001/*
002 * Copyright (C) 2008 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.base;
016
017import static com.google.common.base.Preconditions.checkNotNull;
018import static java.util.Objects.requireNonNull;
019
020import com.google.common.annotations.Beta;
021import com.google.common.annotations.GwtCompatible;
022import com.google.errorprone.annotations.CanIgnoreReturnValue;
023import java.io.IOException;
024import java.util.AbstractList;
025import java.util.Arrays;
026import java.util.Iterator;
027import java.util.Map;
028import java.util.Map.Entry;
029import javax.annotation.CheckForNull;
030import org.checkerframework.checker.nullness.qual.Nullable;
031
032/**
033 * An object which joins pieces of text (specified as an array, {@link Iterable}, varargs or even a
034 * {@link Map}) with a separator. It either appends the results to an {@link Appendable} or returns
035 * them as a {@link String}. Example:
036 *
037 * <pre>{@code
038 * Joiner joiner = Joiner.on("; ").skipNulls();
039 *  . . .
040 * return joiner.join("Harry", null, "Ron", "Hermione");
041 * }</pre>
042 *
043 * <p>This returns the string {@code "Harry; Ron; Hermione"}. Note that all input elements are
044 * converted to strings using {@link Object#toString()} before being appended.
045 *
046 * <p>If neither {@link #skipNulls()} nor {@link #useForNull(String)} is specified, the joining
047 * methods will throw {@link NullPointerException} if any given element is null.
048 *
049 * <p><b>Warning: joiner instances are always immutable</b>; a configuration method such as {@code
050 * useForNull} has no effect on the instance it is invoked on! You must store and use the new joiner
051 * instance returned by the method. This makes joiners thread-safe, and safe to store as {@code
052 * static final} constants.
053 *
054 * <pre>{@code
055 * // Bad! Do not do this!
056 * Joiner joiner = Joiner.on(',');
057 * joiner.skipNulls(); // does nothing!
058 * return joiner.join("wrong", null, "wrong");
059 * }</pre>
060 *
061 * <p>See the Guava User Guide article on <a
062 * href="https://github.com/google/guava/wiki/StringsExplained#joiner">{@code Joiner}</a>.
063 *
064 * @author Kevin Bourrillion
065 * @since 2.0
066 */
067@GwtCompatible
068@ElementTypesAreNonnullByDefault
069public class Joiner {
070  /** Returns a joiner which automatically places {@code separator} between consecutive elements. */
071  public static Joiner on(String separator) {
072    return new Joiner(separator);
073  }
074
075  /** Returns a joiner which automatically places {@code separator} between consecutive elements. */
076  public static Joiner on(char separator) {
077    return new Joiner(String.valueOf(separator));
078  }
079
080  private final String separator;
081
082  private Joiner(String separator) {
083    this.separator = checkNotNull(separator);
084  }
085
086  private Joiner(Joiner prototype) {
087    this.separator = prototype.separator;
088  }
089
090  /*
091   * In this file, we use <? extends @Nullable Object> instead of <?> to work around a Kotlin bug
092   * (see b/189937072 until we file a bug against Kotlin itself). (The two should be equivalent, so
093   * we normally prefer the shorter one.)
094   */
095
096  /**
097   * Appends the string representation of each of {@code parts}, using the previously configured
098   * separator between each, to {@code appendable}.
099   */
100  @CanIgnoreReturnValue
101  public <A extends Appendable> A appendTo(A appendable, Iterable<? extends @Nullable Object> parts)
102      throws IOException {
103    return appendTo(appendable, parts.iterator());
104  }
105
106  /**
107   * Appends the string representation of each of {@code parts}, using the previously configured
108   * separator between each, to {@code appendable}.
109   *
110   * @since 11.0
111   */
112  @CanIgnoreReturnValue
113  public <A extends Appendable> A appendTo(A appendable, Iterator<? extends @Nullable Object> parts)
114      throws IOException {
115    checkNotNull(appendable);
116    if (parts.hasNext()) {
117      appendable.append(toString(parts.next()));
118      while (parts.hasNext()) {
119        appendable.append(separator);
120        appendable.append(toString(parts.next()));
121      }
122    }
123    return appendable;
124  }
125
126  /**
127   * Appends the string representation of each of {@code parts}, using the previously configured
128   * separator between each, to {@code appendable}.
