What exactly are wildcards in JAVA generics?

for example, there is such a generic class

public class Gen<T>{
    
}

then there is such a method in other classes

public void showKeyValue(Gen gen){}

public void showKeyValue(Gen<?> gen){}

also, why Gen < Object > and Gen < String > are of the same category (Gen < Object > .Class = Gen < String > .class)
, but Gen < String > can"t point to a reference to Gen < Object > (Gen < Object > gen = new Gen < String > will make an error). Is there any difference between these two methods? if not, what"s the point of this?
A novice, hoping to have a great god to explain

Generic java

Mar.02,2021

official document contains this problem. Do a porter here, roughly translated as follows:

The

unbounded wildcard (unbounded wildcard) is used in the following two scenarios:

if you need to call a method in the Object class, such as toString () .
if the code does not depend on a specific type. A typical example is Class : Class < T > class most methods do not need to know the type of T .

for an example of the first case, there are the following ways to print element information in List :

  guidelines for using wildcards in official documents . 
< hr >
 I just saw that you have a second question.  Gen < Object > gen = new Gen < String >  will report an error. This problem is easy to understand:  gen.add (new Object ())  this operation is illegal, because the newly added  Object  instance may not be  String  instance.


										
												Previous: Is there a plug-in for phpstorm to make magento's method jump automatically?
												                		Next: Doubts about the nesting of setTimeout Multi-layer callback functions
                							
					
						
					
															
						
														
								Java returns objects based on generics
								
 I want to write a public method that passes in two objects of the same type an and b, and returns b  if an is null
public static ifObj(Object a, Object b) {
    if(null == a) {
        return b;
    }else {
        return a;
    }
}

Obj obj = ifObj(xxx...
								
									
																														Generic
																				java
																			
									
										Mar.20,2021
									
								
							
														
								How JAVA uses ParameterizedType in classes to get generic types
								
 use ParameterizedType in the class to get the generic class of the entity class of the class  has the following code: 
public class Demo<T> {
    
    private Class<T> clazz;
    
    public T getDemo() throws InstantiationException, Illegal...
								
									
																														Reflection
																				generic
																				java
																			
									
										Mar.25,2021
									
								
							
														
								Generics and lambda expressions
								
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.function.Consumer;


public class User {

    private <T> void test(Consumer<T> request) {
        Type[] interfaces = request.getClass().getGenericIn...
								
									
																														Function-expression
																				lambda
																				generic
																				java
																			
									
										May.28,2021
									
								
							
														
								Declaring whether a class is generic or not affects other generic methods of the class (unlike generic parameters on this class)
								
 the JDK8, code is as follows: 
public class Foo<E> {

    public <T extends List> T get(T list) {
        return null;
    }

    public void pass(Foo<?> foo) {
        ArrayList arrayList = foo.get(new ArrayList());
    }

    public ...
								
									
																														Generic
																				java
																			
									
										Sep.17,2021
									
								
							
														
								The problem of Java generics is the same as the code in the book, but the result is different?
								
 parent: Fruit  subclass: Apple 
 List < Fruit > fruitList = new ArrayList < Fruit > (); 
 fruitList.add (new Apple ()); is OK, but I don  t want Apple to fit in 
 so create a function 
 public static < T > void add (List < T > list,T item) {
    list.ad...
								
									
																														Generic
																				java
																			
									
										Oct.19,2021


				
					
						
	
						
		css
		mysql
		arrays
		josn
		react
		html
		typescript
		webpack
		npm
		sass
		R
		objective-c
		.net
		sql-server
		jquery
		python-3.x
		angularjs
		django
		angular
		excel
		regex
		iphone
		ajax
		linux
		xml
		pandas
		vba
		spring
		database
		wordpress
		string
		wpf
		xcode
		windows
		bash
		postgresql
		oracle
		multithreading
		eclipse
		list
		firebase
		algorithm
		macos
		forms
		image
		scala
		visual-studio
		azure
		bootstrap
		spring-boot
		react-native
		python-2.7
		docker
		performance
		function
		winforms
		matlab
		powershell
		apache
		dataframe
		api
		sqlite
		numpy
		rest
		shell
		selenium
		flutter
		dart
		maven
		loops
		qt
		swing
		android-studio
		csv
		express
		file
		class
		tensorflow
		sorting
		codeigniter
		perl
	
						MySQL Query :  SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5 
 MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b34b25-2b480.MAI); waiting for someone to free some space... (errno: 28 "No space left on device") 
 MySQL Errno : 1021 
 Message :  Disk full (/tmp/#sql-temptable-64f5-1b34b25-2b480.MAI); waiting for someone to free some space... (errno: 28 "No space left on device") 
Need Help?