java11-StringJoiner

Showcase of StringJoiner API.

preface

Please refer my other github project about joining Strings (using Collectors.joining or String.join): Collectors.joining.

• constructors

  public StringJoiner(CharSequence delimiter)
  
  public StringJoiner(CharSequence delimiter,
                          CharSequence prefix,
                          CharSequence suffix)
  

• methods
  • public StringJoiner setEmptyValue(CharSequence emptyValue) - Sets the sequence of characters to be used when determining the string representation of this StringJoiner and no elements have been added yet, that is, when it is empty. Note that once an add method has been called, the StringJoiner is no longer considered empty, even if the element(s) added correspond to the empty String.
  • public StringJoiner add(CharSequence newElement)
  • public StringJoiner merge(StringJoiner other) - Merge adds the contents of the given StringJoiner without prefix and suffix as the next element if it is non-empty. If the given StringJoiner is empty, the call has no effect.

project description

We provide tests for mentioned above methods:

• "a", "b", "c" -> "a,b,c"

  var stringJoiner = new StringJoiner(",");
      
  List.of("a", "b", "c").forEach(stringJoiner::add);    
  

• "a", "b", "c" -> "prefix-a,b,c-suffix"

  var stringJoiner = new StringJoiner(",", "prefix-", "-suffix");
  
  List.of("a", "b", "c").forEach(stringJoiner::add);    
  

• merge two different StringJoiners

  var stringJoinerComma = new StringJoiner(",");
  List.of("a", "b", "c").forEach(stringJoinerComma::add);
  
  var stringJoinerDot = new StringJoiner(".");
  List.of("d", "e", "f").forEach(stringJoinerDot::add);
  
  var merged = stringJoinerComma.merge(stringJoinerDot);
  
  assertThat(merged.toString(), is("a,b,c,d.e.f"));
  

• empty StringJoiner with / without prefix and suffix

  var stringJoiner = new StringJoiner(",");
      
  assertThat(stringJoiner.toString(), is(""));    
  

  var stringJoiner = new StringJoiner(",", "prefix-", "suffix");
  
  assertThat(stringJoiner.toString(), is("prefix-suffix"));
  

• configure empty StringJoiner - setEmptyValues

  var stringJoiner = new StringJoiner(",");
  stringJoiner.setEmptyValue("empty");
          
  assertThat(stringJoiner.toString(), is("empty"));
  

  var stringJoiner = new StringJoiner(",", "prefix-", "-suffix");
  stringJoiner.setEmptyValue("empty");
  
  assertThat(stringJoiner.toString(), is("empty"));
  

remark

Collectors.joining(...) and String.join(...) are using internally StringJoiner (or StringBuilder in the easiest case):

• Collectors

  public static Collector<CharSequence, ?, String> joining() {
      return new CollectorImpl<CharSequence, StringBuilder, String>(
              StringBuilder::new, StringBuilder::append,
              (r1, r2) -> { r1.append(r2); return r1; },
              StringBuilder::toString, CH_NOID);
  }
  

  public static Collector<CharSequence, ?, String> joining(CharSequence delimiter,
                                                           CharSequence prefix,
                                                           CharSequence suffix) {
      return new CollectorImpl<>(
              () -> new StringJoiner(delimiter, prefix, suffix),
              StringJoiner::add, StringJoiner::merge,
              StringJoiner::toString, CH_NOID);
  }
  

• String

  public static String join(CharSequence delimiter, CharSequence... elements) {
          Objects.requireNonNull(delimiter);
          Objects.requireNonNull(elements);
          // Number of elements not likely worth Arrays.stream overhead.
          StringJoiner joiner = new StringJoiner(delimiter);
          for (CharSequence cs: elements) {
              joiner.add(cs);
          }
          return joiner.toString();
      }