StdIn.java

Below is the syntax highlighted version of StdIn.java from § Standard Libraries.   Here is the Javadoc.

/******************************************************************************
 *  Compilation:  javac StdIn.java
 *  Execution:    java StdIn   (interactive test of basic functionality)
 *  Dependencies: none
 *
 *  Reads in data of various types from standard input.
 *
 ******************************************************************************/

import java.util.ArrayList;
import java.util.InputMismatchException;
import java.util.Locale;
import java.util.NoSuchElementException;
import java.util.Scanner;
import java.util.regex.Pattern;

/**
 *  The {@code StdIn} class provides static methods for reading strings
 *  and numbers from standard input.
 *  These functions fall into one of four categories:
 *  <ul>
 *  <li>those for reading individual tokens from standard input, one at a time,
 *      and converting each to a number, string, or boolean
 *  <li>those for reading characters from standard input, one at a time
 *  <li>those for reading lines from standard input, one at a time
 *  <li>those for reading a sequence of values of the same type from standard input,
 *      and returning the values in an array
 *  </ul>
 *  <p>
 *  Generally, it is best not to mix functions from the different
 *  categories in the same program.
 *  <p>
 *  <b>Getting started.</b>
 *  To use this class, you must have {@code StdIn.class} in your
 *  Java classpath. If you used our autoinstaller, you should be all set.
 *  Otherwise, either download
 *  <a href = "https://introcs.cs.princeton.edu/java/code/stdlib.jar">stdlib.jar</a>
 *  and add to your Java classpath or download
 *  <a href = "https://introcs.cs.princeton.edu/java/stdlib/StdIn.java">StdIn.java</a>
 *  and put a copy in your working directory.
 *  <p>
 *  <b>Reading tokens from standard input and converting to numbers and strings.</b>
 *  You can use the following methods to read numbers, strings, and booleans
 *  from standard input one at a time:
 *  <ul>
 *  <li> {@link #isEmpty()}
 *  <li> {@link #readInt()}
 *  <li> {@link #readDouble()}
 *  <li> {@link #readString()}
 *  <li> {@link #readShort()}
 *  <li> {@link #readLong()}
 *  <li> {@link #readFloat()}
 *  <li> {@link #readByte()}
 *  <li> {@link #readBoolean()}
 *  </ul>
 *  <p>
 *  The first method returns true if standard input has no more tokens.
 *  Each other method skips over any input that is whitespace. Then, it reads
 *  the next token and attempts to convert it into a value of the specified
 *  type. If it succeeds, it returns that value; otherwise, it
 *  throws an {@link InputMismatchException}.
 *  <p>
 *  <em>Whitespace</em> includes spaces, tabs, and newlines; the full definition
 *  is inherited from {@link Character#isWhitespace(char)}.
 *  A <em>token</em> is a maximal sequence of non-whitespace characters.
 *  The precise rules for describing which tokens can be converted to
 *  integers and floating-point numbers are inherited from
 *  <a href = "http://docs.oracle.com/javase/7/docs/api/java/util/Scanner.html#number-syntax">Scanner</a>,
 *  using the locale {@link Locale#US}; the rules
 *  for floating-point numbers are slightly different
 *  from those in {@link Double#valueOf(String)},
 *  but unlikely to be of concern to most programmers.
 *  <p>
 *  As an example, the following code fragment reads integers from standard input,
 *  one at a time, and prints them one per line.
 *  <pre>
 *  while (!StdIn.isEmpty()) {
 *      double value = StdIn.readDouble();
 *      StdOut.println(value);
 *  }
 *  </pre>
 *  <p>
 *  <b>Reading characters from standard input.</b>
 *  You can use the following two methods to read characters from standard input one at a time:
 *  <ul>
 *  <li> {@link #hasNextChar()}
 *  <li> {@link #readChar()}
 *  </ul>
 *  <p>
 *  The first method returns true if standard input has more input (including whitespace).
 *  The second method reads and returns the next character of input on standard
 *  input (possibly a whitespace character).
 *  <p>
 *  As an example, the following code fragment reads characters from standard input,
 *  one character at a time, and prints it to standard output.
 *  <pre>
 *  while (StdIn.hasNextChar()) {
 *      char c = StdIn.readChar();
 *      StdOut.print(c);
 *  }
 *  </pre>
 *  <p>
 *  <b>Reading lines from standard input.</b>
 *  You can use the following two methods to read lines from standard input:
 *  <ul>
 *  <li> {@link #hasNextLine()}
 *  <li> {@link #readLine()}
 *  </ul>
 *  <p>
