Add the R generic "toString" to "jobjRef" methods #348

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1. s-u

   added

   enhancement

   on May 16, 2026
2. s-u commented on May 16, 2026

   

   s-u

   on May 16, 2026

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   Good idea, but I think it may be more R-like to simply use the as.character generic for this instead (which is currently unimplemented) - I'd rather not introduce a new generic unless it is required. That said, there is a slight difference in Java's toString() notion in that as.character is vectorized, so one may think of arrays behaving differently between as.character and toString.
3. rikivillalba commented on May 17, 2026

   

   rikivillalba

   on May 17, 2026

   Last edited by rikivillalba

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   Yes, as.character seems natural and automatically works with other R functions such as paste.
   On the other hand, R has a generic toString since 1.2 (probably a concept borrowed from Java). The default uses paste to concatenate vectors to a length one output, the manpages describes this single element semantics explicitly:

   This is a helper function for [format] to produce a single character string describing an R object.

   However the default does not use format at all, but as.character through paste coercing of its argument...

   toString(matrix(1:10,2,5))
   # [1] "1, 2, 3, 4, 5, 6, 7, 8, 9, 10"
   

   One may think in separate methods for jarrayRef and jobjRef to obey to the vector nature of as.character:

   setMethod("as.character", "jarrayRef", function(x, ...) {
     r <- as.list(x)    # <- method rJava:::as.list.jarrayRef
     if (!is.recursive(r)) 
       return(as.character(r, ...))
     vapply(r, toString, character(1), ..., USE.NAMES = FALSE)
   })
   
   setMethod("as.character", "jobjRef", function(x, ...) {
     toString(x, ...)
   })
   
   setMethod("toString", "jobjRef", function(x, ...) {
     r<- .jcall(x, "S", "toString")
     toString(r, ...)
   })
   

   However it should be noted that java object arrays (not from primitive types) are more akin of lists, and as.character over lists does not dispatch any generic (not sure)...

   So the results would be:

   # array of objects
   jarray_list <- .jarray(list(
     new(J("java.lang.Integer"), 10L),
     new(J("java.lang.String"), "hello"),
     .jarray(letters[1:3])
   ))
   
   # array of primitive
   jarray_primitive <- .jarray(1:5)
   
   as.character(jarray_list)
   # [1] "10"   "hello"    
   # [3] "[Ljava.lang.String;@2cdf8d8a"
   
   as.character(jarray_primitive)
   # [1] "1" "2" "3" "4" "5"
   

   and r's toString is equivalent to Object.toString.