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.Beta; import com.google.common.annotations.GwtCompatible; import javax.annotation.CheckReturnValue; /** * Static methods pertaining to ASCII characters (those in the range of values * {@code 0x00} through {@code 0x7F}), and to strings containing such * characters. * * <p>ASCII utilities also exist in other classes of this package: * <ul> * <!-- TODO(kevinb): how can we make this not produce a warning when building gwt javadoc? --> * <li>{@link Charsets#US_ASCII} specifies the {@code Charset} of ASCII characters. * <li>{@link CharMatcher#ASCII} matches ASCII characters and provides text processing methods * which operate only on the ASCII characters of a string. * </ul> * * @author Craig Berry * @author Gregory Kick * @since 7.0 */ @CheckReturnValue @GwtCompatible public final class Ascii { private Ascii() { } /* The ASCII control characters, per RFC 20. */ /** * Null ('\0'): The all-zeros character which may serve to accomplish * time fill and media fill. Normally used as a C string terminator. * <p>Although RFC 20 names this as "Null", note that it is distinct * from the C/C++ "NULL" pointer. * * @since 8.0 */ public static final byte NUL = 0; /** * Start of Heading: A communication control character used at * the beginning of a sequence of characters which constitute a * machine-sensible address or routing information. Such a sequence is * referred to as the "heading." An STX character has the effect of * terminating a heading. * * @since 8.0 */ public static final byte SOH = 1; /** * Start of Text: A communication control character which * precedes a sequence of characters that is to be treated as an entity * and entirely transmitted through to the ultimate destination. Such a * sequence is referred to as "text." STX may be used to terminate a * sequence of characters started by SOH. * * @since 8.0 */ public static final byte STX = 2; /** * End of Text: A communication control character used to * terminate a sequence of characters started with STX and transmitted * as an entity. * * @since 8.0 */ public static final byte ETX = 3; /** * End of Transmission: A communication control character used * to indicate the conclusion of a transmission, which may have * contained one or more texts and any associated headings. * * @since 8.0 */ public static final byte EOT = 4; /** * Enquiry: A communication control character used in data * communication systems as a request for a response from a remote * station. It may be used as a "Who Are You" (WRU) to obtain * identification, or may be used to obtain station status, or both. * * @since 8.0 */ public static final byte ENQ = 5; /** * Acknowledge: A communication control character transmitted * by a receiver as an affirmative response to a sender. * * @since 8.0 */ public static final byte ACK = 6; /** * Bell ('\a'): A character for use when there is a need to call for * human attention. It may control alarm or attention devices. * * @since 8.0 */ public static final byte BEL = 7; /** * Backspace ('\b'): A format effector which controls the movement of * the printing position one printing space backward on the same * printing line. (Applicable also to display devices.) * * @since 8.0 */ public static final byte BS = 8; /** * Horizontal Tabulation ('\t'): A format effector which controls the * movement of the printing position to the next in a series of * predetermined positions along the printing line. (Applicable also to * display devices and the skip function on punched cards.) * * @since 8.0 */ public static final byte HT = 9; /** * Line Feed ('\n'): A format effector which controls the movement of * the printing position to the next printing line. (Applicable also to * display devices.) Where appropriate, this character may have the * meaning "New Line" (NL), a format effector which controls the * movement of the printing point to the first printing position on the * next printing line. Use of this convention requires agreement * between sender and recipient of data. * * @since 8.0 */ public static final byte LF = 10; /** * Alternate name for {@link #LF}. ({@code LF} is preferred.) * * @since 8.0 */ public static final byte NL = 10; /** * Vertical Tabulation ('\v'): A format effector which controls the * movement of the printing position to the next in a series of * predetermined printing lines. (Applicable also to display devices.) * * @since 8.0 */ public static final byte VT = 11; /** * Form Feed ('\f'): A format effector which controls the movement of * the printing position to the first pre-determined printing line on * the next form or page. (Applicable also to display devices.) * * @since 8.0 */ public static final byte FF = 12; /** * Carriage Return ('\r'): A format effector which controls the * movement of the printing position to the first printing position on * the same printing line. (Applicable also