 *  The first method returns true if standard input has more input (including whitespace).
 *  The second method reads and returns the remaining portion of
 *  the next line of input on standard input (possibly whitespace),
 *  discarding the trailing line separator.
 *  <p>
 *  A <em>line separator</em> is defined to be one of the following strings:
 *  {@code \n} (Linux), {@code \r} (old Macintosh),
 *  {@code \r\n} (Windows),
 *  {@code \}{@code u2028}, {@code \}{@code u2029}, or {@code \}{@code u0085}.
 *  <p>
 *  As an example, the following code fragment reads text from standard input,
 *  one line at a time, and prints it to standard output.
 *  <pre>
 *  while (StdIn.hasNextLine()) {
 *      String line = StdIn.readLine();
 *      StdOut.println(line);
 *  }
 *  </pre>
 *  <p>
 *  <b>Reading a sequence of values of the same type from standard input.</b>
 *  You can use the following methods to read a sequence numbers, strings,
 *  or booleans (all of the same type) from standard input:
 *  <ul>
 *  <li> {@link #readAllDoubles()}
 *  <li> {@link #readAllInts()}
 *  <li> {@link #readAllLongs()}
 *  <li> {@link #readAllStrings()}
 *  <li> {@link #readAllLines()}
 *  <li> {@link #readAll()}
 *  </ul>
 *  <p>
 *  The first three methods read of all of remaining token on standard input
 *  and converts the tokens to values of
 *  the specified type, as in the corresponding
 *  {@code readDouble}, {@code readInt}, and {@code readString()} methods.
 *  The {@code readAllLines()} method reads all remaining lines on standard
 *  input and returns them as an array of strings.
 *  The {@code readAll()} method reads all remaining input on standard
 *  input and returns it as a string.
 *  <p>
 *  As an example, the following code fragment reads all of the remaining
 *  tokens from standard input and returns them as an array of strings.
 *  <pre>
 *  String[] words = StdIn.readAllStrings();
 *  </pre>
 *  <p>
 *  <b>Differences with Scanner.</b>
 *  {@code StdIn} and {@link Scanner} are both designed to parse
 *  tokens and convert them to primitive types and strings.
 *  The main differences are summarized below:
 *  <ul>
 *  <li> {@code StdIn} is a set of static methods and reads
 *       reads input from only standard input. It is suitable for use before
 *       a programmer knows about objects.
 *       See {@link In} for an object-oriented version that handles
 *       input from files, URLs,
 *       and sockets.
 *  <li> {@code StdIn} uses whitespace as the delimiter pattern
 *       that separates tokens.
 *       {@link Scanner} supports arbitrary delimiter patterns.
 *  <li> {@code StdIn} coerces the character-set encoding to UTF-8,
 *       which is the most widely used character encoding for Unicode.
 *  <li> {@code StdIn} coerces the locale to {@link Locale#US},
 *       for consistency with {@link StdOut}, {@link Double#parseDouble(String)},
 *       and floating-point literals.
 *  <li> {@code StdIn} has convenient methods for reading a single
 *       character; reading in sequences of integers, doubles, or strings;
 *       and reading in all of the remaining input.
 *  </ul>
 *  <p>
 *  Historical note: {@code StdIn} preceded {@code Scanner}; when
 *  {@code Scanner} was introduced, this class was re-implemented to use {@code Scanner}.
 *  <p>
 *  <b>Using standard input.</b>
 *  Standard input is a fundamental operating system abstraction on Mac OS X,
 *  Windows, and Linux.
 *  The methods in {@code StdIn} are <em>blocking</em>, which means that they
 *  will wait until you enter input on standard input.
 *  If your program has a loop that repeats until standard input is empty,
 *  you must signal that the input is finished.
 *  To do so, depending on your operating system and IDE,
 *  use either {@code <Ctrl-d>} or {@code <Ctrl-z>}, on its own line.
 *  If you are redirecting standard input from a file, you will not need
 *  to do anything to signal that the input is finished.
 *  <p>
 *  <b>Known bugs.</b>
 *  Java's UTF-8 encoding does not recognize the optional
 *  <a href = "http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4508058">byte-order mask</a>.
 *  If the input begins with the optional byte-order mask, {@code StdIn}
 *  will have an extra character {@code \}{@code uFEFF} at the beginning.
 *  <p>
 *  <b>Reference.</b>
 *  For additional documentation,
 *  see <a href="https://introcs.cs.princeton.edu/15inout">Section 1.5</a> of
 *  <em>Computer Science: An Interdisciplinary Approach</em>
 *  by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 *  @author David Pritchard
 */
public final class StdIn {

