How to work with an expected return type of "java.lang.Class"?

Question

I thought I'd try to write an R interface to Scribe (mature OAuth library for Java by Pablo Fernandez) as a way of refreshing myself on Java (not used it in 8 years), learning rJava and to make better use of the Twitter API. But mostly because it's Friday afternoon and I thought it'd be fun. :)

Unfortunately I haven't got very far...

I downloaded the .jar file for scribe and also commons-condec (its only dependency, which I subsequently unzipped). I've ran the code in Java using netbeans and it works fine using his twitter example.

I was OK for the first few lines of code by just following the rJava documenation:

# load R packages
library(rJava)

# Initialise
.jinit()

# Add class paths
d1 <- "C:/Users/Tony/Documents/R/java/scribe-1.1.0.jar"
d2 <- "C:/Users/Tony/Documents/R/java/commons-codec-1.4/"
.jaddClassPath(path=c(d1, d2))

But then scribe quick start guide says the following is needed:

// Java Code
OAuthService service = new ServiceBuilder()
                                .provider(TwitterApi.class)
                                .apiKey("6icbcAXyZx67r8uTAUM5Qw")
                                .apiSecret("SCCAdUUc6LXxiazxH3N0QfpNUvlUy84mZ2XZKiv39s")
                                .build();

I can't figure out how to rewrite that into rJava parlance. A little web searching suggests I should do it in parts, so first I did:

# Create object (back to R code again)
( service <- .jnew("org.scribe.builder.ServiceBuilder") )
[1] "Java-Object{org.scribe.builder.ServiceBuilder@58fe64b9}"

# Set up apiKey and apiSecret using "$" shortcut
service$apiKey("6icbcAXyZx67r8uTAUM5Qw")
service$apiSecret("SCCAdUUc6LXxiazxH3N0QfpNUvlUy84mZ2XZKiv39s")

Good so far. Then I need to figure out what return type is expected from the provider function:

# Inspect return type
.jmethods(service, "provider")
[1] "public org.scribe.builder.ServiceBuilder org.scribe.builder.ServiceBuilder.provider(java.lang.Class)"

It needs "java.lang.Class". This is where I get confused. What does that mean? I guess, looking at the source, it needs a return type of type "ServiceBuilder", but how to do that? This was my best guess after looking at ?.jcall (note: 'use.true.class = TRUE' didn't do anything):

> .jcall(obj = service, returnSig = "Lorg.scribe.builder.ServiceBuilder;", method = "org.scribe.builder.ServiceBuilder.provider", "org.scribe.builder.api.TwitterApi")

Error in .jcall(obj = service, returnSig = "Lorg.scribe.builder.ServiceBuilder;",  : 
  method org.scribe.builder.ServiceBuilder.provider with signature (Ljava/lang/String;)Lorg.scribe.builder.ServiceBuilder; not found

Any ideas?

Answer 1

It looks to me like the provider method returns ServiceBuilder and takes a Class as a parameter.

In Java if you put the classname followed by .class that makes a class literal object in the code. If you instead load the class using reflection you can refer to a class by its string name. I'm not sure how this works in R but in Java the syntax is:

Class c = Class.forName("org.scribe.builder.api.TwitterApi");

This puts the class instance into the variable c. Then you could call the provider method:

service$provider(c);