Java 101: The ins and outs of standard input/output

Acquire the basics of standard input, standard output, and standard mistake

In prior Java 101 articles, I referred to the concepts of redirection, standard input device, and standard output device. To demonstrate inputting information, several examples called System.in.read(). It turns out that Arrangement.in.read() inputs information from the standard input device. To demonstrate outputting information, examples called System.out.impress() and System.out.println(). In contrast to System.in.read(), those methods -- named sequences of executable code (to be explored in next calendar month'southward article) -- ship their output to the standard output device. Desire to know more most standard I/O concepts? Read on!

Standard I/O is a standardized input/output machinery that originates from the Unix operating system. Although this mechanism is mostly used with older non-GUI operating systems, standard I/O still plays a part in mod GUI (graphical user interface) operating systems, where people use it to debug malfunctioning programs and to teach input/output in entry-level programming courses.

As you've probably guessed, standard I/O uses devices for inputting and outputting data. These devices include standard input, standard output, and standard error.

Standard input

The standard input device is that function of the operating system that controls from where a program receives its input. Past default, the standard input device reads that input from a device commuter attached to the keyboard. Yet, you can redirect, or switch, the input source to a device driver attached to a file and so that input seems to exist "magically" coming from a file -- instead of the keyboard.

A program inputs its data from the standard input device by calling Java's System.in.read() method. Look in the SDK documentation and y'all'll observe a form called Organization. That class contains a variable chosen in -- an object created from a bracket of InputStream. The period character after Organization states that in belongs to System, and the period character after in states that read() belongs to in. In other words, read() is a method that belongs to an object called in, which in turn belongs to a class called Arrangement. (I will hash out more nearly classes, objects, and "belonging to" next month.)

System.in.read() takes no arguments and returns an integer, which has led some to believe that Arrangement.in.read() returns user-entered integer numbers. To analyze, Organization.in.read() either returns a key's vii-scrap ASCII lawmaking (if the standard input device is set to the keyboard) or an eight-bit byte from a file (if the standard input device has been redirected from the keyboard to a file). In either case, Organization.in.read() converts the code to a 32-bit integer and returns the result.

Presume that the standard input device is set to the keyboard. The following is a description of what happens under Windows: When y'all type a central on a Windows-controlled keyboard, the operating arrangement stores that fundamental's 7-bit ASCII lawmaking in an internal key buffer. That key buffer holds up to roughly 16 ASCII codes and is organized as a first-in/outset-out circular queue data structure. System.in.read() retrieves the ASCII code from the head of the key buffer and then removes that code from the fundamental buffer. That 7-scrap ASCII code then converts to an int -- by System.in.read() prepending 25 nix bits to the lawmaking -- and returns to the method's caller. A second System.in.read() method call retrieves the side by side ASCII code, which is at present at the head of the key buffer, and and then on.

Suppose there are no ASCII codes in the key buffer. What happens? System.in.read() waits for the user to blazon keys and press the terminator. Under Windows, that terminator is the Enter key. Pressing Enter causes Windows to store a carriage return lawmaking (ASCII 13) followed by a new-line code (ASCII 10) in the key buffer. Therefore, the key buffer might contain several ASCII codes followed by a carriage return and a new-line character. The first of those codes returns from System.in.read(). Check out that activity by keying in, compiling, and running the Echo awarding; its source lawmaking appears in List 1.

List 1. Echo.coffee 

// Echo.java class Echo {    public static void primary (String [] args) throws java.io.IOException    {       int ch;       Organization.out.print ("Enter some text: ");       while ((ch = System.in.read ()) != '\north')          System.out.print ((char) ch);    } }

Echo completes the following steps:

  1. Calls the Arrangement.out.print() method, which takes a Cord argument, to output a prompt
  2. Calls System.in.read() to input ASCII codes from the standard input device as 32-fleck integers
  3. Converts those 32-scrap integers to 16-chip Unicode characters by way of the (char) cast
  4. Calls the System.out.print() method, which takes a char argument, to echo those Unicode characters to the standard output device

The concluding three steps in the previous four steps have place in a while loop, and continue until a new-line character is read. To run Echo and so that it inputs from the keyboard and outputs to the screen, issue the post-obit command line: java Repeat.

Although System.in.read() never throws an exception (see the give-and-take-counting topic in this article for a definition of that term), when the standard input device is set to the keyboard, it might throw an exception when you redirect the standard input device from the keyboard to a file. For instance, suppose you redirect the standard input device to a file, and Organization.in.read() reads content from the file. Now suppose that the file is situated on a floppy disk, and the user ejects that disk during the read operation. When the ejection takes identify, Organisation.in.read() throws an exception, informing the program that it can't read the file. That provides the reason for appending the throws java.io.IOException clause to the main() method header. (You volition explore exceptions, throwing exceptions, and related concepts in a future article.)

How exercise you redirect the standard input device and then that input originates from a file? The answer is to introduce a less-than sign, <, on the command line and follow that symbol with a filename. To run across how that works, consequence the following command line: java Echo <Echo.coffee. The control line redirects the standard input device to a file chosen Echo.java. When Repeat runs, because each line ends in a new-line grapheme, merely the first line of text in Echo.java appears on the screen.