    /*** begin: section (1 of 2) of code duplicated from In to StdIn. */

    // assume Unicode UTF-8 encoding
    private static final String CHARSET_NAME = "UTF-8";

    // assume language = English, country = US for consistency with System.out.
    private static final Locale LOCALE = Locale.US;

    // the default token separator; we maintain the invariant that this value
    // is held by the scanner's delimiter between calls
    private static final Pattern WHITESPACE_PATTERN = Pattern.compile("\\p{javaWhitespace}+");

    // makes whitespace significant
    private static final Pattern EMPTY_PATTERN = Pattern.compile("");

    // used to read the entire input
    private static final Pattern EVERYTHING_PATTERN = Pattern.compile("\\A");

    /*** end: section (1 of 2) of code duplicated from In to StdIn. */

    private static Scanner scanner;

    // it doesn't make sense to instantiate this class
    private StdIn() { }

    //// begin: section (2 of 2) of code duplicated from In to StdIn,
    //// with all methods changed from "public" to "public static"

   /**
     * Returns true if standard input is empty (except possibly for whitespace).
     * Use this method to know whether the next call to {@link #readString()},
     * {@link #readDouble()}, etc. will succeed.
     *
     * @return {@code true} if standard input is empty (except possibly
     *         for whitespace); {@code false} otherwise
     */
    public static boolean isEmpty() {
        return !scanner.hasNext();
    }

   /**
     * Returns true if standard input has a next line.
     * Use this method to know whether the
     * next call to {@link #readLine()} will succeed.
     * This method is functionally equivalent to {@link #hasNextChar()}.
     *
     * @return {@code true} if standard input has more input (including whitespace);
     *         {@code false} otherwise
     */
    public static boolean hasNextLine() {
        return scanner.hasNextLine();
    }

    /**
     * Returns true if standard input has more input (including whitespace).
     * Use this method to know whether the next call to {@link #readChar()} will succeed.
     * This method is functionally equivalent to {@link #hasNextLine()}.
     *
     * @return {@code true} if standard input has more input (including whitespace);
     *         {@code false} otherwise
     */
    public static boolean hasNextChar() {
        scanner.useDelimiter(EMPTY_PATTERN);
        boolean result = scanner.hasNext();
        scanner.useDelimiter(WHITESPACE_PATTERN);
        return result;
    }


   /**
     * Reads and returns the next line, excluding the line separator if present.
     *
     * @return the next line, excluding the line separator if present;
     *         {@code null} if no such line
     */
    public static String readLine() {
        String line;
        try {
            line = scanner.nextLine();
        }
        catch (NoSuchElementException e) {
            line = null;
        }
        return line;
    }

