Fix lazy jfield, add test (#164)

Author: mkitti

README.md

@@ -39,12 +39,12 @@ OrderedCollections.OrderedSet{String} with 2 elements:
   "-Xmx1024M"
   "-Xrs"
 
-julia> JavaCall.init() # Optionally, explicitly initialize the JVM. Do not use this in package `__init__()` to allow other packages to add class paths or options.
+julia> JavaCall.init() # Call before using `jcall` or `jfield`. Do not use this in package `__init__()` to allow other packages to add classpaths or options.
 
 julia> jls = @jimport java.lang.System
 JavaObject{Symbol("java.lang.System")}
 
-julia> out = jfield(jls, "out", @jimport java.io.PrintStream)
+julia> out = jfield(jls, "out", @jimport java.io.PrintStream) # Third arg is optional, but helps type stability.
 JavaObject{Symbol("java.io.PrintStream")}(JavaCall.JavaLocalRef(Ptr{Nothing} @0x0000000003ecda38))
 
 julia> jcall(out, "println", Nothing, (JString,), "Hello World")

src/core.jl

@@ -424,7 +424,7 @@ function jfield(ref, field::AbstractString)
     assertroottask_or_goodenv() && assertloaded()
     field = listfields(ref, field)[]
     fieldType = jimport(gettype(field))
-    jfieldID = get_field_id(ref, field, fieldType)
+    jfieldID = get_field_id(ref, field)
     _jfield(_jcallable(ref), jfieldID, fieldType)
 end
 

test/runtests.jl

@@ -169,6 +169,9 @@ end
     t=JTest(())
     t_fields = Dict(getname(f) => f for f in listfields(t))
 
+    lazy_out = jfield(System, "out") # Not type stable
+    @test jcall(System_out, "equals", jboolean, (JObject,), lazy_out) == 0x01
+
     @testset "$ftype" for (name, ftype, valtest) in [ ("booleanField", jboolean, ==(true)) ,
                                 ("integerField", jint, ==(100)) ,
                                 ("stringField", JString, ==("A STRING")) ,