how to detect if a class implements an interface

[memcallen]

I've made a battery class which implements a custom interface "IPowerProvider" and a cable which also implements the same interface, when I want to add more cables how could I detect if the class implements my interface?

 

sidenote-I've tried to use instanceof with an instance rather than the class but it didn't work

[strumshot]

I dont know if this is the best way by you could try to cast it as an instance of your interface and wrap it in a try/catch.

[memcallen]

what do you mean by wrap it?

[strumshot]

Try {
IPowerProvider p = (IPowerProvider) var;
// success, is IPowerProvider
}
Catch {
// not IPowerProvider
}

 

Im on a phone, lol so sorry for any goofed syntax!

[memcallen]

do you know what the catch would be?

[strumshot]

I would put a generic Exception, or you could run it once with something you know will fail and it will tell you theeexception type. I dont know right offhand.

[strumshot]

something to the effect of invalidCast

 

edit: I probably should have waited till I got in front of my computer, sorry for being short with those responses. Anyway I know this method would work, but back in the day I was taught to avoid catch statements whenever possible, as they were taxing on performance. I'm not sure if that's still the case, or if Java has a specific implementation for the kind of check you are looking for. Maybe someone who drinks more Java can chime in on the %100 official of if this is the best practice, but again, I know it would work. Also, I'm still used to C# at the end of the day where you don't necessarily have to provide the explicit exception type. I was trying to suggest doing it without the try/catch and trying to cast something you know is not IPowerProvider and letting it crash and tell you the exception type; but again, it should be "InvalidCastException", so the code might look like this:

 

try {
   IPowerProvider p = (IPowerProvider) var;
    // success, is IPowerProvider
}
catch(InvalidCastException){
    // not IPowerProvider
}

[jabelar]

I've made a battery class which implements a custom interface "IPowerProvider" and a cable which also implements the same interface, when I want to add more cables how could I detect if the class implements my interface?

 

sidenote-I've tried to use instanceof with an instance rather than the class but it didn't work

 

instanceof is the proper way to do it.  What do you mean it didn't work?

[coolAlias]

Assuming you have an instance of your class (or any instance of any class, really), such as 'Battery battery = new Battery()', then to check if it implements an interface and then use those interface methods, you use the following, which is what everyone else has been trying to say:

// checking if the object implements the interface
if (battery instanceof IPowerProvider) {
// now casting the object to the interface class so you can use the interface methods
IPowerProvider power = (IPowerProvider) battery;
power.someInterfaceMethod();

// which you can also write as one line (after checking instanceof) if you only need to call one method:
((IPowerProvider) battery).someInterfaceMethod();
}

[strumshot]

Assuming you have an instance of your class (or any instance of any class, really), such as 'Battery battery = new Battery()', then to check if it implements an interface and then use those interface methods, you use the following, which is what everyone else has been trying to say:

// checking if the object implements the interface
if (battery instanceof IPowerProvider) {
// now casting the object to the interface class so you can use the interface methods
IPowerProvider power = (IPowerProvider) battery;
power.someInterfaceMethod();

// which you can also write as one line (after checking instanceof) if you only need to call one method:
((IPowerProvider) battery).someInterfaceMethod();
}

 

Definitely this! I'd like to note that I was attempting to offer an alternate solution, assuming instanceof "doesn't work" - but as they pointed out, it should. 

[memcallen]

what I was doing was

 

TileEntity te = world.getTileEntity(x,y-1,z);
if(te!=null)
if(te instanceof IPowerContainer)
System.out.println("yes");

and it wasn't printing anything even though I had a block that properly implemented it and had a tileEntity below my battery

[memcallen]

oops forgot to do "

"

[memcallen]

bbcode why you do this?

[jabelar]

Well even though you might be confident that the tile entity below the position is there, in computer debugging usually you'll find that something you're sure of isn't actually true.

 

So if I were you I'd add more console output to confirm.  I would output the x, y, z coordinates and I would output the name of the block found at the position.  I would also put in the else clauses for each if statement and put a console printout there too.  for example, you test whether the te is null but it is not clear if that is true or not because you don't get any output. 

 

so if you have console statement showing all the information (position, whether te is null), then you'll be able to debug.

