What is a "[Ljava.lang.String", and how to print its content human readably?

Question
1

I think this problem stems from my lack of understanding of Java.
I’d like to print the chars of a string which I’m guessing is some sort of funny java string.

I was given an object of type InputStream an needed to read the lines and parse them by a particular rule.
I used

val lines = Source.createBufferedSource(s).getLines()

to apparently get an Iterator for the lines.
Next, I used the following to get what IntelliJ thinks is an object of type Iterator[Array[String]]

val plists = lines.map(line => line.split(",").map(_.trim)).filter(list => ! list.contains(""))

When I try to print this with println, it prints as something I don’t understand.

println("slists="+slists.toList)

The output is the following.

 slists=List([Ljava.lang.String;@49c7b90e)

Can someone tell me what this means? I notice the opening [ is never closed. Is the type name really “[Ljava.lang.string”? Is there a way to print the actual strings of characters?

2

Could be that that question is really a red herring, although I’d still love to know the answer.

What would also be helpful is to know how to map/flatMap a function across all the lines (as strings) given an object of type InputStream.

3

It’s just what the toString method of a JVM array returns.
The default implementation of toString that gets inherited from java.lang.Object is more or less equal to this:

def toString(): String = this.getClass.getName + "@" + this.hashCode.toHexString

The class name of a String array is [Ljava.lang.String;. [ means it’s an array, L means it can contain references to objects, and java.lang.String means all those objects should be instances of java.lang.String. The ; is just because Java loves its semicolons, I suppose.

If you want to pretty print an array you can use java.util.Arrays.toString.

val list = List(Array(1,2),Array(3,4))
println( list.map(java.util.Arrays.toString) )  // List([1, 2], [3, 4])

When you play around with arrays in the Scala REPL, most of the time you won’t see things like [Ljava.lang.String;@49c7b90e. That’s because the REPL contains special cases for when the results of expressions contain arrays.

4

Note that the Scala repl has some magic to identify arrays when printing results, but it doesn’t work with arrays nested in other data structures:

scala> Array(1, 2)
res0: Array[Int] = Array(1, 2)

scala> Some(Array(1, 2))
res1: Some[Array[Int]] = Some([I@6056232d)
5

For some reason it does work for arrays nested in standard library collections.

6

Are you sure java.util.Arrays.toString is correct? When I try the following:

println("words...="+words.map(java.util.Arrays.toString))

I get an error "cannot resolve symbol toString"

the type of words is Array[String]

I get the same error if I try to convert it to a list first.

println("words...="+words.toList.map(java.util.Arrays.toString))

7

You’re right, it doesn’t work for Array[String]. There is only a method toString for Array[Object] because in Java arrays are covariant…

Then better to ignore java.util.Arrays.toString and do something like

words.map(_.mkString("[", ", ", "]"))
1 Like
8

Another option, if it’s just for debugging (easier to type but runs
slower), is to to .toList.toString on it.
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