Java tutorial
/* * Copyright (C) 2010 The Guava Authors * * 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.common.base; import static com.google.common.base.Preconditions.checkArgument; import static com.google.common.base.Preconditions.checkNotNull; import com.google.common.annotations.GwtCompatible; import com.google.common.annotations.VisibleForTesting; import java.util.Formatter; import javax.annotation.CheckReturnValue; import javax.annotation.Nullable; /** * Static utility methods pertaining to {@code String} or {@code CharSequence} * instances. * * @author Kevin Bourrillion * @since 3.0 */ @CheckReturnValue @GwtCompatible public final class Strings { private Strings() { } /** * Returns the given string if it is non-null; the empty string otherwise. * * @param string the string to test and possibly return * @return {@code string} itself if it is non-null; {@code ""} if it is null */ public static String nullToEmpty(@Nullable String string) { return (string == null) ? "" : string; } /** * Returns the given string if it is nonempty; {@code null} otherwise. * * @param string the string to test and possibly return * @return {@code string} itself if it is nonempty; {@code null} if it is * empty or null */ @Nullable public static String emptyToNull(@Nullable String string) { return isNullOrEmpty(string) ? null : string; } /** * Returns {@code true} if the given string is null or is the empty string. * * <p>Consider normalizing your string references with {@link #nullToEmpty}. * If you do, you can use {@link String#isEmpty()} instead of this * method, and you won't need special null-safe forms of methods like {@link * String#toUpperCase} either. Or, if you'd like to normalize "in the other * direction," converting empty strings to {@code null}, you can use {@link * #emptyToNull}. * * @param string a string reference to check * @return {@code true} if the string is null or is the empty string */ public static boolean isNullOrEmpty(@Nullable String string) { return string == null || string.length() == 0; // string.isEmpty() in Java 6 } /** * Returns a string, of length at least {@code minLength}, consisting of * {@code string} prepended with as many copies of {@code padChar} as are * necessary to reach that length. For example, * * <ul> * <li>{@code padStart("7", 3, '0')} returns {@code "007"} * <li>{@code padStart("2010", 3, '0')} returns {@code "2010"} * </ul> * * <p>See {@link Formatter} for a richer set of formatting capabilities. * * @param string the string which should appear at the end of the result * @param minLength the minimum length the resulting string must have. Can be * zero or negative, in which case the input string is always returned. * @param padChar the character to insert at the beginning of the result until * the minimum length is reached * @return the padded string */ public static String padStart(String string, int minLength, char padChar) { checkNotNull(string); // eager for GWT. if (string.length() >= minLength) { return string; } StringBuilder sb = new StringBuilder(minLength); for (int i = string.length(); i < minLength; i++) { sb.append(padChar); } sb.append(string); return sb.toString(); } /** * Returns a string, of length at least {@code minLength}, consisting of * {@code string} appended with as many copies of {@code padChar} as are * necessary to reach that length. For example, * * <ul> * <li>{@code padEnd("4.", 5, '0')} returns {@code "4.000"} * <li>{@code padEnd("2010", 3, '!')} returns {@code "2010"} * </ul> * * <p>See {@link Formatter} for a richer set of formatting capabilities. * * @param string the string which should appear at the beginning of the result * @param minLength the minimum length the resulting string must have. Can be * zero or negative, in which case the input string is always returned. * @param padChar the character to append to the end of the result until the * minimum length is reached * @return the padded string */ public static String padEnd(String string, int minLength, char padChar) { checkNotNull(string); // eager for GWT. if (string.length() >= minLength) { return string; } StringBuilder sb = new StringBuilder(minLength); sb.append(string); for (int i = string.length(); i < minLength; i++) { sb.append(padChar); } return sb.toString(); } /** * Returns a string consisting of a specific number of concatenated copies of * an input string. For example, {@code repeat("hey", 3)} returns the string * {@code "heyheyhey"}. * * @param string any non-null string * @param count the number of times to repeat it; a nonnegative integer * @return a string containing {@code string} repeated {@code count} times * (the empty string if {@code count} is zero) * @throws IllegalArgumentException if {@code count} is negative */ public static String repeat(String string, int count) { checkNotNull(string); // eager for GWT. if (count <= 1) { checkArgument(count >= 0, "invalid count: %s", count); return (count == 0) ? "" : string; } // IF YOU MODIFY THE CODE HERE, you must update StringsRepeatBenchmark final int len = string.length(); final long longSize = (long) len * (long) count; final int size = (int) longSize; if (size != longSize) { throw new ArrayIndexOutOfBoundsException("Required array size too large: " + longSize); } final char[] array = new char[size]; string.getChars(0, len, array, 0); int n; for (n = len; n < size - n; n <<= 1) { System.arraycopy(array, 0, array, n, n); } System.arraycopy(array, 0, array, n, size - n); return new String(array); } /** * Returns the longest string {@code prefix} such that * {@code a.toString().startsWith(prefix) && b.toString().startsWith(prefix)}, * taking care not to split surrogate pairs. If {@code a} and {@code b} have * no common prefix, returns the empty string. * * @since 11.0 */ public static String commonPrefix(CharSequence a, CharSequence b) { checkNotNull(a); checkNotNull(b); int maxPrefixLength = Math.min(a.length(), b.length()); int p = 0; while (p < maxPrefixLength && a.charAt(p) == b.charAt(p)) { p++; } if (validSurrogatePairAt(a, p - 1) || validSurrogatePairAt(b, p - 1)) { p--; } return a.subSequence(0, p).toString(); } /** * Returns the longest string {@code suffix} such that * {@code a.toString().endsWith(suffix) && b.toString().endsWith(suffix)}, * taking care not to split surrogate pairs. If {@code a} and {@code b} have * no common suffix, returns the empty string. * * @since 11.0 */ public static String commonSuffix(CharSequence a, CharSequence b) { checkNotNull(a); checkNotNull(b); int maxSuffixLength = Math.min(a.length(), b.length()); int s = 0; while (s < maxSuffixLength && a.charAt(a.length() - s - 1) == b.charAt(b.length() - s - 1)) { s++; } if (validSurrogatePairAt(a, a.length() - s - 1) || validSurrogatePairAt(b, b.length() - s - 1)) { s--; } return a.subSequence(a.length() - s, a.length()).toString(); } /** * True when a valid surrogate pair starts at the given {@code index} in the * given {@code string}. Out-of-range indexes return false. */ @VisibleForTesting static boolean validSurrogatePairAt(CharSequence string, int index) { return index >= 0 && index <= (string.length() - 2) && Character.isHighSurrogate(string.charAt(index)) && Character.isLowSurrogate(string.charAt(index + 1)); } }