001    /*
002     * Copyright (C) 2010 The Guava Authors
003     *
004     * Licensed under the Apache License, Version 2.0 (the "License");
005     * you may not use this file except in compliance with the License.
006     * You may obtain a copy of the License at
007     *
008     * http://www.apache.org/licenses/LICENSE-2.0
009     *
010     * Unless required by applicable law or agreed to in writing, software
011     * distributed under the License is distributed on an "AS IS" BASIS,
012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013     * See the License for the specific language governing permissions and
014     * limitations under the License.
015     */
016    
017    package com.google.common.base;
018    
019    import static com.google.common.base.Preconditions.checkArgument;
020    import static com.google.common.base.Preconditions.checkNotNull;
021    
022    import com.google.common.annotations.GwtCompatible;
023    
024    import java.util.Formatter;
025    
026    import javax.annotation.Nullable;
027    
028    /**
029     * Static utility methods pertaining to {@code String} or {@code CharSequence}
030     * instances.
031     *
032     * @author Kevin Bourrillion
033     * @since 3.0
034     */
035    @GwtCompatible
036    public final class Strings {
037      private Strings() {}
038    
039      /**
040       * Returns the given string if it is non-null; the empty string otherwise.
041       *
042       * @param string the string to test and possibly return
043       * @return {@code string} itself if it is non-null; {@code ""} if it is null
044       */
045      public static String nullToEmpty(@Nullable String string) {
046        return (string == null) ? "" : string;
047      }
048    
049      /**
050       * Returns the given string if it is nonempty; {@code null} otherwise.
051       *
052       * @param string the string to test and possibly return
053       * @return {@code string} itself if it is nonempty; {@code null} if it is
054       *     empty or null
055       */
056      public static @Nullable String emptyToNull(@Nullable String string) {
057        return isNullOrEmpty(string) ? null : string;
058      }
059    
060      /**
061       * Returns {@code true} if the given string is null or is the empty string.
062       *
063       * <p>Consider normalizing your string references with {@link #nullToEmpty}.
064       * If you do, you can use {@link String#isEmpty()} instead of this
065       * method, and you won't need special null-safe forms of methods like {@link
066       * String#toUpperCase} either. Or, if you'd like to normalize "in the other
067       * direction," converting empty strings to {@code null}, you can use {@link
068       * #emptyToNull}.
069       *
070       * @param string a string reference to check
071       * @return {@code true} if the string is null or is the empty string
072       */
073      public static boolean isNullOrEmpty(@Nullable String string) {
074        return string == null || string.length() == 0; // string.isEmpty() in Java 6
075      }
076    
077      /**
078       * Returns a string, of length at least {@code minLength}, consisting of
079       * {@code string} prepended with as many copies of {@code padChar} as are
080       * necessary to reach that length. For example,
081       *
082       * <ul>
083       * <li>{@code padStart("7", 3, '0')} returns {@code "007"}
084       * <li>{@code padStart("2010", 3, '0')} returns {@code "2010"}
085       * </ul>
086       *
087       * <p>See {@link Formatter} for a richer set of formatting capabilities.
088       *
089       * @param string the string which should appear at the end of the result
090       * @param minLength the minimum length the resulting string must have. Can be
091       *     zero or negative, in which case the input string is always returned.
092       * @param padChar the character to insert at the beginning of the result until
093       *     the minimum length is reached
094       * @return the padded string
095       */
096      public static String padStart(String string, int minLength, char padChar) {
097        checkNotNull(string);  // eager for GWT.
098        if (string.length() >= minLength) {
099          return string;
100        }
101        StringBuilder sb = new StringBuilder(minLength);
102        for (int i = string.length(); i < minLength; i++) {
103          sb.append(padChar);
104        }
105        sb.append(string);
106        return sb.toString();
107      }
108    
109      /**
110       * Returns a string, of length at least {@code minLength}, consisting of
111       * {@code string} appended with as many copies of {@code padChar} as are
112       * necessary to reach that length. For example,
113       *
114       * <ul>
115       * <li>{@code padEnd("4.", 5, '0')} returns {@code "4.000"}
116       * <li>{@code padEnd("2010", 3, '!')} returns {@code "2010"}
117       * </ul>
118       *
119       * <p>See {@link Formatter} for a richer set of formatting capabilities.
120       *
121       * @param string the string which should appear at the beginning of the result
122       * @param minLength the minimum length the resulting string must have. Can be
123       *     zero or negative, in which case the input string is always returned.
124       * @param padChar the character to append to the end of the result until the
125       *     minimum length is reached
126       * @return the padded string
127       */
128      public static String padEnd(String string, int minLength, char padChar) {
129        checkNotNull(string);  // eager for GWT.
130        if (string.length() >= minLength) {
131          return string;
132        }
133        StringBuilder sb = new StringBuilder(minLength);
134        sb.append(string);
135        for (int i = string.length(); i < minLength; i++) {
136          sb.append(padChar);
137        }
138        return sb.toString();
139      }
140    
141      /**
142       * Returns a string consisting of a specific number of concatenated copies of
143       * an input string. For example, {@code repeat("hey", 3)} returns the string
144       * {@code "heyheyhey"}.
145       *
146       * @param string any non-null string
147       * @param count the number of times to repeat it; a nonnegative integer
148       * @return a string containing {@code string} repeated {@code count} times
149       *     (the empty string if {@code count} is zero)
150       * @throws IllegalArgumentException if {@code count} is negative
151       */
152      public static String repeat(String string, int count) {
153        checkNotNull(string);  // eager for GWT.
154        checkArgument(count >= 0, "invalid count: %s", count);
155    
156        // If this multiplication overflows, a NegativeArraySizeException or
157        // OutOfMemoryError is not far behind
158        StringBuilder builder = new StringBuilder(string.length() * count);
159        for (int i = 0; i < count; i++) {
160          builder.append(string);
161        }
162        return builder.toString();
163      }
164    }