Joiner.java

/* Copyright (c) 2008 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


package com.google.gdata.util.common.base;

import static com.google.gdata.util.common.base.Preconditions.checkNotNull;

import java.io.IOException;
import java.util.AbstractList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;

import javax.annotation.Nullable;

/**
 * An object which joins pieces of text (specified as an array, {@link
 * Iterable}, varargs or even a {@link Map}) with a separator. It either
 * appends the results to an {@link Appendable} or returns them as a {@link
 * String}. Example: <pre>   {@code
 *
 *   Joiner joiner = Joiner.on("; ").skipNulls();
 *    . . .
 *   return joiner.join("Harry", null, "Ron", "Hermione");}</pre>
 *
 * This returns the string {@code "Harry; Ron; Hermione"}. Note that all input
 * elements are converted to strings using {@link Object#toString()} before
 * being appended.
 *
 * <p>If neither {@link #skipNulls()} nor {@link #useForNull(String)} is
 * specified, the joining methods will throw {@link NullPointerException} if any
 * given element is null.
 *
 * 
 */
public class Joiner {
  /**
   * Returns a joiner which automatically places {@code separator} between
   * consecutive elements.
   */
  public static Joiner on(String separator) {
    return new Joiner(separator);
  }

  private final String separator;

  private Joiner(String separator) {
    this.separator = checkNotNull(separator);
  }

  private Joiner(Joiner prototype) {
    this.separator = prototype.separator;
  }

  /**
   * Appends the string representation of each of {@code parts}, using the
   * previously configured separator between each, to {@code appendable}.
   */
  public <A extends Appendable> A appendTo(A appendable, Iterable<?> parts)
      throws IOException {
    checkNotNull(appendable);
    Iterator<?> iterator = parts.iterator();
    if (iterator.hasNext()) {
      appendable.append(toString(iterator.next()));
      while (iterator.hasNext()) {
        appendable.append(separator);
        appendable.append(toString(iterator.next()));
      }
    }
    return appendable;
  }

  /**
   * Appends the string representation of each of {@code parts}, using the
   * previously configured separator between each, to {@code appendable}.
   */
  public final <A extends Appendable> A appendTo(
      A appendable, Object[] parts) throws IOException {
    return appendTo(appendable, Arrays.asList(parts));
  }

  /**
   * Appends to {@code appendable} the string representation of each of the
   * remaining arguments.
   */
  public final <A extends Appendable> A appendTo(A appendable,
      Object first, Object second, Object... rest)
      throws IOException {
    return appendTo(appendable, iterable(first, second, rest));
  }

  /**
   * Appends the string representation of each of {@code parts}, using the
   * previously configured separator between each, to {@code builder}. Identical
   * to {@link #appendTo(Appendable, Iterable)}, except that it does not throw
   * {@link IOException}.
   */
  public final StringBuilder appendTo(StringBuilder builder, Iterable<?> parts)
  {
    try {
      appendTo((Appendable) builder, parts);
    } catch (IOException impossible) {
      throw new AssertionError(impossible);
    }
    return builder;
  }

  /**
   * Appends the string representation of each of {@code parts}, using the
   * previously configured separator between each, to {@code builder}. Identical
   * to {@link #appendTo(Appendable, Iterable)}, except that it does not throw
   * {@link IOException}.
   */
  public final StringBuilder appendTo(StringBuilder builder, Object[] parts) {
    return appendTo(builder, Arrays.asList(parts));
  }

  /**
   * Appends to {@code builder} the string representation of each of the
   * remaining arguments. Identical to {@link #appendTo(Appendable, Object,
   * Object, Object[])}, except that it does not throw {@link IOException}.
   */
  public final StringBuilder appendTo(StringBuilder builder,
      Object first, Object second, Object... rest) {
    return appendTo(builder, iterable(first, second, rest));
  }

  /**
   * Returns a string containing the string representation of each of {@code
   * parts}, using the previously configured separator between each.
   */
  public final String join(Iterable<?> parts) {
    return appendTo(new StringBuilder(), parts).toString();
  }

  /**
   * Returns a string containing the string representation of each of {@code
   * parts}, using the previously configured separator between each.
   */
  public final String join(Object[] parts) {
    return join(Arrays.asList(parts));
  }

  /**
   * Returns a string containing the string representation of each argument,
   * using the previously configured separator between each.
   */
  public final String join(
      Object first, Object second, Object... rest) {
    return join(iterable(first, second, rest));
  }

