unexpected String.join() behavior

Question

I tried to print contents of a List<String> with a "," in between them. Followed the book Functional Programming in Java by Venkat Subramaniam, on page 54 , he did exactly what I am doing but my print result is coming as a single string without a comma(,).

String[][] deepArrayStr = new String[][]{{"Alex", "nullpointer"}, {"Eran", "George"}};
 List<String> str= Arrays.stream(deepArrayStr)
     .flatMap(Arrays::stream)
     .collect(Collectors.toList());
for(String s:str){
    System.out.print(String.join(",",s));
}

Output: AlexnullpointerEranGeorge

Expected: Alex,nullpointer,Eran,George

I tried another way and it works fine, I just wanted to know is there anything that I am doing wrong in the above approach whereas in the book Venkat also did the same thing that I am doing?

 System.out.println(Arrays.stream(deepArrayStr)
        .flatMap(Arrays::stream)
        .collect(Collectors.joining(",")));

Output: Alex,nullpointer,Eran,George

Expected: Alex,nullpointer,Eran,George

My question is not to convert from List to string but to add delimiter while printing.

please see Venkat's example , and correct me if i have done anything different. — Vishwa Ratna, Feb 26 '19 at 11:43
Possible duplicate of [Java: convert List to a String](https://stackoverflow.com/questions/1751844/java-convert-liststring-to-a-string) — Dimitris, Feb 26 '19 at 11:48
Note that you can also join the strings using a collector: `.collect(Collectors.joining(","))`. Of course, the resulting object will be `String` instead of `List`. — MC Emperor, Feb 26 '19 at 11:48

Jon Skeet · Accepted Answer · 2019-02-26T12:11:42.910

23

You're iterating over the list, and calling String.join with a single entry each time. That's never going to use the delimiter.

To make this clear, consider these two examples:

String x1 = String.join(",", "a");
String x2 = String.join(",", "a", "b");

Then the value of x1 will be "a", and the value of x2 will be "a,b". String.join only adds the delimiter between items - so if you only pass it a single item, it doesn't need to add any delimiters.

You can just remove your for loop, and replace it with:

System.out.println(String.join(",", str));

edited Feb 26 '19 at 12:11

answered Feb 26 '19 at 11:46

Jon Skeet

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sir, i wonder why in loop each time the delimiter is not getting added?? – Vishwa Ratna Feb 26 '19 at 12:01
1

@CommonMan: Because you're only passing in a single string. If you call `string.Join(",", "foo")` the result is always just "foo". – Jon Skeet Feb 26 '19 at 12:10
1

@CommonMan: I've edited my answer to clarify the behavior of String.join with a single element. – Jon Skeet Feb 26 '19 at 12:11

Eran · Answer 2 · 2019-02-26T11:49:46.500

11

System.out.print(String.join(",",s));

You are passing a single String to each String.join call, so of course it won't add any delimiter. It only adds the delimiter (comma in your case), if you pass multiple Strings.

If, for example, you'd collect your Stream into a String array instead of a List, you can pass that array to String.join:

String[] str = Arrays.stream(deepArrayStr)
                     .flatMap(Arrays::stream)
                     .toArray(String[]::new);
System.out.println (String.join (",", str));

This will output:

Alex,nullpointer,Eran,George

edited Feb 26 '19 at 11:49

answered Feb 26 '19 at 11:46

Eran

387,369
54
702
768

1

You can pass a list into `String.join` as well - there's an overload accepting an `Iterable` – Jon Skeet Feb 26 '19 at 11:56
@JonSkeet thanks for the comment. I missed that overload. – Eran Feb 26 '19 at 11:57

score 5 · Answer 3 · answered Feb 26 '19 at 12:32

Though the answers already highlight the difference and the fix possible, this is just to make it more functional in comparison:

Your first block of code can be transformed as:

Arrays.stream(deepArrayStr)
        .flatMap(Arrays::stream)
        .map(s -> String.join(",", s)) //map each string 'if possible' joined by delimeter ,
        .forEach(System.out::print); //print the above mapped string

while on the other hand the latter simply does this:

System.out.println(Arrays.stream(deepArrayStr)
    .flatMap(Arrays::stream)
    .collect(Collectors.joining(","))); //join strings and collect to print (after that)

Reference to String.join and its relevant implementation and Collectors.joining and its functionality.

unexpected String.join() behavior

3 Answers3