#2628 How to use Java constucts?

isomorph Wed 16 Aug

I do want to use some Java stuff in the fantom code. The documentation says something about using Java-apis, so i want to change this code:

  • Int[]? list := [,] to something like this:
  • Int[]? list := new ArrayList<Int>() by adding "using [java] java.util::ArrayList". (<- seems to work - no errors, but java.util.ArrayList wouldn't throw errors too) The change doesn't work of course, but how can i create objects und use them from the Java-api?

(yes, i know that the list fantom is using is an arraylist, but i just want to know how i can use Java stuff in the fantom code with this example)

SlimerDude Wed 16 Aug

You're almost there - the answer is a lot simpler than you think!

using [java] java.util::ArrayList  // (this was correct!)


list := ArrayList()

echo( list[0]     )  // --> "Hello!"
echo( list.get(0) )  // --> "Hello!"
echo( list.typeof )  // --> [java]java.util::ArrayList

Note there are no user generics in Fantom, so in the case of ArrayList, the values are always of type Obj?.

isomorph Wed 16 Aug

Should have told that i want to declare this variable direct in a class not in a method - i realized now there is a difference. How you show it (and i already tried) does not work outside a method. It says then: "Type inference not supported for fields" (haven't recognized this earlier, course i prefer to declare the type...). The fantom code

  • Int[]? list := [,]

works perfectly fine there. So i think i have to declare the data type course the same error appears when i do this:

  • list := [,]

However this is the code i searched and found with your help:

  • Obj? list := ArrayList()

Thank you!

SlimerDude Wed 16 Aug

Try this instead:

ArrayList list := ArrayList()

If you declare a field to be of type Obj? then the compiler will just treat it as such, which (usually) isn't very useful!

isomorph Mon 21 Aug

Yeah of course - just haven't thought that it recognize this type.

Do you know why it's a bit slower when i use the Java-ArrayList manually like this than using the fantom code (the fantom code will use an Java-ArrayList too - so what's the difference)?

SlimerDude Tue 22 Aug

I am not aware that there is, or should be, a speed difference between using JavaFFI and not. I'll have to defer to someone more knowledgeable.

brian Wed 23 Aug

Do you know why it's a bit slower when i use the Java-ArrayList manually like this than using the fantom code

Fantom does not use ArrayList, it has its own low level implementation of sys::List based on a Java array. In general unless you are trying to interop with Java code, its better to prefer Fantom's built-in APIs

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