Java Community | Help. Code. Learn.•10mo ago

HELP WITH getClass() method

Consider this class:

public class MyClass {...}

public class MyClass {...}

what is the difference between using getClass() and using MyClass.class, I heard somewhere that getClass() can cause unexpected behaviour when a class extends MyClass but why would that happen ?

49 Replies

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Kyo-chan•10mo ago

You may call getClass() on any object, including one of which you don't know what's it class Example:

Object o = getMysteryObject(); // this time it gives a LocalDate... But on another call it gives a BigDecimal. You never know
Class<?> objClass = o.getClass();
System.out.println(objClass); // prints LocalDate fully qualified name

Object o = getMysteryObject(); // this time it gives a LocalDate... But on another call it gives a BigDecimal. You never know
Class<?> objClass = o.getClass();
System.out.println(objClass); // prints LocalDate fully qualified name

userexitOP•10mo ago

userexitOP•10mo ago

then what is this guy here saying ? @Kyo-chan

Kyo-chan•10mo ago

He's saying "I'm a beginner" If the class you want is MyClass, use MyClass.class If the class you want is that of an object you got, use getClass() on that object

userexitOP•10mo ago

I think he means if you have for example:

public class Shape {...}
public class Rectangle extends Shape {...}

public class Shape {...}
public class Rectangle extends Shape {...}

and somewhere in your code you have: Shape shape = new Rectangle(); and you do shape.getClass(); you get the Rectangle class and if its in another package then you cant find resources anymore in your package lamo do you think this is what he means ?

Kyo-chan•10mo ago

He doesn't mean much other than how if you use getClass() on an object whose variable type isn't a final class, then the possible results are more than just the one. ...... And yeah, that's the point. It's not an issue.

userexitOP•10mo ago

ah okay and btw getClass().getResource() will try to find it in the package of the class and getClass().getClassLoader().getResource() will try to find it outside right what I dont get is how can you have package based resources there is only 1 resource folder in java for example running this:

public class classInsideTestPackage {
    public void checkPath() {
        URL url = getClass().getClassLoader().getResource("resource.txt");
        System.out.println(url.getPath());
    }

    public static void main(String[] args) {
        new classInsideTestPackage().checkPath();
    }
}

public class classInsideTestPackage {
    public void checkPath() {
        URL url = getClass().getClassLoader().getResource("resource.txt");
        System.out.println(url.getPath());
    }

    public static void main(String[] args) {
        new classInsideTestPackage().checkPath();
    }
}

doesnt throw a null error

userexitOP•10mo ago

because it looked for it here

Kyo-chan•10mo ago

Actually, the resources folder is a Maven thing to better organize project. Natively, Java has no such notion

userexitOP•10mo ago

outside all packages if I delete getClassLoader() I get an error if I delete getClassLoader() and I put the resource inside the testPackage folder I get no error no more yeah so do you know how I can put resources in a specific package only ?

Kyo-chan•10mo ago

Isn't that exactly what you just demonstrated you know how to do?

userexitOP•10mo ago

yes but only after compilation because then I get the folder of the package in the target/classes folder and I can do wahtever I want with it but before compiling How can I decide a resource will go inside a package folder

Kyo-chan•10mo ago

Just put the resources in folders named exactly like that inside the resources folder (Though nobody but the Java base library team does that)

userexitOP•10mo ago

so inside src/main/resources I put a folder named testPackage and inside it it will put all the resources inside the testPackage inside target/classes ?

Kyo-chan•10mo ago

yup

userexitOP•10mo ago

why do you say that ?

Kyo-chan•10mo ago

I often mention various things that are true in this world. Don't mind it

userexitOP•10mo ago

no but I mean is there any logicla reason for which normal people wouldnt do ut

Kyo-chan•10mo ago

There doesn't exactly seem to be a point to bother with that. There is a little bit more of a point with Java 9+ modules isolation, but even then you can just name folders rather than actual packages.

userexitOP•10mo ago

isnt getting resources an enough point ?

Kyo-chan•10mo ago

You can get resources without putting them in packages, and with putting them in packages named differently to the ones you put your classes in. Otherwise Maven would have simplified putting resources in the same packages as classes

userexitOP•10mo ago

really how can you get such resources

Kyo-chan•10mo ago

Again, did you not demonstrate you know how?

userexitOP•10mo ago

I mean u can have a folder inside resource named: jeff and then do getClass().getClassLoader().getResource("/jeff"); but do u have naother way ? @Kyo-chan

Kyo-chan•10mo ago

I don't remember all the details, but if you include the starting / I don't think you need the classLoader

userexitOP•10mo ago

why, what does the starting / change lol

Kyo-chan•10mo ago

It changes where it looks for the resource

userexitOP•10mo ago

yeah so if you do /jeff where will it look

Kyo-chan•10mo ago

In a package named jeff

userexitOP•10mo ago

even wi thout classloader ill check damn ur right why does a / change so much ? does it mean look from behind ? @Kyo-chan

userexitOP•10mo ago

I guess when u do jeff/resourcetest.txt it tries to find folder jeff in the current folder which is the testPackage folder if u do /jeff/resourcetest.txt it tries to find it on the parent

userexitOP•10mo ago

so what if i have a package inside a package

userexitOP•10mo ago

wait why does it work

userexitOP•10mo ago

? does adding a / simply get me all out, inside the target/classes folder ? yes it does what if I have ar esource in parent package that I want to access can I access it without using getClassLoader ? also justl earned using getSystemResource deos the same nice damn

Kyo-chan•10mo ago

It means look from the root

userexitOP•10mo ago

what do u use to look from behind so i can look in resources of parent package

Kyo-chan•10mo ago

I don't do that. I'm not convinced there is something for it. I'd try .. just in case

userexitOP•10mo ago

yeah URL url = getClass().getResource("..New Text Document.txt"); this here doesnt work wait adding a slash works URL url = getClass().getResource("../New Text Document.txt"); @Kyo-chan

Kyo-chan•10mo ago

My bad, I assumed that as obvious

userexitOP•10mo ago

can I chain double dots ?

Kyo-chan•10mo ago

I don't know, but it would be very weird if you couldn't

userexitOP•10mo ago

so .... this here puts me 2 packages abvoe ?

Kyo-chan•10mo ago

../../name.text Not very used to navigate filesystem trees, are we?

userexitOP•10mo ago

yeah not used to none of that yeah it works i can double dot chain i guess triple, quadruple etc all work the same I was in a state of confusion but thanks to this conversation im in a state of clarity now, thank you for the help @Kyo-chan

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