1 /* 2 * Copyright (c) 2013, 2014, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 package java.util; 26 27 /** 28 * {@code StringJoiner} is used to construct a sequence of characters separated 29 * by a delimiter and optionally starting with a supplied prefix 30 * and ending with a supplied suffix. 31 * <p> 32 * Prior to adding something to the {@code StringJoiner}, its 33 * {@code sj.toString()} method will, by default, return {@code prefix + suffix}. 34 * However, if the {@code setEmptyValue} method is called, the {@code emptyValue} 35 * supplied will be returned instead. This can be used, for example, when 36 * creating a string using set notation to indicate an empty set, i.e. 37 * <code>"{}"</code>, where the {@code prefix} is <code>"{"</code>, the 38 * {@code suffix} is <code>"}"</code> and nothing has been added to the 39 * {@code StringJoiner}. 40 * 41 * @apiNote 42 * <p>The String {@code "[George:Sally:Fred]"} may be constructed as follows: 43 * 44 * <pre> {@code 45 * StringJoiner sj = new StringJoiner(":", "[", "]"); 46 * sj.add("George").add("Sally").add("Fred"); 47 * String desiredString = sj.toString(); 48 * }</pre> 49 * <p> 50 * A {@code StringJoiner} may be employed to create formatted output from a 51 * {@link java.util.stream.Stream} using 52 * {@link java.util.stream.Collectors#joining(CharSequence)}. For example: 53 * 54 * <pre> {@code 55 * List<Integer> numbers = Arrays.asList(1, 2, 3, 4); 56 * String commaSeparatedNumbers = numbers.stream() 57 * .map(i -> i.toString()) 58 * .collect(Collectors.joining(", ")); 59 * }</pre> 60 * 61 * @see java.util.stream.Collectors#joining(CharSequence) 62 * @see java.util.stream.Collectors#joining(CharSequence, CharSequence, CharSequence) 63 * @since 1.8 64 */ 65 public final class StringJoiner { 66 private final String prefix; 67 private final String delimiter; 68 private final String suffix; 69 70 /** Contains all the string components added so far. */ 71 private String[] elts; 72 73 /** The number of string components added so far. */ 74 private int size; 75 76 /** Total length in chars so far, excluding prefix and suffix. */ 77 private int len; 78 79 /** 80 * When overriden by the user to be non-null via {@link setEmptyValue}, the 81 * string returned by toString() when no elements have yet been added. 82 * When null, prefix + suffix is used as the empty value. 83 */ 84 private String emptyValue; 85 86 /** 87 * Constructs a {@code StringJoiner} with no characters in it, with no 88 * {@code prefix} or {@code suffix}, and a copy of the supplied 89 * {@code delimiter}. 90 * If no characters are added to the {@code StringJoiner} and methods 91 * accessing the value of it are invoked, it will not return a 92 * {@code prefix} or {@code suffix} (or properties thereof) in the result, 93 * unless {@code setEmptyValue} has first been called. 94 * 95 * @param delimiter the sequence of characters to be used between each 96 * element added to the {@code StringJoiner} value 97 * @throws NullPointerException if {@code delimiter} is {@code null} 98 */ StringJoiner(CharSequence delimiter)99 public StringJoiner(CharSequence delimiter) { 100 this(delimiter, "", ""); 101 } 102 103 /** 104 * Constructs a {@code StringJoiner} with no characters in it using copies 105 * of the supplied {@code prefix}, {@code delimiter} and {@code suffix}. 106 * If no characters are added to the {@code StringJoiner} and methods 107 * accessing the string value of it are invoked, it will return the 108 * {@code prefix + suffix} (or properties thereof) in the result, unless 109 * {@code setEmptyValue} has first been called. 110 * 111 * @param delimiter the sequence of characters to be used between each 112 * element added to the {@code StringJoiner} 113 * @param prefix the sequence of characters to be used at the beginning 114 * @param suffix the sequence of characters to be used at the end 115 * @throws NullPointerException if {@code prefix}, {@code delimiter}, or 116 * {@code suffix} is {@code null} 117 */ StringJoiner(CharSequence delimiter, CharSequence prefix, CharSequence suffix)118 public StringJoiner(CharSequence delimiter, 119 CharSequence prefix, 120 CharSequence suffix) { 121 Objects.requireNonNull(prefix, "The prefix must not be null"); 122 Objects.requireNonNull(delimiter, "The delimiter must not be null"); 123 Objects.requireNonNull(suffix, "The suffix must not be null"); 124 // make defensive copies of arguments 125 this.prefix = prefix.toString(); 126 this.delimiter = delimiter.toString(); 127 this.suffix = suffix.toString(); 128 } 129 130 /** 131 * Sets the sequence of characters to be used when determining the string 132 * representation of this {@code StringJoiner} and no elements have been 133 * added yet, that is, when it is empty. A copy of the {@code emptyValue} 134 * parameter is made for this purpose. Note that once an add method has been 135 * called, the {@code StringJoiner} is no longer considered empty, even if 136 * the element(s) added correspond to the empty {@code String}. 