* Prior to adding something to the {@code StringJoiner}, its
* {@code sj.toString()} method will, by default, return {@code prefix + suffix}.
* However, if the {@code setEmptyValue} method is called, the {@code 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 {@code prefix} is "{", the
* {@code suffix} is "}" and nothing has been added to the
* {@code StringJoiner}.
*
* @apiNote
*
The String {@code "[George:Sally:Fred]"} may be constructed as follows: * *
{@code
* StringJoiner sj = new StringJoiner(":", "[", "]");
* sj.add("George").add("Sally").add("Fred");
* String desiredString = sj.toString();
* }
* * A {@code StringJoiner} may be employed to create formatted output from a * {@link java.util.stream.Stream} using * {@link java.util.stream.Collectors#joining(CharSequence)}. For example: * *
{@code
* List numbers = Arrays.asList(1, 2, 3, 4);
* String commaSeparatedNumbers = numbers.stream()
* .map(i -> i.toString())
* .collect(Collectors.joining(", "));
* }
*
* @see java.util.stream.Collectors#joining(CharSequence)
* @see java.util.stream.Collectors#joining(CharSequence, CharSequence, CharSequence)
* @since 1.8
*/
public final class StringJoiner {
private final String prefix;
private final String delimiter;
private final String suffix;
/*
* StringBuilder value -- at any time, the characters constructed from the
* prefix, the added element separated by the delimiter, but without the
* suffix, so that we can more easily add elements without having to jigger
* the suffix each time.
*/
private StringBuilder value;
/*
* By default, the string consisting of prefix+suffix, returned by
* toString(), or properties of value, when no elements have yet been added,
* i.e. when it is empty. This may be overridden by the user to be some
* other value including the empty String.
*/
private String emptyValue;
/**
* Constructs a {@code StringJoiner} with no characters in it, with no
* {@code prefix} or {@code suffix}, and a copy of the supplied
* {@code delimiter}.
* If no characters are added to the {@code StringJoiner} and methods
* accessing the value of it are invoked, it will not return a
* {@code prefix} or {@code suffix} (or properties thereof) in the result,
* unless {@code setEmptyValue} has first been called.
*
* @param delimiter the sequence of characters to be used between each
* element added to the {@code StringJoiner} value
* @throws NullPointerException if {@code delimiter} is {@code null}
*/
public StringJoiner(CharSequence delimiter) {
this(delimiter, "", "");
}
/**
* Constructs a {@code StringJoiner} with no characters in it using copies
* of the supplied {@code prefix}, {@code delimiter} and {@code suffix}.
* If no characters are added to the {@code StringJoiner} and methods
* accessing the string value of it are invoked, it will return the
* {@code prefix + suffix} (or properties thereof) in the result, unless
* {@code setEmptyValue} has first been called.
*
* @param delimiter the sequence of characters to be used between each
* element added to the {@code StringJoiner}
* @param prefix the sequence of characters to be used at the beginning
* @param suffix the sequence of characters to be used at the end
* @throws NullPointerException if {@code prefix}, {@code delimiter}, or
* {@code suffix} is {@code null}
*/
public StringJoiner(CharSequence delimiter,
CharSequence prefix,
CharSequence suffix) {
Objects.requireNonNull(prefix, "The prefix must not be null");
Objects.requireNonNull(delimiter, "The delimiter must not be null");
Objects.requireNonNull(suffix, "The suffix must not be null");
// make defensive copies of arguments
this.prefix = prefix.toString();
this.delimiter = delimiter.toString();
this.suffix = suffix.toString();
this.emptyValue = this.prefix + this.suffix;
}
/**
* Sets the sequence of characters to be used when determining the string
* representation of this {@code StringJoiner} and no elements have been
* added yet, that is, when it is empty. A copy of the {@code emptyValue}
* parameter is made for this purpose. Note that once an add method has been
* called, the {@code StringJoiner} is no longer considered empty, even if
* the element(s) added correspond to the empty {@code String}.
*
* @param emptyValue the characters to return as the value of an empty
* {@code StringJoiner}
* @return this {@code StringJoiner} itself so the calls may be chained
* @throws NullPointerException when the {@code emptyValue} parameter is
* {@code null}
*/
public StringJoiner setEmptyValue(CharSequence emptyValue) {
this.emptyValue = Objects.requireNonNull(emptyValue,
"The empty value must not be null").toString();
return this;
}
/**
* Returns the current value, consisting of the {@code prefix}, the values
* added so far separated by the {@code delimiter}, and the {@code suffix},
* unless no elements have been added in which case, the
* {@code prefix + suffix} or the {@code emptyValue} characters are returned
*
* @return the string representation of this {@code StringJoiner}
*/
@Override
public String toString() {
if (value == null) {
return emptyValue;
} else {
if (suffix.equals("")) {
return value.toString();
} else {
int initialLength = value.length();
String result = value.append(suffix).toString();
// reset value to pre-append initialLength
value.setLength(initialLength);
return result;
}
}
}
/**
* Adds a copy of the given {@code CharSequence} value as the next
* element of the {@code StringJoiner} value. If {@code newElement} is
* {@code null}, then {@code "null"} is added.
*
* @param newElement The element to add
* @return a reference to this {@code StringJoiner}
*/
public StringJoiner add(CharSequence newElement) {
prepareBuilder().append(newElement);
return this;
}
/**
* Adds the contents of the given {@code StringJoiner} without prefix and
* suffix as the next element if it is non-empty. If the given {@code
* StringJoiner} is empty, the call has no effect.
*
* A {@code StringJoiner} is empty if {@link #add(CharSequence) add()} * has never been called, and if {@code merge()} has never been called * with a non-empty {@code StringJoiner} argument. * *
If the other {@code StringJoiner} is using a different delimiter, * then elements from the other {@code StringJoiner} are concatenated with * that delimiter and the result is appended to this {@code StringJoiner} * as a single element. * * @param other The {@code StringJoiner} whose contents should be merged * into this one * @throws NullPointerException if the other {@code StringJoiner} is null * @return This {@code StringJoiner} */ public StringJoiner merge(StringJoiner other) { Objects.requireNonNull(other); if (other.value != null) { final int length = other.value.length(); // lock the length so that we can seize the data to be appended // before initiate copying to avoid interference, especially when // merge 'this' StringBuilder builder = prepareBuilder(); builder.append(other.value, other.prefix.length(), length); } return this; } private StringBuilder prepareBuilder() { if (value != null) { value.append(delimiter); } else { value = new StringBuilder().append(prefix); } return value; } /** * Returns the length of the {@code String} representation * of this {@code StringJoiner}. Note that if * no add methods have been called, then the length of the {@code String} * representation (either {@code prefix + suffix} or {@code emptyValue}) * will be returned. The value should be equivalent to * {@code toString().length()}. * * @return the length of the current value of {@code StringJoiner} */ public int length() { // Remember that we never actually append the suffix unless we return // the full (present) value or some sub-string or length of it, so that // we can add on more if we need to. return (value != null ? value.length() + suffix.length() : emptyValue.length()); } }