1. What is a packaging class

A new class generated by encapsulating the basic data type corresponding to a package ----> packaging class

Int,byte.....--->Basic data type

Wrapper class ---> reference data type

1. Correspondence of basic data

basic data type	Corresponding wrapper class	inheritance relationship
byte	Byte	--->Number--->Object
short	Short	--->Number--->Objec
int	Integer	--->Number--->Objec
long	Long	--->Number--->Objec
float	Float	--->Number--->Objec
double	Double	--->Number--->Objec
char	Character	Object
boolean	Boolean	Object

2, the reason for the existence of packaging

On the basis of basic data types, the reasons why we create basic classes:

(1) java language: object-oriented language, best at operating various classes.

(2) I used to learn to load data ---" array, int[] String[] double[] Student[]

The ---" collection of loaded data that you will learn in the future has a feature that only data of reference data type can be loaded;

2. Integer class (int)

The wrapper class is the encapsulation of the basic data type: the int type encapsulation produces an Integer

The Integer class wraps a value of the primitive type int in an object. An object of type Integer contains a field of type int.

1. Class inheritance relationship:

2. Implemented interface:

3. This class is modified by final, then this class cannot have subclasses and cannot be inherited:

4. Code example (attribute)

Determine the maximum (minimum) value that an Integer can enter:

{
    public static void main(String[] args) {
        //Attributes:
        System.out.println(Integer.MAX_VALUE);
        System.out.println(Integer.MIN_VALUE);
        //The principle of "extremes must be reversed"
        System.out.println(Integer.MAX_VALUE+1);
        System.out.println(Integer.MIN_VALUE-1);
    }
}

Output result:

5. Constructor (found no null parameter constructor)

(1) Use int as a constructor parameter:

The underlying method outputs the int type data you input

(2) Use String type data as the constructor parameter:

First, you pass in a string, and the underlying method will determine whether the string you input can be converted to int type;

<1> can be transferred out: direct output

<2> Unable to transfer out: An exception occurred (thrown by the underlying layer)

6. Unique to packaging: automatic packing (automatic unpacking)

Convert primitive data types to wrapper classes

{
    public static void main(String[] args) {
        //Autoboxing: int ----->Integer
        Integer i = 12;
        System.out.println(i);
        
        //Automatic unboxing: Integer----->int
        Integer i2 = new Integer(12);
        int num = i2;
        System.out.println(num);
    }
}

7. Common methods:

(Main) The bottom layer of the Value of method:

compareTo:

Returns only three values ​​(1, 0, -1)

equals:

Integer rewrites the equals method in Object, and compares the value of the value encapsulated by the underlying layer.

==:

Because == compares the addresses of two objects

Autoboxing:

If the autoboxed value is between -128~127, then the comparison is the specific value, otherwise, the comparison is the address of the object intValue():

The effect will be Intger--->int

parseInt(String s) :

        String--->int:

toString() :

Will Integer--->String

Method code display:

public class Test04 {
    public static void main(String[] args) {
        //compareTo: only returns three values
        Integer i1 = new Integer(16);
        Integer i2 = new Integer(12);
        System.out.println(i1.compareTo(i2));
        //equals:Integer overrides the equals method in Object, and compares the value of the value encapsulated by the underlying layer.
        //Integer objects are objects created with the new keyword:
        Integer i3 = new Integer(12);
        Integer i4 = new Integer(12);
        System.out.println(i3 == i4);//false because == compares the addresses of two objects
      	//System.out.println(i3.equals(i4));
        boolean flag = i3.equals(i4);
        System.out.println(flag);

        //Integer objects are done with autoboxing:
        Integer i5 = 11;
        Integer i6 = 11;
        System.out.println(i5.equals(i6));
        System.out.println(i5 == i6);
        /*
        If the autoboxed value is between -128 and 127, then the specific value is compared
        Otherwise, the comparison is against the address of the object
         */
        
        //intValue(): The role will be Intger--->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() : Convert Integer--->String
        Integer i9 = 130;
        System.out.println(i9.toString());
    }
}