java/util/StringJoiner.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * accompanied this code).
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package java.util;

/**
 * {@code 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.
 * <p>
 * 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.
 * <code>"{}"</code>, where the {@code prefix} is <code>"{"</code>, the
 * {@code suffix} is <code>"}"</code> and nothing has been added to the
 * {@code StringJoiner}.
 *
 * @apiNote
 * <p>The String {@code "[George:Sally:Fred]"} may be constructed as follows:
 *
 * <pre> {@code
 * StringJoiner sj = new StringJoiner(":", "[", "]");
 * sj.add("George").add("Sally").add("Fred");
 * String desiredString = sj.toString();
 * }</pre>
 * <p>
 * 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:
 *
 * <pre> {@code
 * List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
 * String commaSeparatedNumbers = numbers.stream()
 *     .map(i -> i.toString())
 *     .collect(Collectors.joining(", "));
 * }</pre>
 *
 * @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.
     *
     * <p>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.
     *
     * <p>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());
    }
}