to display devices.) * * @since 8.0 */ public static final byte CR = 13; /** * Shift Out: A control character indicating that the code * combinations which follow shall be interpreted as outside of the * character set of the standard code table until a Shift In character * is reached. * * @since 8.0 */ public static final byte SO = 14; /** * Shift In: A control character indicating that the code * combinations which follow shall be interpreted according to the * standard code table. * * @since 8.0 */ public static final byte SI = 15; /** * Data Link Escape: A communication control character which * will change the meaning of a limited number of contiguously following * characters. It is used exclusively to provide supplementary controls * in data communication networks. * * @since 8.0 */ public static final byte DLE = 16; /** * Device Control 1. Characters for the control * of ancillary devices associated with data processing or * telecommunication systems, more especially switching devices "on" or * "off." (If a single "stop" control is required to interrupt or turn * off ancillary devices, DC4 is the preferred assignment.) * * @since 8.0 */ public static final byte DC1 = 17; // aka XON /** * Transmission On: Although originally defined as DC1, this ASCII * control character is now better known as the XON code used for software * flow control in serial communications. The main use is restarting * the transmission after the communication has been stopped by the XOFF * control code. * * @since 8.0 */ public static final byte XON = 17; // aka DC1 /** * Device Control 2. Characters for the control * of ancillary devices associated with data processing or * telecommunication systems, more especially switching devices "on" or * "off." (If a single "stop" control is required to interrupt or turn * off ancillary devices, DC4 is the preferred assignment.) * * @since 8.0 */ public static final byte DC2 = 18; /** * Device Control 3. Characters for the control * of ancillary devices associated with data processing or * telecommunication systems, more especially switching devices "on" or * "off." (If a single "stop" control is required to interrupt or turn * off ancillary devices, DC4 is the preferred assignment.) * * @since 8.0 */ public static final byte DC3 = 19; // aka XOFF /** * Transmission off. See {@link #XON} for explanation. * * @since 8.0 */ public static final byte XOFF = 19; // aka DC3 /** * Device Control 4. Characters for the control * of ancillary devices associated with data processing or * telecommunication systems, more especially switching devices "on" or * "off." (If a single "stop" control is required to interrupt or turn * off ancillary devices, DC4 is the preferred assignment.) * * @since 8.0 */ public static final byte DC4 = 20; /** * Negative Acknowledge: A communication control character * transmitted by a receiver as a negative response to the sender. * * @since 8.0 */ public static final byte NAK = 21; /** * Synchronous Idle: A communication control character used by * a synchronous transmission system in the absence of any other * character to provide a signal from which synchronism may be achieved * or retained. * * @since 8.0 */ public static final byte SYN = 22; /** * End of Transmission Block: A communication control character * used to indicate the end of a block of data for communication * purposes. ETB is used for blocking data where the block structure is * not necessarily related to the processing format. * * @since 8.0 */ public static final byte ETB = 23; /** * Cancel: A control character used to indicate that the data * with which it is sent is in error or is to be disregarded. * * @since 8.0 */ public static final byte CAN = 24; /** * End of Medium: A control character associated with the sent * data which may be used to identify the physical end of the medium, or * the end of the used, or wanted, portion of information recorded on a * medium. (The position of this character does not necessarily * correspond to the physical end of the medium.) * * @since 8.0 */ public static final byte EM = 25; /** * Substitute: A character that may be substituted for a * character which is determined to be invalid or in error. * * @since 8.0 */ public static final byte SUB = 26; /** * Escape: A control character intended to provide code * extension (supplementary characters) in general information * interchange. The Escape character itself is a prefix affecting the * interpretation of a limited number of contiguously following * characters. * * @since 8.0 */ public static final byte ESC = 27; /** * File Separator: These four information separators may be * used within data in optional fashion, except that their hierarchical * relationship shall be: FS is the most inclusive, then GS, then RS, * and US is least inclusive. (The content and length of a File, Group, * Record, or Unit are not