  /**
   * Returns a joiner with the same behavior as this one, except automatically
   * substituting {@code nullText} for any provided null elements.
   */
  public Joiner useForNull(final String nullText) {
    checkNotNull(nullText);
    return new Joiner(this) {
      @Override CharSequence toString(Object part) {
        return (part == null) ? nullText : part.toString();
      }
      @Override public Joiner useForNull(String nullText) {
        checkNotNull(nullText); // weird, just to satisfy NullPointerTester!
        throw new UnsupportedOperationException("already specified useForNull");
      }
      @Override public Joiner skipNulls() {
        throw new UnsupportedOperationException("already specified useForNull");
      }
    };
  }

  /**
   * Returns a joiner with the same behavior as this joiner, except
   * automatically skipping over any provided null elements.
   */
  public Joiner skipNulls() {
    return new Joiner(this) {
      @Override public <A extends Appendable> A appendTo(
          A appendable, Iterable<?> parts) throws IOException {
        checkNotNull(appendable, "appendable");
        checkNotNull(parts, "parts");
        Iterator<?> iterator = parts.iterator();
        while (iterator.hasNext()) {
          Object part = iterator.next();
          if (part != null) {
            appendable.append(Joiner.this.toString(part));
            break;
          }
        }
        while (iterator.hasNext()) {
          Object part = iterator.next();
          if (part != null) {
            appendable.append(separator);
            appendable.append(Joiner.this.toString(part));
          }
        }
        return appendable;
      }
      @Override public Joiner useForNull(String nullText) {
        checkNotNull(nullText); // weird, just to satisfy NullPointerTester!
        throw new UnsupportedOperationException("already specified skipNulls");
      }
      @Override public MapJoiner withKeyValueSeparator(String kvs) {
        checkNotNull(kvs); // weird, just to satisfy NullPointerTester!
        throw new UnsupportedOperationException(
            "can't use .skipNulls() with maps");
      }
    };
  }

  /**
   * Returns a {@code MapJoiner} using the given key-value separator, and the
   * same configuration as this {@code Joiner} otherwise.
   */
  public MapJoiner withKeyValueSeparator(String keyValueSeparator) {
    return new MapJoiner(this, checkNotNull(keyValueSeparator));
  }

  /**
   * An object that joins map entries in the same manner as {@code Joiner} joins
   * iterables and arrays.
   */
  public static class MapJoiner {
    private Joiner joiner;
    private String keyValueSeparator;

    private MapJoiner(Joiner joiner, String keyValueSeparator) {
      this.joiner = joiner;
      this.keyValueSeparator = keyValueSeparator;
    }

    /**
     * Appends the string representation of each entry of {@code map}, using the
     * previously configured separator and key-value separator, to {@code
     * appendable}.
     */
    public <A extends Appendable> A appendTo(A appendable, Map<?, ?> map)
        throws IOException {
      checkNotNull(appendable);
      Iterator<? extends Map.Entry<?, ?>> iterator = map.entrySet().iterator();
      if (iterator.hasNext()) {
        Entry<?, ?> entry = iterator.next();
        appendable.append(joiner.toString(entry.getKey()));
        appendable.append(keyValueSeparator);
        appendable.append(joiner.toString(entry.getValue()));
        while (iterator.hasNext()) {
          appendable.append(joiner.separator);
          Entry<?, ?> e = iterator.next();
          appendable.append(joiner.toString(e.getKey()));
          appendable.append(keyValueSeparator);
          appendable.append(joiner.toString(e.getValue()));
        }
      }
      return appendable;
    }

    /**
     * Appends the string representation of each entry of {@code map}, using the
     * previously configured separator and key-value separator, to {@code
     * builder}. Identical to {@link #appendTo(Appendable, Map)}, except that it
     * does not throw {@link IOException}.
     */
    public StringBuilder appendTo(StringBuilder builder, Map<?, ?> map) {
      try {
        appendTo((Appendable) builder, map);
      } catch (IOException impossible) {
        throw new AssertionError(impossible);
      }
      return builder;
    }

    /**
     * Returns a string containing the string representation of each entry of
     * {@code map}, using the previously configured separator and key-value
     * separator.
     */
    public String join(Map<?, ?> map) {
      return appendTo(new StringBuilder(), map).toString();
    }

    /**
     * Returns a map joiner with the same behavior as this one, except
     * automatically substituting {@code nullText} for any provided null keys or
     * values.
     */
    public MapJoiner useForNull(String nullText) {
      return new MapJoiner(joiner.useForNull(nullText), keyValueSeparator);
    }
  }

  CharSequence toString(Object part) {
    return part.toString();
  }

  private static Iterable<Object> iterable(
      final Object first, final Object second, final Object[] rest) {
    checkNotNull(rest);
    return new AbstractList<Object>() {
      @Override public int size() {
        return rest.length + 2;
      }
      @Override public Object get(int index) {
        switch (index) {
          case 0:
            return first;
          case 1:
            return second;
          default:
            return rest[index - 2];
        }
      }
    };
  }
}