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 * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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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, i.e. 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;

    }

    /**

     * The length of the {@code StringJoiner} value, i.e. the length of

     * {@code String} representation of the {@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());

    }

}