2021-08-07 native and package types

Writing Java In the process of code , Has been to Java Some of the class parameters provided are questionable , I checked it today , Learn to understand .

type argument cannot be of primitive type

problem ：

1. Java What are native types in ？
2. Java What is the encapsulation type in ？

Java Primitive types and primitive encapsulation types in

Java   There are two different types ： Reference types and primitive types （ Or built-in type ）. such as :Int yes java Original data type of ,Integer yes java by int Provided encapsulation class .Java A wrapper class is provided for each primitive type , Common primitive and corresponding encapsulation classes are as follows :

  The original type            Wrapper class    
  boolean             Boolean   
  char                   Character   
  byte                   Byte   
  short                 Short   
  int                     Integer   
  long                   Long   
  float                 Float   
  double               Double  

The difference between the reference type and the original type :

1. The initialization methods are different

int i = 5;    //  The original type 

Integer j = new Integer(10); //  Object reference  java 1.5 In the future, it supports automatic packing, so  Integer j = 10;  It's fine too 

Using primitive types does not require calling new, There is no need to create objects . This saves time and space . Mixing primitive types and objects can also lead to unexpected results related to assignment . 

2. Original type is class , Reference types are objects

The original type size is compared with "==" , The reference type size is compared with "equals"

3. Reference types can be serialized , The original type cannot .

4. The methods provided by reference types can be flexibly converted , You can extend the , The original type cannot

5. Only reference types can be used in collection classes , Cannot use primitive type

6. Original type has no null The concept of , There are , In some cases, it is necessary to identify whether a parameter has been initialized , If you use the original type , that 0 I don't know whether it is the initial value or not. The system automatically gives it .

7. Sometimes you have to use encapsulated classes , For example, if you want to use
request.setAttribute(String key ,Object value); This method , The second parameter is Object type , When you want to put an integer , Then you can only put Integer Can't put int.

summary :

  The behavior of primitive and encapsulated types is completely different , And they have different semantics . Reference types and primitive types have different characteristics and usages , They include ： Size and speed issues , What kind of data structure does this type store , The default value specified when the reference type and the original type are used as instance data for a class . The default value of the object reference instance variable is    null, The default values of primitive type instance variables are related to their types .
  int( The original type )   Generally, it is enough to be a numerical parameter    
  integer ( Package type )  It is generally used for type conversion