    /**
     * Reads and returns the next character.
     *
     * @return the next {@code char}
     * @throws NoSuchElementException if standard input is empty
     */
    public static char readChar() {
        try {
            scanner.useDelimiter(EMPTY_PATTERN);
            String ch = scanner.next();
            assert ch.length() == 1 : "Internal (Std)In.readChar() error!"
                + " Please contact the authors.";
            scanner.useDelimiter(WHITESPACE_PATTERN);
            return ch.charAt(0);
        }
        catch (NoSuchElementException e) {
            throw new NoSuchElementException("attempts to read a 'char' value from standard input, "
                                           + "but no more tokens are available");
        }
    }

   /**
     * Reads and returns the remainder of the input, as a string.
     *
     * @return the remainder of the input, as a string
     * @throws NoSuchElementException if standard input is empty
     */
    public static String readAll() {
        if (!scanner.hasNextLine())
            return "";

        String result = scanner.useDelimiter(EVERYTHING_PATTERN).next();
        // not that important to reset delimiter, since now scanner is empty
        scanner.useDelimiter(WHITESPACE_PATTERN); // but let's do it anyway
        return result;
    }


   /**
     * Reads the next token from standard input and returns it as a {@code String}.
     *
     * @return the next {@code String}
     * @throws NoSuchElementException if standard input is empty
     */
    public static String readString() {
        try {
            return scanner.next();
        }
        catch (NoSuchElementException e) {
            throw new NoSuchElementException("attempts to read a 'String' value from standard input, "
                                           + "but no more tokens are available");
        }
    }

   /**
     * Reads the next token from standard input, parses it as an integer, and returns the integer.
     *
     * @return the next integer on standard input
     * @throws NoSuchElementException if standard input is empty
     * @throws InputMismatchException if the next token cannot be parsed as an {@code int}
     */
    public static int readInt() {
        try {
            return scanner.nextInt();
        }
        catch (InputMismatchException e) {
            String token = scanner.next();
            throw new InputMismatchException("attempts to read an 'int' value from standard input, "
                                           + "but the next token is \"" + token + "\"");
        }
        catch (NoSuchElementException e) {
            throw new NoSuchElementException("attemps to read an 'int' value from standard input, "
                                           + "but no more tokens are available");
        }

    }

   /**
     * Reads the next token from standard input, parses it as a double, and returns the double.
     *
     * @return the next double on standard input
     * @throws NoSuchElementException if standard input is empty
     * @throws InputMismatchException if the next token cannot be parsed as a {@code double}
     */
    public static double readDouble() {
        try {
            return scanner.nextDouble();
        }
        catch (InputMismatchException e) {
            String token = scanner.next();
            throw new InputMismatchException("attempts to read a 'double' value from standard input, "
                                           + "but the next token is \"" + token + "\"");
        }
        catch (NoSuchElementException e) {
            throw new NoSuchElementException("attempts to read a 'double' value from standard input, "
                                           + "but no more tokens are available");
        }
    }

   /**
     * Reads the next token from standard input, parses it as a float, and returns the float.
     *
     * @return the next float on standard input
     * @throws NoSuchElementException if standard input is empty
     * @throws InputMismatchException if the next token cannot be parsed as a {@code float}
     */
    public static float readFloat() {
        try {
            return scanner.nextFloat();
        }
        catch (InputMismatchException e) {
            String token = scanner.next();
            throw new InputMismatchException("attempts to read a 'float' value from standard input, "
                                           + "but the next token is \"" + token + "\"");
        }
        catch (NoSuchElementException e) {
            throw new NoSuchElementException("attempts to read a 'float' value from standard input, "
                                           + "but there no more tokens are available");
        }
    }

