picture mean? What does
< U > mean? Can
be removed?
if you can get rid of < U >, what"s the difference from the original comparison?
U has not indicated that the return value is U? So what is the function of the front < U > of U? There is no < T > in front of T in the second picture, which indicates that the return value is T
. 
the first < U > is declared, indicating that this type parameter is unique to this method, and the latter U indicates that the method return type is U
it is recommended that the subject read more related basic books. The following screenshots are taken from the 10th edition of core java I
the first < U > cannot be deleted. If deleted, the compiler will assume that your method returns a class named U (U.class) and will not be found.
this < U > is used to tell the compiler that I'm going to return a generic type here, never mind that I
second update:
The declaration of < T > is in this class. Do you think the declaration of this class is:
public Something <T> {
///
}something like this.
you directly use U to identify the return value, and JVM will look for the type U.class , but obviously it doesn't exist, so use < U > to identify the actual type for the virtual machine to wait until the method is run and then look at the actual type corresponding to U . Here is the issue of generic erasure of Java . You can check out some articles on your own.
Update:
generics can be understood as "allow any type of parameter to be accepted here", but the general usage is for types that require multiple parameters, and generic parameter types have certain associations.
for example, after using generics in a picture, identity must be of the same type as one of the two input parameters of accumulator and its return value, and the parameter types accepted by combiner must also be the same as identity .
eg.
when identity is a String , then the first parameter and return value of accumulator must be of type String , otherwise an error will be reported.
this is the expressive power that cannot be achieved when Object is used to express "allow any type of parameter to be accepted here".
Baidu "generic Parameter Type" for more details.
Previous: Why is the content of < script id= "layoutHeaderTpl" type= "text/html" > not parsed under ie?
The horizontal coordinates of echarts are not specifically marked, and the X coordinates do not have a grid to indicate. Here is a figure that specifies . js * * createChartSix() { this.$http .get(this.$api.dataChart) .then(rsp => { ...
sincerely ask for advice: < hr > I want to use node as the background to build a video streaming server. The front end is similar to Youku VOD. It can record the playback node function, and load the progress bar at any point (starting from the c...
I need to implement in a chained promise function, any function error in the middle terminates the program, and can catch the error function, and then perform different operations according to different error functions. How can the following code be imp...
I would like to ask what is wrong with this code. Thank you for your answers. ...
is doing a question on Niuke.com. I encounter a problem: to achieve the function callIt, after the call meets the following conditions 1, the returned result is the result after calling fn 2, and the call parameter of fn is all the parameters after th...
A timing examination system is triggered if the click event is triggered within 5 seconds, and if it is not triggered, the system marks the correct answer at the end of the countdown. How to implement ...
description: a regular match is given to the content of an input box, and the matching content is the product activation code. looks like this: "0C31-0B81-BB32-3094-0C31-0B81-BB32-3094 " Code: $( -sharplicenseCode ).keyup(function () { le...
1. The custom event triggers the click event of .cpcstartrefresh, but triggers 2 click events each time 2. The code is as follows: window.onmessage = function (e) { create an event object, var myEvent = document.createEvent( Event ); m...
I configured the MIME type of the file with the amr suffix in the apache configuration as application ms-download, Why it is a garbled page when opening a file in amr format using window.open in chrome, while a file in amr format can be successfully dow...
I got a set of data, which is to choose the type of question. How can I tell if I have chosen the right one answera: "Olympic Games " answerb: "Asian Games " answerc: "Paralympic Games " answerd: "University Games " id: "1772 " question: "f...
read a number, such as 521 change this number to 0521 but put it into four div separately, how to realize it? 0 < div > 5 < div > 2 < div > 1 < div > ...
react projects cannot save store? using redux, data index.js ReactDOM.render( <Provider store={store}> <Router > < Provider>, document.getElementById( root )) registerServiceWorker() actions export const SET_USER = ...
FileInputStream fis = new FileInputStream (); int t = fis.read; how to see that the read method returns a number of type int? The source code is not implemented? ...
prepare to start an app background development. The basic framework is spring, spring mvc, mybatis (write), jdbctemplate (read). in order to deal with the problem of high concurrency in the future, what designs or technologies are best carried out in ...
how to find all the NA characters in a HMLT page and replace it with "and invalid " with native JS ...
...
suppose there are only a.js and b.js (only two js and nothing else) b.js export A variable is used for a.js can you directly use import and export without using packaging and compilation tools such as webpack or babel can it be achieved with the h...
such as the question, with asynchronous functions also deal with data, but how to feedback to the foreground? It would be nice to give some ideas ...
do the gods have plug-ins for uploading attachments in the mobile version that can also be uploaded more than one? ...
what does the code circled in the following picture mean? ...