Suppose you demand a utility programme that reads an entire file and either displays the file's contents on the screen, copies those contents to another file, or copies those contents to a printer. Unfortunately, the Echo programme only performs that job until information technology encounters the first new-line grapheme. What exercise you exercise? The answer to the problem lies in the Blazon application. Listing 2 provides the source code:

List 2. Type.java 

// Blazon.coffee class Blazon {    public static void main (String [] args) throws java.io.IOException    {       int ch;       while ((ch = Organisation.in.read ()) != -i)          System.out.print ((char) ch);    } }

Type resembles Echo, however, there is no prompt, and the while loop tests against -1 (which indicates finish of file) instead of \n (which indicates end of line). To run Type, issue the following command line: java Type <Blazon.java. The contents of Type.java -- or whatsoever file is specified -- will display. As an experiment, try specifying java Type. What practice you recollect volition happen? (Hint: this program resembles Echo but doesn't terminate until you press Ctrl+C.)

Earlier, I mentioned that some programmers mistakenly retrieve that Organisation.in.read() returns a user-entered number. Equally you've just seen, that isn't the case. But what must you do if you want to employ System.in.read() to recollect a number? Have a look at the Catechumen awarding, whose source lawmaking is presented in Listing 3.

Listing 3. Convert.coffee 

// Convert.java class Convert {    public static void principal (Cord [] args) throws java.io.IOException    {       System.out.impress ("Please enter a number: ");       int num = 0;       int ch;       while ((ch = System.in.read ()) != '\n')          if (ch >= '0' && ch <= '9')          {              num *= 10;              num += ch - '0';          }          else              suspension;       Organization.out.println ("num = " + num);       Organisation.out.println ("num squared = " + num * num);    } }

Listing 3'southward Catechumen plan prompts the user to enter a number (via Organization.out.print ("Delight enter a number: ");). It reads these digits -- one at a time -- and converts each digit'due south numeric code to a binary number that is added to a variable called num. Finally, calls to Organization.out.println() output the value within num and the foursquare of that value to the standard output device.

Catechumen demonstrates the fourth dimension-honored technique of using a while loop to test for a digit, premultiplying a variable past x (to make room for the incoming digit), converting a digit to its binary equivalent, and adding that binary equivalent to the variable. However, that technique is non a sound technique to use if you're writing a program for deployment in different countries equally some countries utilise digits other than 0 through 9 -- such as Tamil digits. To make the program operate with other digits, you need to aggrandize the if statement to examination for those digits and alter the ch - '0' expression. Fortunately, Java simplifies that task past providing a Character class, which you'll explore in a future commodity.

Standard output

The standard output device is that part of the operating organisation that controls where a program sends its output. By default, the standard output device sends the output to a device driver fastened to the screen. Nevertheless, the output destination can be redirected to a device driver attached to a file or printer, which results in the same program displaying its findings on the screen, saving them in a file, or providing a hardcopy listing of the results.

You reach standard output by calling Java'southward Organization.out.print() and System.out.println() methods. Except for the fact that print() methods don't output a new-line character later on the data, the two method groups are equivalent. Methods exist to output Boolean, graphic symbol, character assortment, double-precision floating-bespeak, floating-point, integer, long integer, string, and object values. To demonstrate these methods, Listing 4 presents source code to the Print application.

Listing iv. Impress.coffee 

// Impress.java course Impress {    public static void chief (Cord [] args)    {       boolean b = truthful;       System.out.println (b);       char c = 'A';       System.out.println (c);       char [] carray = { 'A', 'B', 'C' };       System.out.println (carray);       double d = 3.5;       Organization.out.println (d);       float f = -9.3f;       Organisation.out.println (f);       int i = 'X';       Arrangement.out.println (i);       long l = 9000000;       System.out.println (l);       Cord s = "abc";       Organization.out.println (s);       Arrangement.out.println (new Print ());    } }

Listing 4 has probably triggered some questions for you. Start, what is all that System.out. business doing in forepart of println()? Again, refer to the Organization grade in the SDK documentation. The grade contains a variable chosen out -- an object created from a course called PrintStream. The menses character after System indicates that out belongs to System. The menstruation graphic symbol subsequently out states that println() belongs to out. In other words, println() is a method that belongs to an object called out, which in plough belongs to a class called System.

The second question you might be asking yourself involves println() argument data types: how is information technology possible for the same println() method to be chosen with different types of argument data? The answer: because there are several println() methods in the PrintStream grade. At runtime, the JVM knows which println() method to call past examining the number of method-phone call arguments and their data types. (Declaring several methods with the same name but different numbers of arguments and argument data types is known as method overloading. I will talk over that concept side by side calendar month.)

Finally, you lot might be wondering about System.out.println (new Print ());. That method phone call illustrates the println() method, which takes an Object argument. First, the creation operator new creates an object from the Print class and returns a reference to -- also known equally the accost of -- that object. Finally, that address passes as an argument to the println() method, which takes an Object argument. The method converts the object's contents to a string and outputs that string. By default, the cord consists of the proper noun of the object'south course, followed past an @ (at) graphic symbol, followed past a hexadecimal-formatted integer that represents the object's hashcode. (I will present hashcodes and the conversion of objects to strings in an upcoming commodity.)

Compile Impress.java and run the program by issuing the following command line: java Print. You lot should meet 9 lines of output. Redirect that output to the out.dat file by issuing the following command line: coffee Print >out.dat. You can now view the contents of the file.

The greater-than sign, >, indicates standard output redirection. Whenever you want to redirect the standard output device from the screen to a file or printer, specify that symbol followed by the file or printer name on the command line. For example, redirect Print's output to a Windows printer by issuing the following command line: java Print >prn.