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






False


Django, Why does the system automatically generate an empty session? every time the request is made?


Use python to convert dict1 to dict2, in the example. I tried for a long time but couldn't find a solution.


How to self-start the php-fpm service when building a LNMP environment with docker?


Can an array in react state be bound to a dom state using an element in the array?


Wechat browser version 6.6.5 cannot prevent the springback of pull-up and drop-down to the top under ios system.


In express+hbs, how do you define the routed data as a variable on the page?


How to use symbol reference of Iconfont in dva


How to elegantly add data to JSON objects?


Using node to build video streaming server


The proportional width of the echarts graph is inconsistent.


React uses axios to call the background data. After success, how to jump to the page based on the returned value?


I don't understand what a linux command means. Please explain it.


Vue introduces multiple components in vue-router by nesting named views, and does not refresh the same reference in other pages.


Running nodejs reports fewer errors ")"



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



Ask an IdentityServer4 question, close and reopen the browser will automatically log in?


How does WeChat Mini Programs return to Mini Program after calling the dial-up interface wx.makePhoneCall?


'@ src/hoc/withPage' how to let vim recognize @ src as the current working directory


Vue2.5.2 reported an error!


The bizarre questions of the interview




© 2021 CodesHelper


Advertising
Contact us
About us


Menu