The Standard Input, Output, and Error Streams in Java

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The Standard Input, Output, and Error Streams
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The standard output stream (usually the display) is represented by the PrintStream object Systemout The standard input stream (usually the keyboard) is represented by the InputStream object Systemin In other words, it is a byte input stream The standard error stream (also usually the display) is represented by Systemerr which is another object of the PrintStream class The PrintStream class offers print() methods which act as corresponding print() methods from the PrintWriter class These methods can be used to write output to Systemout and Systemerr In other words, both Systemout and Systemerr act like PrintWriter, but in addition they have write() methods for writing bytes
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CHAPTER 11: FILES AND STREAMS
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In order to read characters typed by the user, the Console class is recommended (see the next section)
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Comparison of Byte Streams and Character Streams
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It is instructive to see which byte streams correspond to which character streams Table 117 shows the correspondence between byte and character streams Note that not all classes have a corresponding counterpart
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Table 117
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Correspondence Between Selected Byte and Character Streams Byte Streams
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OutputStream InputStream
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Character Streams
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Writer Reader OutputStreamWriter InputStreamReader FileWriter FileReader BufferedWriter BufferedReader PrintWriter
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No counterpart No counterpart
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FileOutputStream FileInputStream BufferedOutputStream BufferedInputStream PrintStream DataOutputStream DataInputStream ObjectOutputStream ObjectInputStream
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No counterpart No counterpart No counterpart No counterpart
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115 The Console Class
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A console is a unique character-based device associated with a JVM Whether a JVM has a console depends on the platform, and also on the manner in which the JVM is invoked When the JVM is started from a command line, and the standard input and output streams have not been redirected, the console will normally correspond to the keyboard and the display (Figure 118) In any case, the console will be represented by an instance of the class Console This Console instance is obtained by calling the static method console() of the System class If there is no console associated with the JVM, the null value is returned by this method
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// Obtaining the console: Console console = Systemconsole();
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115: THE Console CLASS if (console == null) { Systemerrprintln("No console available"); return; } // Continue Figure 118
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Keyboard and Display as Console
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Object of class PrintWriter Console
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I t ' s n ow o r n e ve r
Flaming
Display
Star M i n d s
S u s p i o u s
format() printf()
Angel
String readLine() char[] readPassword()
Keyboard
Object of class Reader
For creating dialog for console-based applications, the Console class provides the following functionality: Prompt and read a line of character-based response
String username = consolereadLine("Enter the user name (%d chars): ", 4);
The readLine() method first prints the formatted prompt on the console, and then returns the characters typed at the console when the line is terminated by the ENTER key Prompt and read passwords without echoing the characters on the console
char[] password; do { password = consolereadPassword("Enter password (min %d chars): ", 6); } while (passwordlength < 6);
The readPassword() method first prints the formatted prompt, and returns the password characters typed by the user in an array of char when the line is terminated by the ENTER key The password characters are not echoed on the display Since a password is sensitive data, the recommended practice is to have it stored in memory only as long as it is necessary and to zero-fill the char array as soon as possible in order to overwrite the password characters Print formatted strings to the console The Console class provides the format() and the printf() methods for this purpose Using these methods and creating formatted strings are covered in Section 127, p 593
CHAPTER 11: FILES AND STREAMS
Note that the console only returns character-based input For reading other types of values from the standard input stream, the Scanner class (p 571) can be considered The Console class provides methods for formatted prompting and reading from the console, and obtaining the reader associated with it
String readLine() String readLine(String format, Object args)
The first method reads a single line of text from the console The second method prints a formatted prompt first, then reads a single line of text from the console The prompt is constructed by formatting the specified args according to the specified format
char[] readPassword() char[] readPassword(String format, Object args)
The first method reads a password or a password phrase from the console with echoing disabled The second method does the same, but first prints a formatted prompt
Reader reader()
This retrieves the unique Reader object associated with this console The Console class provides the following methods for writing formatted strings to the console, and obtaining the writer associated with it:
Console format(String format, Object args) Console printf(String format, Object args)
These methods write a formatted string to this console's output stream using the specified format string and arguments, according to the default locale
PrintWriter writer()