137 * 138 * @param emptyValue the characters to return as the value of an empty 139 * {@code StringJoiner} 140 * @return this {@code StringJoiner} itself so the calls may be chained 141 * @throws NullPointerException when the {@code emptyValue} parameter is 142 * {@code null} 143 */ setEmptyValue(CharSequence emptyValue)144 public StringJoiner setEmptyValue(CharSequence emptyValue) { 145 this.emptyValue = Objects.requireNonNull(emptyValue, 146 "The empty value must not be null").toString(); 147 return this; 148 } 149 getChars(String s, char[] chars, int start)150 private static int getChars(String s, char[] chars, int start) { 151 int len = s.length(); 152 s.getChars(0, len, chars, start); 153 return len; 154 } 155 156 /** 157 * Returns the current value, consisting of the {@code prefix}, the values 158 * added so far separated by the {@code delimiter}, and the {@code suffix}, 159 * unless no elements have been added in which case, the 160 * {@code prefix + suffix} or the {@code emptyValue} characters are returned. 161 * 162 * @return the string representation of this {@code StringJoiner} 163 */ 164 @Override toString()165 public String toString() { 166 final String[] elts = this.elts; 167 if (elts == null && emptyValue != null) { 168 return emptyValue; 169 } 170 final int size = this.size; 171 final int addLen = prefix.length() + suffix.length(); 172 if (addLen == 0) { 173 compactElts(); 174 return size == 0 ? "" : elts[0]; 175 } 176 final String delimiter = this.delimiter; 177 final char[] chars = new char[len + addLen]; 178 int k = getChars(prefix, chars, 0); 179 if (size > 0) { 180 k += getChars(elts[0], chars, k); 181 for (int i = 1; i < size; i++) { 182 k += getChars(delimiter, chars, k); 183 k += getChars(elts[i], chars, k); 184 } 185 } 186 k += getChars(suffix, chars, k); 187 return new String(chars); 188 } 189 190 /** 191 * Adds a copy of the given {@code CharSequence} value as the next 192 * element of the {@code StringJoiner} value. If {@code newElement} is 193 * {@code null}, then {@code "null"} is added. 194 * 195 * @param newElement The element to add 196 * @return a reference to this {@code StringJoiner} 197 */ add(CharSequence newElement)198 public StringJoiner add(CharSequence newElement) { 199 final String elt = String.valueOf(newElement); 200 if (elts == null) { 201 elts = new String[8]; 202 } else { 203 if (size == elts.length) 204 elts = Arrays.copyOf(elts, 2 * size); 205 len += delimiter.length(); 206 } 207 len += elt.length(); 208 elts[size++] = elt; 209 return this; 210 } 211 212 /** 213 * Adds the contents of the given {@code StringJoiner} without prefix and 214 * suffix as the next element if it is non-empty. If the given {@code 215 * StringJoiner} is empty, the call has no effect. 216 * 217 * <p>A {@code StringJoiner} is empty if {@link #add(CharSequence) add()} 218 * has never been called, and if {@code merge()} has never been called 219 * with a non-empty {@code StringJoiner} argument. 220 * 221 * <p>If the other {@code StringJoiner} is using a different delimiter, 222 * then elements from the other {@code StringJoiner} are concatenated with 223 * that delimiter and the result is appended to this {@code StringJoiner} 224 * as a single element. 225 * 226 * @param other The {@code StringJoiner} whose contents should be merged 227 * into this one 228 * @throws NullPointerException if the other {@code StringJoiner} is null 229 * @return This {@code StringJoiner} 230 */ merge(StringJoiner other)231 public StringJoiner merge(StringJoiner other) { 232 Objects.requireNonNull(other); 233 if (other.elts == null) { 234 return this; 235 } 236 other.compactElts(); 237 return add(other.elts[0]); 238 } 239 compactElts()240 private void compactElts() { 241 if (size > 1) { 242 final char[] chars = new char[len]; 243 int i = 1, k = getChars(elts[0], chars, 0); 244 do { 245 k += getChars(delimiter, chars, k); 246 k += getChars(elts[i], chars, k); 247 elts[i] = null; 248 } while (++i < size); 249 size = 1; 250 elts[0] = new String(chars); 251 } 252 } 253 254 /** 255 * Returns the length of the {@code String} representation 256 * of this {@code StringJoiner}. Note that if 257 * no add methods have been called, then the length of the {@code String} 258 * representation (either {@code prefix + suffix} or {@code emptyValue}) 259 * will be returned. The value should be equivalent to 260 * {@code toString().length()}. 261 * 262 * @return the length of the current value of {@code StringJoiner} 263 */ length()264 public int length() { 265 return (size == 0 && emptyValue != null) ? emptyValue.length() : 266 len + prefix.length() + suffix.length(); 267 } 268 } 269