[Jwosty]

Have you set up your tile entity properly? Try putting a print statement in all of its constructors.

[memcallen]

yeah I have setup my TileEntity correctly it can already store power and save to nbt and all the normal tileEntity stuff also I'm pretty sure that the tileEntity isn't null because I replace the blocks every time I test it

[jabelar]

yeah I have setup my TileEntity correctly it can already store power and save to nbt and all the normal tileEntity stuff also I'm pretty sure that the tileEntity isn't null because I replace the blocks every time I test it

 

In programming, if something isn't working the way you expect then one of your assumptions is wrong.  You have to check everything explicitly.  You think you have a tile entity there, but obviously it isn't being detected.  So you need to prove that the position you're checking is correct, that the tile entity is actually not null, and so forth.

 

This is debugging.  You don't fix it by asking "what's wrong with my code", you fix it by *tracing* your code exactly.  At some point your code is going wrong, so you need to go step by step and prove where it goes wrong.  You have to prove that the te is not null at that position before you complain about the instanceof not working.  Just saying "I'm sure" isn't enough -- print it out right at the point in the code where you are checking it.

[Master801]

*Face-palm*

 

Interface.class.isAssignableFrom(this.getClass());

 

Or

 

Interface.class.isAssignableFrom(SomeClass.class);

[TheGreyGhost]

Hi

 

Instance of should work fine for your code.  Your te is almost certainly null.

 

http://stackoverflow.com/questions/496928/what-is-the-difference-between-instanceof-and-class-isassignablefrom

 

Do you know how to use the debugger?  If not, I reckon you should spend a bit of time learning how, because it'll make your life a whole lot easier, you'll wonder how you ever managed without it.

 

For example

http://www.vogella.com/tutorials/EclipseDebugging/article.html

 

In this case, I'd suggest you put a breakpoint on this line

TileEntity te = world.getTileEntity(x,y-1,z);

 

and it will show you in 30 seconds flat what type of TileEntity you have found (if any).

 

-TGG

 

 

 

 

 

[memcallen]

I know how to use the debugger mode, and I've done System.out.println(te) before and it wasn't null and instanceof wasn't returning true still...

[memcallen]

I put this in my onBlockActivated method:

if(world.getTileEntity(x, y+1, z)!=null)System.out.println("not Null");
if(world.getTileEntity(x, y+1, z) instanceof IPowerContainer)System.out.println("Is Power Container");

 

the block that I placed on top implemented IPowerContainer and had a TileEntity this is what I got in the console:

not Null

 

[Jwosty]

What happens when you print the TE's class? e.g. ...println(world.getTileEntity(...).getClass())

[jabelar]

We're just saying that it should work.  And when something that should work doesn't work, then it is time to trace things really carefully.  The code you've posted it is not at all obvious that you've confirmed that the te is really what you think it is.  Maybe you did try it, but you haven't shown any proof.

 

Based on the code you listed, there is is only a few possible causes:

1) the class isn't actually an instance of that interface.  Can you post the code for the tile entity class?

2) the tile entity is null.  Again the code you've shown so far would not prevent that case, so it would be good to show proof.

3) there is a tile entity there (not null) but not the one you expect.

 

Maybe you have this last case.

 

As strumshot mentions, another way to confirm whether it is actually an instance is to try to cast it into the interface.  If that fails then it would tell you something.

 

Anyway, debugging isn't about trusting that you know something is correct.  Obviously something you think is correct is actually wrong.  This happens all the time in programming -- every bug is something you think that should work but doesn't.  The only thing to do is to exactly trace everything all at the same time.

 

You can check any source on Java programming and instanceof should work for testing classes that implement an interface, as per the stackoverflow link above also attests.  You can also try the isApplicable() method alternative.

[TheGreyGhost]

I know how to use the debugger mode, and I've done System.out.println(te) before and it wasn't null and instanceof wasn't returning true still...

OK.  So what is the reason you haven't added a breakpoint to this line

TileEntity te = world.getTileEntity(x,y-1,z);

and discovered what kind of TileEntity you actually get in te?

 

-TGG

[larsgerrits]

Did you register your TileEntitiy? Of the Battery and the Cable?