Fundamental Programming Structures in Java

If you run this program from a Windows terminal, special characters in your name may not show up correctly. By default, Windows terminals use an archaic character encoding. To fix this, switch the terminal to the UTF-8 encoding, by issuing the following command prior to running the program:

chcp 65001

Then, if you use Java 18 or above, all will be well. With older versions of Java, run the program as:

java -Dfile.encoding=utf-8 InputDemo

If you use a development environment, you should not have to worry about this issue.

Prior to Java 25, reading console input was not so easy. To use an older version of Java, make these adaptations:

First first construct a Scanner object that is attached to System.in:

Scanner in = new Scanner(System.in);

(Objects, constructors, and the new operator are discussed in detail in Chapter 4.)

The nextLine method reads a line of input.

System.out.print("What is your name? ");
String name = in.nextLine();

To read an integer, use the nextInt method.

System.out.print("How old are you? ");
int age = in.nextInt();

Similarly, the nextDouble method reads the next floating-point number.

Finally, include the line

import java.util.Scanner;

at the beginning of the program, to tell the compiler that the Scanner class is defined in the java.util package. Packages and import directives are covered in more detail in Chapter 4.

The parseInt and parseDouble methods are not intended for parsing user input, and IO.println is not intended for presenting numbers to a general audience. They use the number format for decimal Java literals. That’s ok for sample programs in a programming book. However, most users expect to see the decimal digits and separators to which they are accustomed.

To parse human input, use the nextInt and nextDouble methods of the Scanner class. For output, use the formatted method that you will see in Section 3.7.2. These methods use the number format of the host system.

The IO.readLine method is not suitable for reading a password from a console since the input is plainly visible to anyone. Use the readPassword method of the Console class to read a password while hiding the user input:

String username = System.console().readLine("User name: ");
char[] passwd = System.console().readPassword("Password: ");
. . .
Arrays.fill(passwd, '*');

For security reasons, the password is returned in an array of characters rather than a string. After you are done processing the password, you should immediately overwrite the array elements with a filler value.

java.lang.IO25

• println(Object obj)

  Converts the object to a string and prints it on the console, followed by a line separator.

• print(Object ob)

  Converts the object to a string and prints it on the console without a line separator.

• println()

  Prints a line separator.

• String readln(String prompt)

  Prints a prompt on the console and returns one line of user input.

• String readln()

  Returns one line of user input without printing a prompt.

java.util.Scanner5.0

• Scanner(InputStream in)

  constructs a Scanner object from the given input stream.

• String nextLine()

  reads the next line of input.

• String next()

  reads the next word of input (delimited by whitespace).

• int nextInt()

• double nextDouble()

  read and convert the next character sequence that represents an integer or floating-point number.

• boolean hasNext()

  tests whether there is another word in the input.

• boolean hasNextInt()

• boolean hasNextDouble()

  test whether the next character sequence represents an integer or floating-point number.

java.lang.System1.0

• static Console console()6

  returns a Console object for interacting with the user through a console window if such interaction is possible, null otherwise. A Console object is available for any program that is launched in a console window. Otherwise, the availability is system-dependent.

java.io.Console6

• char[] readPassword(String prompt, Object... args)

• String readLine(String prompt, Object... args)

  display the prompt and read the user input until the end of the input line. The optional args parameters are used to supply formatting arguments, as described in the next section.

You can use the s conversion to format arbitrary objects. If an arbitrary object implements the Formattable interface, the object’s formatTo method is invoked. Otherwise, the toString method is invoked to turn the object into a string. The toString method is discussed in Chapter 5 and interfaces in Chapter 6.

Formatting is locale-specific. For example, in Germany, the group separator is a period, not a comma. On a computer with a German locale, the call

double x = 10000.0 / 3.0;
IO.print("%8.2f".formatted(x));

yields the output

 3333,33

This locale-specific behavior is normally what you want when you communicate with users. However, if you produce a file that is later consumed by a computer program, you may need to choose a fixed locale for the output. Specify the locale as the first argument to the static format method of the String class:

IO.print(String.format(Locale.US, "%8.2f", x));