Class IO

java.lang.Object

java.lang.IO

public final class IO extends Object

A collection of static methods that provide convenient access to System.in and System.out for line-oriented input and output.

The readln() and readln(String) methods decode bytes read from System.in into characters. The charset used for decoding is specified by the stdin.encoding property. If this property is not present, or if the charset it names cannot be loaded, then UTF-8 is used instead. Decoding always replaces malformed and unmappable byte sequences with the charset's default replacement string.

Charset decoding is set up upon the first call to one of the readln methods. Decoding may buffer additional bytes beyond those that have been decoded to characters returned to the application. After the first call to one of the readln methods, any subsequent use of System.in results in unspecified behavior.

API Note:: The expected use case is that certain applications will use only the readln methods to read from the standard input, and they will not mix these calls with other techniques for reading from System.in.
Since:: 25

Method Summary

Modifier and Type	Method	Description
`static void`	`print(Object obj)`	Writes a string representation of the specified object to the standard output.
`static void`	`println()`	Writes a line separator to the standard output.
`static void`	`println(Object obj)`	Writes a string representation of the specified object and then writes a line separator to the standard output.
`static String`	`readln()`	Reads a single line of text from the standard input.
`static String`	`readln(String prompt)`	Writes a prompt and then reads a line of input.

Methods declared in class Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Details

println

public static void println(Object obj)

Writes a string representation of the specified object and then writes a line separator to the standard output.

The effect is as if println(obj) had been called on System.out.

Parameters:: obj - the object to print, may be null

println

public static void println()

Writes a line separator to the standard output.

The effect is as if println() had been called on System.out.

print

public static void print(Object obj)

Writes a string representation of the specified object to the standard output.

The effect is as if print(obj) had been called on System.out.

Parameters:: obj - the object to print, may be null

readln

public static String readln()

Reads a single line of text from the standard input.

One line is read from the decoded input as if by BufferedReader.readLine() and then the result is returned.

If necessary, this method first sets up charset decoding, as described in above in the class specification.

Returns:: a string containing the line read from the standard input, not including any line separator characters. Returns null if an end of stream has been reached without having read any characters.
Throws:: IOError - if an I/O error occurs

readln

public static String readln(String prompt)

Writes a prompt and then reads a line of input.

Writes a prompt as if by calling print, and then reads a single line of text as if by calling readln.

If necessary, this method first sets up charset decoding, as described in above in the class specification.

Parameters:: prompt - the prompt string, may be null
Returns:: a string containing the line read from the standard input, not including any line separator characters. Returns null if an end of stream has been reached without having read any characters.
Throws:: IOError - if an I/O error occurs

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/IO.html