integer(12);
        Integer i4 = new Integer(12);
        System.out.println(i3 == i4);//false  because == Compare the addresses of the two objects 
        boolean flag = i3.equals(i4);
        System.out.println(flag);
        //Integer Objects are automatically boxed ：
        Integer i5 = 130;
        Integer i6 = 130;
        System.out.println(i5.equals(i6));//true
        System.out.println(i5 == i6);
        /*  If the auto packing value is -128~127 Between , Then the comparison is the specific value   No in , The comparison is the address of the object  */
        //intValue() : The effect will be Integer--->int
        Integer i7 = 130;
        int i = i7.intValue();
        System.out.println(i);
        //parseInt(String s) :String--->int:
        int i8 = Integer.parseInt("12");
        System.out.println(i8);
        //toString:Integer--->String
        Integer i10 = 130;
        System.out.println(i10.toString());
    }
}
 Running results ：
-1
false
true
true
false
130
12
130