129   */
130  @CanIgnoreReturnValue
131  public final <A extends Appendable> A appendTo(A appendable, @Nullable Object[] parts)
132      throws IOException {
133    return appendTo(appendable, Arrays.asList(parts));
134  }
135
136  /** Appends to {@code appendable} the string representation of each of the remaining arguments. */
137  @CanIgnoreReturnValue
138  public final <A extends Appendable> A appendTo(
139      A appendable,
140      @CheckForNull Object first,
141      @CheckForNull Object second,
142      @Nullable Object... rest)
143      throws IOException {
144    return appendTo(appendable, iterable(first, second, rest));
145  }
146
147  /**
148   * Appends the string representation of each of {@code parts}, using the previously configured
149   * separator between each, to {@code builder}. Identical to {@link #appendTo(Appendable,
150   * Iterable)}, except that it does not throw {@link IOException}.
151   */
152  @CanIgnoreReturnValue
153  public final StringBuilder appendTo(
154      StringBuilder builder, Iterable<? extends @Nullable Object> parts) {
155    return appendTo(builder, parts.iterator());
156  }
157
158  /**
159   * Appends the string representation of each of {@code parts}, using the previously configured
160   * separator between each, to {@code builder}. Identical to {@link #appendTo(Appendable,
161   * Iterable)}, except that it does not throw {@link IOException}.
162   *
163   * @since 11.0
164   */
165  @CanIgnoreReturnValue
166  public final StringBuilder appendTo(
167      StringBuilder builder, Iterator<? extends @Nullable Object> parts) {
168    try {
169      appendTo((Appendable) builder, parts);
170    } catch (IOException impossible) {
171      throw new AssertionError(impossible);
172    }
173    return builder;
174  }
175
176  /**
177   * Appends the string representation of each of {@code parts}, using the previously configured
178   * separator between each, to {@code builder}. Identical to {@link #appendTo(Appendable,
179   * Iterable)}, except that it does not throw {@link IOException}.
180   */
181  @CanIgnoreReturnValue
182  public final StringBuilder appendTo(StringBuilder builder, @Nullable Object[] parts) {
183    return appendTo(builder, Arrays.asList(parts));
184  }
185
186  /**
187   * Appends to {@code builder} the string representation of each of the remaining arguments.
188   * Identical to {@link #appendTo(Appendable, Object, Object, Object...)}, except that it does not
189   * throw {@link IOException}.
190   */
191  @CanIgnoreReturnValue
192  public final StringBuilder appendTo(
193      StringBuilder builder,
194      @CheckForNull Object first,
195      @CheckForNull Object second,
196      @Nullable Object... rest) {
197    return appendTo(builder, iterable(first, second, rest));
198  }
199
200  /**
201   * Returns a string containing the string representation of each of {@code parts}, using the
202   * previously configured separator between each.
203   */
204  public final String join(Iterable<? extends @Nullable Object> parts) {
205    return join(parts.iterator());
206  }
207
208  /**
209   * Returns a string containing the string representation of each of {@code parts}, using the
210   * previously configured separator between each.
211   *
212   * @since 11.0
213   */
214  public final String join(Iterator<? extends @Nullable Object> parts) {
215    return appendTo(new StringBuilder(), parts).toString();
216  }
217
218  /**
219   * Returns a string containing the string representation of each of {@code parts}, using the
220   * previously configured separator between each.
221   */
222  public final String join(@Nullable Object[] parts) {
223    return join(Arrays.asList(parts));
224  }
225
226  /**
227   * Returns a string containing the string representation of each argument, using the previously
228   * configured separator between each.
229   */
230  public final String join(
231      @CheckForNull Object first, @CheckForNull Object second, @Nullable Object... rest) {
232    return join(iterable(first, second, rest));
233  }
234
235  /**
236   * Returns a joiner with the same behavior as this one, except automatically substituting {@code
237   * nullText} for any provided null elements.
238   */
239  public Joiner useForNull(String nullText) {
240    checkNotNull(nullText);
241    return new Joiner(this) {
242      @Override
243      CharSequence toString(@CheckForNull Object part) {
244        return (part == null) ? nullText : Joiner.this.toString(part);
245      }
246
247      @Override
248      public Joiner useForNull(String nullText) {
249        throw new UnsupportedOperationException("already specified useForNull");
250      }
251
252      @Override
253      public Joiner skipNulls() {
254        throw new UnsupportedOperationException("already specified useForNull");
255      }
256    };
257  }
258
259  /**
260   * Returns a joiner with the same behavior as this joiner, except automatically skipping over any
261   * provided null elements.