specified.) * * @since 8.0 */ public static final byte FS = 28; /** * Group Separator: These four information separators may be * used within data in optional fashion, except that their hierarchical * relationship shall be: FS is the most inclusive, then GS, then RS, * and US is least inclusive. (The content and length of a File, Group, * Record, or Unit are not specified.) * * @since 8.0 */ public static final byte GS = 29; /** * Record Separator: These four information separators may be * used within data in optional fashion, except that their hierarchical * relationship shall be: FS is the most inclusive, then GS, then RS, * and US is least inclusive. (The content and length of a File, Group, * Record, or Unit are not specified.) * * @since 8.0 */ public static final byte RS = 30; /** * Unit Separator: These four information separators may be * used within data in optional fashion, except that their hierarchical * relationship shall be: FS is the most inclusive, then GS, then RS, * and US is least inclusive. (The content and length of a File, Group, * Record, or Unit are not specified.) * * @since 8.0 */ public static final byte US = 31; /** * Space: A normally non-printing graphic character used to * separate words. It is also a format effector which controls the * movement of the printing position, one printing position forward. * (Applicable also to display devices.) * * @since 8.0 */ public static final byte SP = 32; /** * Alternate name for {@link #SP}. * * @since 8.0 */ public static final byte SPACE = 32; /** * Delete: This character is used primarily to "erase" or * "obliterate" erroneous or unwanted characters in perforated tape. * * @since 8.0 */ public static final byte DEL = 127; /** * The minimum value of an ASCII character. * * @since 9.0 (was type {@code int} before 12.0) */ public static final char MIN = 0; /** * The maximum value of an ASCII character. * * @since 9.0 (was type {@code int} before 12.0) */ public static final char MAX = 127; /** * Returns a copy of the input string in which all {@linkplain #isUpperCase(char) uppercase ASCII * characters} have been converted to lowercase. All other characters are copied without * modification. */ public static String toLowerCase(String string) { int length = string.length(); for (int i = 0; i < length; i++) { if (isUpperCase(string.charAt(i))) { char[] chars = string.toCharArray(); for (; i < length; i++) { char c = chars[i]; if (isUpperCase(c)) { chars[i] = (char) (c ^ 0x20); } } return String.valueOf(chars); } } return string; } /** * Returns a copy of the input character sequence in which all {@linkplain #isUpperCase(char) * uppercase ASCII characters} have been converted to lowercase. All other characters are copied * without modification. * * @since 14.0 */ public static String toLowerCase(CharSequence chars) { if (chars instanceof String) { return toLowerCase((String) chars); } int length = chars.length(); StringBuilder builder = new StringBuilder(length); for (int i = 0; i < length; i++) { builder.append(toLowerCase(chars.charAt(i))); } return builder.toString(); } /** * If the argument is an {@linkplain #isUpperCase(char) uppercase ASCII character} returns the * lowercase equivalent. Otherwise returns the argument. */ public static char toLowerCase(char c) { return isUpperCase(c) ? (char) (c ^ 0x20) : c; } /** * Returns a copy of the input string in which all {@linkplain #isLowerCase(char) lowercase ASCII * characters} have been converted to uppercase. All other characters are copied without * modification. */ public static String toUpperCase(String string) { int length = string.length(); for (int i = 0; i < length; i++) { if (isLowerCase(string.charAt(i))) { char[] chars = string.toCharArray(); for (; i < length; i++) { char c = chars[i]; if (isLowerCase(c)) { chars[i] = (char) (c & 0x5f); } } return String.valueOf(chars); } } return string; } /** * Returns a copy of the input character sequence in which all {@linkplain #isLowerCase(char) * lowercase ASCII characters} have been converted to uppercase. All other characters are copied * without modification. * * @since 14.0 */ public static String toUpperCase(CharSequence chars) { if (chars instanceof String) { return toUpperCase((String) chars); } int length = chars.length(); StringBuilder builder = new StringBuilder(length); for (int i = 0; i < length; i++) { builder.append(toUpperCase(chars.charAt(i))); } return builder.toString(); } /** * If the argument is a {@linkplain #isLowerCase(char) lowercase ASCII character} returns the * uppercase equivalent. Otherwise returns the argument. */ public static char toUpperCase(char c) { return isLowerCase(c) ? (char) (c & 0x5f) : c; } /** * Indicates whether {@code c} is one of the twenty-six lowercase ASCII alphabetic characters * between {@code 'a'} and {@code 'z'} inclusive. All others (including non-ASCII characters) * return {@code false}. */ public