JUnit's Assertions.assertThrows first argument?
Hi,
I am trying to understand the first argument
Assertions.assertThrows takes.
I know it has to be the expected exception that will be thrown, and in this case we expect it to be RuntimeException.
I am confused as to what the .class means? I know what a class is but never seen it used with a dot after some object.32 Replies
⌛ This post has been reserved for your question.
Hey @Steadhaven! Please useTIP: Narrow down your issue to simple and precise questions to maximize the chance that others will reply in here./closeor theClose Postbutton above when your problem is solved. Please remember to follow the help guidelines. This post will be automatically closed after 300 minutes of inactivity.
If you write
.class, you get a Class object containing information about the class
It is the type then? e.g.
RuntimeException.class returns the type RuntimeException instead of the object itself?If you have an object of a class, you can also get the
Class<> object of that class using yourObject.getClass()
it is an object with information about the typeah
hm
so its not quite the type, but info about the type
and assertThrows can use that
yes
Is there a way to see an easy example of what
.class might contain?
perhaps its possible to call it on String or Integeryou can use it to access metadata, get its methods/constructors, etc
you can do things like that
Here's what you can do with a
Class<>: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Class.htmlClass (Java SE 21 & JDK 21)
declaration: module: java.base, package: java.lang, class: Class
yes
result
wow wtf
yes
There are some methods coming from
Object
if you just want the declared methods, you can use cl.getDeclaredMethods()
you can also access fields and other stuffbesides using
.class for assertThrows (it expected the Class<T> expectedType as an argument), will I run into .class and Class<T> elsewhere?
So its a long winded way to get access to the fields of a class
or the methods
like saying
cl.getDeclaredMethods()
instead of directly using the methods on the class itselffrom time to time
Well it's made for code that doesn't know (at compile-time) what class it is used for
and the
Class<> object contains that information at runtime
Gives me:
So it seems that this method returns only the top level, whereas the former is recursively going deeper in the nesting and looks at Main as well
you mena it looks at
Object?
getMethods also includes inherited methodsYes
so declaredmethods is looking at the first level
but getmethods goes deeper
looking at inhereted stuff too
yes, it gives you all methods inherited from any level
ok
but there are more methods than that
this is hard to understand, but perhaps the topic to learn about is "reflections"? I can't find further info other than the Java doc about
Class Class<T>
no videos/articles about Class Class<T> itselfYes, it is reflection
at least what I can see
But it isn't that much of an important topic when you are a beginner
apparently its important in testing
You can just use
SomeClass.class
and that gives you an object representing the class/type
and you can pass that object to assertThrows or other thingsThanks
If you are finished with your post, please close it.
If you are not, please ignore this message.
Note that you will not be able to send further messages here after this post have been closed but you will be able to create new posts.
Post Closed
This post has been closed by <@305362010374406144>.
if you want to learn about reflection https://dev.java/learn/introduction_to_java_reflection/
Dev.java: The Destination for Java Developers
Introduction to Java Reflection - Dev.java
Java reflection allows an object to look in the mirror and discover what fields, methods, and constructors it has. We can read and write fields, invoke methods, and even create new objects by calling the constructors.