262   */
263  public Joiner skipNulls() {
264    return new Joiner(this) {
265      @Override
266      public <A extends Appendable> A appendTo(
267          A appendable, Iterator<? extends @Nullable Object> parts) throws IOException {
268        checkNotNull(appendable, "appendable");
269        checkNotNull(parts, "parts");
270        while (parts.hasNext()) {
271          Object part = parts.next();
272          if (part != null) {
273            appendable.append(Joiner.this.toString(part));
274            break;
275          }
276        }
277        while (parts.hasNext()) {
278          Object part = parts.next();
279          if (part != null) {
280            appendable.append(separator);
281            appendable.append(Joiner.this.toString(part));
282          }
283        }
284        return appendable;
285      }
286
287      @Override
288      public Joiner useForNull(String nullText) {
289        throw new UnsupportedOperationException("already specified skipNulls");
290      }
291
292      @Override
293      public MapJoiner withKeyValueSeparator(String kvs) {
294        throw new UnsupportedOperationException("can't use .skipNulls() with maps");
295      }
296    };
297  }
298
299  /**
300   * Returns a {@code MapJoiner} using the given key-value separator, and the same configuration as
301   * this {@code Joiner} otherwise.
302   *
303   * @since 20.0
304   */
305  public MapJoiner withKeyValueSeparator(char keyValueSeparator) {
306    return withKeyValueSeparator(String.valueOf(keyValueSeparator));
307  }
308
309  /**
310   * Returns a {@code MapJoiner} using the given key-value separator, and the same configuration as
311   * this {@code Joiner} otherwise.
312   */
313  public MapJoiner withKeyValueSeparator(String keyValueSeparator) {
314    return new MapJoiner(this, keyValueSeparator);
315  }
316
317  /**
318   * An object that joins map entries in the same manner as {@code Joiner} joins iterables and
319   * arrays. Like {@code Joiner}, it is thread-safe and immutable.
320   *
321   * <p>In addition to operating on {@code Map} instances, {@code MapJoiner} can operate on {@code
322   * Multimap} entries in two distinct modes:
323   *
324   * <ul>
325   *   <li>To output a separate entry for each key-value pair, pass {@code multimap.entries()} to a
326   *       {@code MapJoiner} method that accepts entries as input, and receive output of the form
327   *       {@code key1=A&key1=B&key2=C}.
328   *   <li>To output a single entry for each key, pass {@code multimap.asMap()} to a {@code
329   *       MapJoiner} method that accepts a map as input, and receive output of the form {@code
330   *       key1=[A, B]&key2=C}.
331   * </ul>
332   *
333   * @since 2.0
334   */
335  public static final class MapJoiner {
336    private final Joiner joiner;
337    private final String keyValueSeparator;
338
339    private MapJoiner(Joiner joiner, String keyValueSeparator) {
340      this.joiner = joiner; // only "this" is ever passed, so don't checkNotNull
341      this.keyValueSeparator = checkNotNull(keyValueSeparator);
342    }
343
344    /**
345     * Appends the string representation of each entry of {@code map}, using the previously
346     * configured separator and key-value separator, to {@code appendable}.
347     */
348    @CanIgnoreReturnValue
349    public <A extends Appendable> A appendTo(A appendable, Map<?, ?> map) throws IOException {
350      return appendTo(appendable, map.entrySet());
351    }
352
353    /**
354     * Appends the string representation of each entry of {@code map}, using the previously
355     * configured separator and key-value separator, to {@code builder}. Identical to {@link
356     * #appendTo(Appendable, Map)}, except that it does not throw {@link IOException}.
357     */
358    @CanIgnoreReturnValue
359    public StringBuilder appendTo(StringBuilder builder, Map<?, ?> map) {
360      return appendTo(builder, map.entrySet());
361    }
362
363    /**
364     * Appends the string representation of each entry in {@code entries}, using the previously
365     * configured separator and key-value separator, to {@code appendable}.
366     *
367     * @since 10.0
368     */
369    @Beta
370    @CanIgnoreReturnValue
371    public <A extends Appendable> A appendTo(A appendable, Iterable<? extends Entry<?, ?>> entries)
372        throws IOException {
373      return appendTo(appendable, entries.iterator());
374    }
375
376    /**
377     * Appends the string representation of each entry in {@code entries}, using the previously
378     * configured separator and key-value separator, to {@code appendable}.
