Joining Strings in Java 8

Java is addressing issue of boilerplate code which developers can hardly avoid. One such boilerplate code is concatenating list of strings with a delimiter. This has been mainly used by developers to write formatted output.

For example:

For a list of city names we may want to print as

[Bangalore, Delhi, Mumbai, Chennai, Kolkata]

Traditional Way

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String[] cities = { "Bangalore", "Delhi", "Mumbai", "Chennai", "Kolkata" };

StringBuilder output = new StringBuilder();
 output.append("[");

for (int i = 0; i < cities.length; i++) {
    output.append(cities[i]);
    if(i<cities.length-1)
    output.append(",");
 }
 output.append("\]");

System.out.println(output);

Output:

[Bangalore,Delhi,Mumbai,Chennai,Kolkata]

With StringJoiner API

With Java 8 we have java.util.StringJoiner.StringJoiner API. This does the same functionality as above but being less verbose.

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String[] cities = { "Bangalore", "Delhi", "Mumbai", "Chennai", "Kolkata" };

StringJoiner joiner = new StringJoiner(",", "[", "]");

for (String city : cities) {
 joiner.add(city);
 }

System.out.println(joiner);

Output:

[Bangalore,Delhi,Mumbai,Chennai,Kolkata]

StringJoiner API needs three main inputs:

delimiter
prefix of the list (optional)
suffix of the list (optional)

as indicated by its constructor:

java.util.StringJoiner.StringJoiner(CharSequence **delimiter**, CharSequence **prefix**, CharSequence **suffix**)

StringJoiner API can be used in various contexts where list of strings needs to be concatenated.

Following examples show few of them.

Joining an array of strings

Class String has join() API to perform joining operation on given CharSequence or Iterable.

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String[] cities = { "Bangalore", "Delhi", "Mumbai", "Chennai", "Kolkata" };

List<String> list = Arrays.asList(cities);

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

Output:

Bangalore,Delhi,Mumbai,Chennai,Kolkata

Joining strings coming from stream

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String[] cities = { "Bangalore", "Delhi", "Mumbai", "Chennai", "Kolkata" };

List<String> list = Arrays.asList(cities);

String output = list.stream().collect(Collectors.joining(",", "[", "]"));

System.out.println(output);

Output:

[Bangalore,Delhi,Mumbai,Chennai,Kolkata]

Setting empty list string message

An empty list message can be set to be shown when joiner is empty.

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String[] cities = {};

StringJoiner joiner = new StringJoiner(",", "[", "]");

joiner.setEmptyValue("EMPTY!");
 for (String city : cities) {
    joiner.add(city);
 }

System.out.println(joiner);

Output:

EMPTY!

Merge of joiners

Given string joiner is added to the next element of the current string joiner.

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StringJoiner joiner1 = new StringJoiner(",", "[", "]");
 joiner1.add("A").add("B").add("C");
 System.out.println(joiner1);

StringJoiner joiner2 = new StringJoiner("|", "{", "}");
 joiner2.add("1").add("2").add("3");
 System.out.println(joiner2);

System.out.println(joiner1.merge(joiner2));

Output: [A,B,C] {1|2|3} [A,B,C,1|2|3]

That’s it !