   /**
     * Reads the next token from standard input, parses it as a long integer, and returns the long integer.
     *
     * @return the next long integer on standard input
     * @throws NoSuchElementException if standard input is empty
     * @throws InputMismatchException if the next token cannot be parsed as a {@code long}
     */
    public static long readLong() {
        try {
            return scanner.nextLong();
        }
        catch (InputMismatchException e) {
            String token = scanner.next();
            throw new InputMismatchException("attempts to read a 'long' value from standard input, "
                                           + "but the next token is \"" + token + "\"");
        }
        catch (NoSuchElementException e) {
            throw new NoSuchElementException("attempts to read a 'long' value from standard input, "
                                           + "but no more tokens are available");
        }
    }

   /**
     * Reads the next token from standard input, parses it as a short integer, and returns the short integer.
     *
     * @return the next short integer on standard input
     * @throws NoSuchElementException if standard input is empty
     * @throws InputMismatchException if the next token cannot be parsed as a {@code short}
     */
    public static short readShort() {
        try {
            return scanner.nextShort();
        }
        catch (InputMismatchException e) {
            String token = scanner.next();
            throw new InputMismatchException("attempts to read a 'short' value from standard input, "
                                           + "but the next token is \"" + token + "\"");
        }
        catch (NoSuchElementException e) {
            throw new NoSuchElementException("attempts to read a 'short' value from standard input, "
                                           + "but no more tokens are available");
        }
    }

   /**
     * Reads the next token from standard input, parses it as a byte, and returns the byte.
     *
     * @return the next byte on standard input
     * @throws NoSuchElementException if standard input is empty
     * @throws InputMismatchException if the next token cannot be parsed as a {@code byte}
     */
    public static byte readByte() {
        try {
            return scanner.nextByte();
        }
        catch (InputMismatchException e) {
            String token = scanner.next();
            throw new InputMismatchException("attempts to read a 'byte' value from standard input, "
                                           + "but the next token is \"" + token + "\"");
        }
        catch (NoSuchElementException e) {
            throw new NoSuchElementException("attempts to read a 'byte' value from standard input, "
                                           + "but no more tokens are available");
        }
    }

    /**
     * Reads the next token from standard input, parses it as a boolean,
     * and returns the boolean.
     *
     * @return the next boolean on standard input
     * @throws NoSuchElementException if standard input is empty
     * @throws InputMismatchException if the next token cannot be parsed as a {@code boolean}:
     *    {@code true} or {@code 1} for true, and {@code false} or {@code 0} for false,
     *    ignoring case
     */
    public static boolean readBoolean() {
        try {
            String token = readString();
            if ("true".equalsIgnoreCase(token))  return true;
            if ("false".equalsIgnoreCase(token)) return false;
            if ("1".equals(token))               return true;
            if ("0".equals(token))               return false;
            throw new InputMismatchException("attempts to read a 'boolean' value from standard input, "
                                           + "but the next token is \"" + token + "\"");
        }
        catch (NoSuchElementException e) {
            throw new NoSuchElementException("attempts to read a 'boolean' value from standard input, "
                                           + "but no more tokens are available");
        }

    }

    /**
     * Reads all remaining tokens from standard input and returns them as an array of strings.
     *
     * @return all remaining tokens on standard input, as an array of strings
     */
    public static String[] readAllStrings() {
        // we could use readAll.trim().split(), but that's not consistent
        // because trim() uses characters 0x00..0x20 as whitespace
        String[] tokens = WHITESPACE_PATTERN.split(readAll());
        if (tokens.length == 0 || tokens[0].length() > 0)
            return tokens;

        // don't include first token if it is leading whitespace
        String[] decapitokens = new String[tokens.length-1];
        for (int i = 0; i < tokens.length - 1; i++)
            decapitokens[i] = tokens[i+1];
        return decapitokens;
    }

