rJava: pass 2 dimensional string array as function input

Question

In my java class I write 3 test functions as follows:

public void try2dStringArray(String[][] arr) {
    System.out.println(arr.length);

}
public void try1dStringArray(String[] arr) {
    System.out.println(arr.length);

}
public void try2dDoubleArray(double[][] arr) {
    System.out.println(arr.length);

}

In R part I run the following:

library(rJava)
.jinit('/path/to/my/jar/app.jar')
obj <- .jnew('somepackage.Someclass')
doubleMatrix <- rbind(c(1,2), c(3,4))
stringMatrix <- rbind(c('a', 'b'), c('c', 'd'))
stringArray <- c('a', 'b')

result <- .jcall(obj,"V","try1dStringArray",
                 .jarray(stringArray, dispatch = T))
result <- .jcall(obj,"V","try2dDoubleArray",
                 .jarray(doubleMatrix, dispatch = T))
result <- .jcall(obj,"V","try2dStringArray",
                 .jarray(stringMatrix, dispatch = T))

Only the last one errors out:

Error in .jcall(obj, "V", "try2dStringArray", .jarray(stringMatrix, dispatch = T)) : 
  method try2dStringArray with signature ([[Ljava.lang.String;)V not found

How is String[][] different from double[][] in this case and how can I fix it?

Does .jarray(lapply(apply(stringMatrix,1,function(x) lapply(x,.jnew,class="java/lang/String")),.jarray)) create a String[][] java object suitable for your method? — nicola
– nicola, Commented Mar 21, 2016 at 21:24
@nicola It doesn't seem to work: Error in .jcall(obj, "V", "try2dStringArray", .jarray(lapply(apply(stringMatrix, : method try2dStringArray with signature ([Ljava/lang/Object;)V not found — Bamqf
– Bamqf, Commented Mar 21, 2016 at 21:48
It seems that you got Object[][] instead of String[][]. I looked at the doc of .jarray and maybe using the argument contents.class will solve: .jarray(apply(stringMatrix,1,.jarray),contents.class="java/lang/String"). I also simplified a little the command. Hope this works. — nicola
– nicola, Commented Mar 21, 2016 at 22:25
@nicola It doesn't work neither: Error in .jcall(obj, "V", "try2dStringArray", .jarray(apply(stringMatrix, : method try2dStringArray with signature ([Ljava/lang/String;)V not found — Bamqf
– Bamqf, Commented Mar 21, 2016 at 22:38
The attempts I was making were wrong. But here is some mistery: if you try obj$try2dStringArray(.jarray(stringMatrix, dispatch = T)) it should work (at least, it works for me, printing 2). Maybe it's a bug in .jcall. — nicola
– nicola, Commented Mar 21, 2016 at 22:55

nicola · Accepted Answer · 2016-03-22 07:05:32Z

2

At first, I thought that .jarray wasn't able to properly create a String[][] java array from a character R matrix. I was wrong and the error received when the code in the OP is run testifies it:

.jcall(obj,"V","try2dStringArray",.jarray(stringMatrix, dispatch = T))
#Error in .jcall(obj, "V", "try2dStringArray", .jarray(stringMatrix, dispatch = T)) : 
#method try2dStringArray with signature ([[Ljava.lang.String;)V not found

As can be seen, the signature (([[Ljava.lang.String;)V) seems correct (the double [[ should say that we actually passed a String[][] object), but for some reason .jcall can't find it.

However, in rJava methods can be called also with the syntax obj$method(arg1,arg2,...), and in this way the method try2dStringArray is correctly called:

obj$try2dStringArray(.jarray(stringMatrix, dispatch = T))
#2

answered Mar 22, 2016 at 7:05

nicola

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1 Comment

Funkwecker Over a year ago

So, this is a bug of rJava, isn't it? with .jcall it does work, with obj$try.. it does...

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