1 /* 2 * Copyright (c) 2013, 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 /* 71 * StringBuilder value -- at any time, the characters constructed from the 72 * prefix, the added element separated by the delimiter, but without the 73 * suffix, so that we can more easily add elements without having to jigger 74 * the suffix each time. 75 */ 76 private StringBuilder value; 77 78 /* 79 * By default, the string consisting of prefix+suffix, returned by 80 * toString(), or properties of value, when no elements have yet been added, 81 * i.e. when it is empty. This may be overridden by the user to be some 82 * other value including the empty String. 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 this.emptyValue = this.prefix + this.suffix; 129 } 130 131 /** 132 * Sets the sequence of characters to be used when determining the string 133 * representation of this {@code StringJoiner} and no elements have been 134 * added yet, that is, when it is empty. A copy of the {@code emptyValue} 135 * parameter is made for this purpose. Note that once an add method has been 136 * called, the {@code StringJoiner} is no longer considered empty, even if 137 * the element(s) added correspond to the empty {@code String}. 138 * 139 * @param emptyValue the characters to return as the value of an empty 140 * {@code StringJoiner} 141 * @return this {@code StringJoiner} itself so the calls may be chained 142 * @throws NullPointerException when the {@code emptyValue} parameter is 143 * {@code null} 144 */ setEmptyValue(CharSequence emptyValue)145 public StringJoiner setEmptyValue(CharSequence emptyValue) { 146 this.emptyValue = Objects.requireNonNull(emptyValue, 147 "The empty value must not be null").toString(); 148 return this; 149 } 150 151 /** 152 * Returns the current value, consisting of the {@code prefix}, the values 153 * added so far separated by the {@code delimiter}, and the {@code suffix}, 154 * unless no elements have been added in which case, the 155 * {@code prefix + suffix} or the {@code emptyValue} characters are returned 156 * 157 * @return the string representation of this {@code StringJoiner} 158 */ 159 @Override toString()160 public String toString() { 161 if (value == null) { 162 return emptyValue; 163 } else { 164 if (suffix.equals("")) { 165 return value.toString(); 166 } else { 167 int initialLength = value.length(); 168 String result = value.append(suffix).toString(); 169 // reset value to pre-append initialLength 170 value.setLength(initialLength); 171 return result; 172 } 173 } 174 } 175 176 /** 177 * Adds a copy of the given {@code CharSequence} value as the next 178 * element of the {@code StringJoiner} value. If {@code newElement} is 179 * {@code null}, then {@code "null"} is added. 180 * 181 * @param newElement The element to add 182 * @return a reference to this {@code StringJoiner} 183 */ add(CharSequence newElement)184 public StringJoiner add(CharSequence newElement) { 185 prepareBuilder().append(newElement); 186 return this; 187 } 188 189 /** 190 * Adds the contents of the given {@code StringJoiner} without prefix and 191 * suffix as the next element if it is non-empty. If the given {@code 192 * StringJoiner} is empty, the call has no effect. 193 * 194 * <p>A {@code StringJoiner} is empty if {@link #add(CharSequence) add()} 195 * has never been called, and if {@code merge()} has never been called 196 * with a non-empty {@code StringJoiner} argument. 197 * 198 * <p>If the other {@code StringJoiner} is using a different delimiter, 199 * then elements from the other {@code StringJoiner} are concatenated with 200 * that delimiter and the result is appended to this {@code StringJoiner} 201 * as a single element. 202 * 203 * @param other The {@code StringJoiner} whose contents should be merged 204 * into this one 205 * @throws NullPointerException if the other {@code StringJoiner} is null 206 * @return This {@code StringJoiner} 207 */ merge(StringJoiner other)208 public StringJoiner merge(StringJoiner other) { 209 Objects.requireNonNull(other); 210 if (other.value != null) { 211 final int length = other.value.length(); 212 // lock the length so that we can seize the data to be appended 213 // before initiate copying to avoid interference, especially when 214 // merge 'this' 215 StringBuilder builder = prepareBuilder(); 216 builder.append(other.value, other.prefix.length(), length); 217 } 218 return this; 219 } 220 prepareBuilder()221 private StringBuilder prepareBuilder() { 222 if (value != null) { 223 value.append(delimiter); 224 } else { 225 value = new StringBuilder().append(prefix); 226 } 227 return value; 228 } 229 230 /** 231 * Returns the length of the {@code String} representation 232 * of this {@code StringJoiner}. Note that if 233 * no add methods have been called, then the length of the {@code String} 234 * representation (either {@code prefix + suffix} or {@code emptyValue}) 235 * will be returned. The value should be equivalent to 236 * {@code toString().length()}. 237 * 238 * @return the length of the current value of {@code StringJoiner} 239 */ length()240 public int length() { 241 // Remember that we never actually append the suffix unless we return 242 // the full (present) value or some sub-string or length of it, so that 243 // we can add on more if we need to. 244 return (value != null ? value.length() + suffix.length() : 245 emptyValue.length()); 246 } 247 } 248