Module java.base
Package java.util

Class StringJoiner

java.lang.Object
java.util.StringJoiner
public final class StringJoiner extends Object
StringJoiner is used to construct a sequence of characters separated by a delimiter and optionally starting with a supplied prefix and ending with a supplied suffix.

Prior to adding something to the StringJoiner, its sj.toString() method will, by default, return prefix + suffix. However, if the setEmptyValue method is called, the emptyValue supplied will be returned instead. This can be used, for example, when creating a string using set notation to indicate an empty set, i.e. "{}", where the prefix is "{", the suffix is "}" and nothing has been added to the StringJoiner.

API Note:

The String "[George:Sally:Fred]" may be constructed as follows: 

 
 StringJoiner sj = new StringJoiner(":", "[", "]");
 sj.add("George").add("Sally").add("Fred");
 String desiredString = sj.toString();

A StringJoiner may be employed to create formatted output from a Stream using Collectors.joining(CharSequence). For example: 

 
 List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
 String commaSeparatedNumbers = numbers.stream()
     .map(i -> i.toString())
     .collect(Collectors.joining(", "));
Since:
1.8
See Also:

  • Constructor Summary

    Constructors
    Constructor
    Description
    StringJoiner(CharSequence delimiter)
    Constructs a StringJoiner with no characters in it, with no prefix or suffix, and a copy of the supplied delimiter.
    StringJoiner(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
    Constructs a StringJoiner with no characters in it using copies of the supplied prefix, delimiter and suffix.

  • Method Summary

    Modifier and Type
    Method
    Description
    StringJoiner
    add(CharSequence newElement)
    Adds a copy of the given CharSequence value as the next element of the StringJoiner value.
    int
    length()
    Returns the length of the String representation of this StringJoiner.
    StringJoiner
    merge(StringJoiner other)
    Adds the contents of the given StringJoiner without prefix and suffix as the next element if it is non-empty.
    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.
    String
    toString()
    Returns the current value, consisting of the prefix, the values added so far separated by the delimiter, and the suffix, unless no elements have been added in which case, the prefix + suffix or the emptyValue characters are returned.

    Methods declared in class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

  • Constructor Details

    • StringJoiner

      public StringJoiner(CharSequence delimiter)
      Constructs a StringJoiner with no characters in it, with no prefix or suffix, and a copy of the supplied delimiter. If no characters are added to the StringJoiner and methods accessing the value of it are invoked, it will not return a prefix or suffix (or properties thereof) in the result, unless setEmptyValue has first been called.
      Parameters:
      delimiter - the sequence of characters to be used between each element added to the StringJoiner value
      Throws:
      NullPointerException - if delimiter is null

    • StringJoiner

      public StringJoiner(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
      Constructs a StringJoiner with no characters in it using copies of the supplied prefix, delimiter and suffix. If no characters are added to the StringJoiner and methods accessing the string value of it are invoked, it will return the prefix + suffix (or properties thereof) in the result, unless setEmptyValue has first been called.
      Parameters:
      delimiter - the sequence of characters to be used between each element added to the StringJoiner
      prefix - the sequence of characters to be used at the beginning
      suffix - the sequence of characters to be used at the end
      Throws:
      NullPointerException - if prefix, delimiter, or suffix is null

  • Method Details

    • setEmptyValue

      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. A copy of the emptyValue parameter is made for this purpose. 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.
      Parameters:
      emptyValue - the characters to return as the value of an empty StringJoiner
      Returns:
      this StringJoiner itself so the calls may be chained
      Throws:
      NullPointerException - when the emptyValue parameter is null

    • toString

      public String toString()
      Returns the current value, consisting of the prefix, the values added so far separated by the delimiter, and the suffix, unless no elements have been added in which case, the prefix + suffix or the emptyValue characters are returned.
      Overrides:
      toString in class Object
      Returns:
      the string representation of this StringJoiner

    • add

      public StringJoiner add(CharSequence newElement)
      Adds a copy of the given CharSequence value as the next element of the StringJoiner value. If newElement is null, then "null" is added.
      Parameters:
      newElement - The element to add
      Returns:
      a reference to this StringJoiner

    • merge

      public StringJoiner merge(StringJoiner other)
      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.

      A StringJoiner is empty if add() has never been called, and if merge() has never been called with a non-empty StringJoiner argument.

      If the other StringJoiner is using a different delimiter, then elements from the other StringJoiner are concatenated with that delimiter and the result is appended to this StringJoiner as a single element.

      Parameters:
      other - The StringJoiner whose contents should be merged into this one
      Returns:
      This StringJoiner
      Throws:
      NullPointerException - if the other StringJoiner is null

    • length

      public int length()
      Returns the length of the String representation of this StringJoiner. Note that if no add methods have been called, then the length of the String representation (either prefix + suffix or emptyValue) will be returned. The value should be equivalent to toString().length().
      Returns:
      the length of the current value of StringJoiner