    /**
     * Reads all remaining lines from standard input and returns them as an array of strings.
     * @return all remaining lines on standard input, as an array of strings
     */
    public static String[] readAllLines() {
        ArrayList<String> lines = new ArrayList<String>();
        while (hasNextLine()) {
            lines.add(readLine());
        }
        return lines.toArray(new String[0]);
    }

    /**
     * Reads all remaining tokens from standard input, parses them as integers, and returns
     * them as an array of integers.
     * @return all remaining integers on standard input, as an array
     * @throws InputMismatchException if any token cannot be parsed as an {@code int}
     */
    public static int[] readAllInts() {
        String[] fields = readAllStrings();
        int[] vals = new int[fields.length];
        for (int i = 0; i < fields.length; i++)
            vals[i] = Integer.parseInt(fields[i]);
        return vals;
    }

    /**
     * Reads all remaining tokens from standard input, parses them as longs, and returns
     * them as an array of longs.
     * @return all remaining longs on standard input, as an array
     * @throws InputMismatchException if any token cannot be parsed as a {@code long}
     */
    public static long[] readAllLongs() {
        String[] fields = readAllStrings();
        long[] vals = new long[fields.length];
        for (int i = 0; i < fields.length; i++)
            vals[i] = Long.parseLong(fields[i]);
        return vals;
    }

    /**
     * Reads all remaining tokens from standard input, parses them as doubles, and returns
     * them as an array of doubles.
     * @return all remaining doubles on standard input, as an array
     * @throws InputMismatchException if any token cannot be parsed as a {@code double}
     */
    public static double[] readAllDoubles() {
        String[] fields = readAllStrings();
        double[] vals = new double[fields.length];
        for (int i = 0; i < fields.length; i++)
            vals[i] = Double.parseDouble(fields[i]);
        return vals;
    }

    //// end: section (2 of 2) of code duplicated from In to StdIn

    // do this once when StdIn is initialized
    static {
        resync();
    }

    /**
     * If StdIn changes, use this to reinitialize the scanner.
     */
    private static void resync() {
        setScanner(new Scanner(new java.io.BufferedInputStream(System.in), CHARSET_NAME));
    }

    private static void setScanner(Scanner scanner) {
        StdIn.scanner = scanner;
        StdIn.scanner.useLocale(LOCALE);
    }

   /**
     * Reads all remaining tokens, parses them as integers, and returns
     * them as an array of integers.
     * @return all remaining integers, as an array
     * @throws InputMismatchException if any token cannot be parsed as an {@code int}
     * @deprecated Replaced by {@link #readAllInts()}.
     */
    @Deprecated
    public static int[] readInts() {
        return readAllInts();
    }

   /**
     * Reads all remaining tokens, parses them as doubles, and returns
     * them as an array of doubles.
     * @return all remaining doubles, as an array
     * @throws InputMismatchException if any token cannot be parsed as a {@code double}
     * @deprecated Replaced by {@link #readAllDoubles()}.
     */
    @Deprecated
    public static double[] readDoubles() {
        return readAllDoubles();
    }

   /**
     * Reads all remaining tokens and returns them as an array of strings.
     * @return all remaining tokens, as an array of strings
     * @deprecated Replaced by {@link #readAllStrings()}.
     */
    @Deprecated
    public static String[] readStrings() {
        return readAllStrings();
    }


    /**
     * Interactive test of basic functionality.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {

        StdOut.print("Type a string: ");
        String s = StdIn.readString();
        StdOut.println("Your string was: " + s);
        StdOut.println();

        StdOut.print("Type an int: ");
        int a = StdIn.readInt();
        StdOut.println("Your int was: " + a);
        StdOut.println();

        StdOut.print("Type a boolean: ");
        boolean b = StdIn.readBoolean();
        StdOut.println("Your boolean was: " + b);
        StdOut.println();

        StdOut.print("Type a double: ");
        double c = StdIn.readDouble();
        StdOut.println("Your double was: " + c);
        StdOut.println();
    }

}