379     *
380     * @since 11.0
381     */
382    @Beta
383    @CanIgnoreReturnValue
384    public <A extends Appendable> A appendTo(A appendable, Iterator<? extends Entry<?, ?>> parts)
385        throws IOException {
386      checkNotNull(appendable);
387      if (parts.hasNext()) {
388        Entry<?, ?> entry = parts.next();
389        appendable.append(joiner.toString(entry.getKey()));
390        appendable.append(keyValueSeparator);
391        appendable.append(joiner.toString(entry.getValue()));
392        while (parts.hasNext()) {
393          appendable.append(joiner.separator);
394          Entry<?, ?> e = parts.next();
395          appendable.append(joiner.toString(e.getKey()));
396          appendable.append(keyValueSeparator);
397          appendable.append(joiner.toString(e.getValue()));
398        }
399      }
400      return appendable;
401    }
402
403    /**
404     * Appends the string representation of each entry in {@code entries}, using the previously
405     * configured separator and key-value separator, to {@code builder}. Identical to {@link
406     * #appendTo(Appendable, Iterable)}, except that it does not throw {@link IOException}.
407     *
408     * @since 10.0
409     */
410    @Beta
411    @CanIgnoreReturnValue
412    public StringBuilder appendTo(StringBuilder builder, Iterable<? extends Entry<?, ?>> entries) {
413      return appendTo(builder, entries.iterator());
414    }
415
416    /**
417     * Appends the string representation of each entry in {@code entries}, using the previously
418     * configured separator and key-value separator, to {@code builder}. Identical to {@link
419     * #appendTo(Appendable, Iterable)}, except that it does not throw {@link IOException}.
420     *
421     * @since 11.0
422     */
423    @Beta
424    @CanIgnoreReturnValue
425    public StringBuilder appendTo(StringBuilder builder, Iterator<? extends Entry<?, ?>> entries) {
426      try {
427        appendTo((Appendable) builder, entries);
428      } catch (IOException impossible) {
429        throw new AssertionError(impossible);
430      }
431      return builder;
432    }
433
434    /**
435     * Returns a string containing the string representation of each entry of {@code map}, using the
436     * previously configured separator and key-value separator.
437     */
438    public String join(Map<?, ?> map) {
439      return join(map.entrySet());
440    }
441
442    /**
443     * Returns a string containing the string representation of each entry in {@code entries}, using
444     * the previously configured separator and key-value separator.
445     *
446     * @since 10.0
447     */
448    @Beta
449    public String join(Iterable<? extends Entry<?, ?>> entries) {
450      return join(entries.iterator());
451    }
452
453    /**
454     * Returns a string containing the string representation of each entry in {@code entries}, using
455     * the previously configured separator and key-value separator.
456     *
457     * @since 11.0
458     */
459    @Beta
460    public String join(Iterator<? extends Entry<?, ?>> entries) {
461      return appendTo(new StringBuilder(), entries).toString();
462    }
463
464    /**
465     * Returns a map joiner with the same behavior as this one, except automatically substituting
466     * {@code nullText} for any provided null keys or values.
467     */
468    public MapJoiner useForNull(String nullText) {
469      return new MapJoiner(joiner.useForNull(nullText), keyValueSeparator);
470    }
471  }
472
473  CharSequence toString(@CheckForNull Object part) {
474    /*
475     * requireNonNull is not safe: Joiner.on(...).join(somethingThatContainsNull) will indeed throw.
476     * However, Joiner.on(...).useForNull(...).join(somethingThatContainsNull) *is* safe -- because
477     * it returns a subclass of Joiner that overrides this method to tolerate null inputs.
478     *
479     * Unfortunately, we don't distinguish between these two cases in our public API: Joiner.on(...)
480     * and Joiner.on(...).useForNull(...) both declare the same return type: plain Joiner. To ensure
481     * that users *can* pass null arguments to Joiner, we annotate it as if it always tolerates null
482     * inputs, rather than as if it never tolerates them.
483     *
484     * We rely on checkers to implement special cases to catch dangerous calls to join(), etc. based
485     * on what they know about the particular Joiner instances the calls are performed on.
486     *
487     * (In addition to useForNull, we also offer skipNulls. It, too, tolerates null inputs, but its
488     * tolerance is implemented differently: Its implementation avoids calling this toString(Object)
489     * method in the first place.)
490     */
491    requireNonNull(part);
492    return (part instanceof CharSequence) ? (CharSequence) part : part.toString();
493  }
494
495  private static Iterable<@Nullable Object> iterable(
496      @CheckForNull Object first, @CheckForNull Object second, @Nullable Object[] rest) {
497    checkNotNull(rest);
498    return new AbstractList<@Nullable Object>() {
499      @Override
500      public int size() {
501        return rest.length + 2;
502      }
503
504      @Override
505      @CheckForNull
506      public Object get(int index) {
507        switch (index) {
508          case 0:
509            return first;
510          case 1:
511            return second;
512          default:
513            return rest[index - 2];
514        }
515      }
516    };
517  }
518}