static boolean isLowerCase(char c) { // Note: This was benchmarked against the alternate expression "(char)(c - 'a') < 26" (Nov '13) // and found to perform at least as well, or better. return (c >= 'a') && (c <= 'z'); } /** * Indicates whether {@code c} is one of the twenty-six uppercase ASCII alphabetic characters * between {@code 'A'} and {@code 'Z'} inclusive. All others (including non-ASCII characters) * return {@code false}. */ public static boolean isUpperCase(char c) { return (c >= 'A') && (c <= 'Z'); } /** * Truncates the given character sequence to the given maximum length. If the length of the * sequence is greater than {@code maxLength}, the returned string will be exactly * {@code maxLength} chars in length and will end with the given {@code truncationIndicator}. * Otherwise, the sequence will be returned as a string with no changes to the content. * * <p>Examples: * * <pre> {@code * Ascii.truncate("foobar", 7, "..."); // returns "foobar" * Ascii.truncate("foobar", 5, "..."); // returns "fo..." }</pre> * * <p><b>Note:</b> This method <i>may</i> work with certain non-ASCII text but is not safe for * use with arbitrary Unicode text. It is mostly intended for use with text that is known to be * safe for use with it (such as all-ASCII text) and for simple debugging text. When using this * method, consider the following: * * <ul> * <li>it may split surrogate pairs</li> * <li>it may split characters and combining characters</li> * <li>it does not consider word boundaries</li> * <li>if truncating for display to users, there are other considerations that must be taken * into account</li> * <li>the appropriate truncation indicator may be locale-dependent</li> * <li>it is safe to use non-ASCII characters in the truncation indicator</li> * </ul> * * * @throws IllegalArgumentException if {@code maxLength} is less than the length of * {@code truncationIndicator} * @since 16.0 */ @Beta public static String truncate(CharSequence seq, int maxLength, String truncationIndicator) { checkNotNull(seq); // length to truncate the sequence to, not including the truncation indicator int truncationLength = maxLength - truncationIndicator.length(); // in this worst case, this allows a maxLength equal to the length of the truncationIndicator, // meaning that a string will be truncated to just the truncation indicator itself checkArgument(truncationLength >= 0, "maxLength (%s) must be >= length of the truncation indicator (%s)", maxLength, truncationIndicator.length()); if (seq.length() <= maxLength) { String string = seq.toString(); if (string.length() <= maxLength) { return string; } // if the length of the toString() result was > maxLength for some reason, truncate that seq = string; } return new StringBuilder(maxLength).append(seq, 0, truncationLength).append(truncationIndicator).toString(); } /** * Indicates whether the contents of the given character sequences {@code s1} and {@code s2} are * equal, ignoring the case of any ASCII alphabetic characters between {@code 'a'} and {@code 'z'} * or {@code 'A'} and {@code 'Z'} inclusive. * * <p>This method is significantly faster than {@link String#equalsIgnoreCase} and should be used * in preference if at least one of the parameters is known to contain only ASCII characters. * * <p>Note however that this method does not always behave identically to expressions such as: * <ul> * <li>{@code string.toUpperCase().equals("UPPER CASE ASCII")} * <li>{@code string.toLowerCase().equals("lower case ascii")} * </ul> * <p>due to case-folding of some non-ASCII characters (which does not occur in * {@link String#equalsIgnoreCase}). However in almost all cases that ASCII strings are used, * the author probably wanted the behavior provided by this method rather than the subtle and * sometimes surprising behavior of {@code toUpperCase()} and {@code toLowerCase()}. * * @since 16.0 */ @Beta public static boolean equalsIgnoreCase(CharSequence s1, CharSequence s2) { // Calling length() is the null pointer check (so do it before we can exit early). int length = s1.length(); if (s1 == s2) { return true; } if (length != s2.length()) { return false; } for (int i = 0; i < length; i++) { char c1 = s1.charAt(i); char c2 = s2.charAt(i); if (c1 == c2) { continue; } int alphaIndex = getAlphaIndex(c1); // This was also benchmarked using '&' to avoid branching (but always evaluate the rhs), // however this showed no obvious improvement. if (alphaIndex < 26 && alphaIndex == getAlphaIndex(c2)) { continue; } return false; } return true; } /** * Returns the non-negative index value of the alpha character {@code c}, regardless of case. * Ie, 'a'/'A' returns 0 and 'z'/'Z' returns 25. Non-alpha characters return a value of 26 or * greater. */ private static int getAlphaIndex(char c) { // Fold upper-case ASCII to lower-case and make zero-indexed and unsigned (by casting to char). return (char) ((c | 0x20) - 'a'); } }