OpenJDK 27 · java.lang.IO · APIdia, the JavaDoc alternative

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.

Modifier and Type	Field and Description
private static BufferedReader	br The BufferedReader used by readln().

Access	Constructor and Description
private	IO()

Method Summary

Modifier and Type	Method and Description
public static void	print(Object the object to print, may be `null` obj) Writes a string representation of the specified object to the standard output.
public static void	println(Object the object to print, may be `null` obj) Writes a string representation of the specified object and then writes a line separator to the standard output.
public static void	println() Writes a line separator to the standard output.
pack-priv static synchronized BufferedReader	Returns: the internal BufferedReader instance reader() On the first call, creates an InputStreamReader to decode characters from System.in, wraps it in a BufferedReader, and returns the BufferedReader.
public static String	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. readln() Reads a single line of text from the standard input.
public static String	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. readln(String the prompt string, may be `null` prompt) Writes a prompt and then reads a line of input.

Inherited from java.lang.Object:: clone equals finalize getClass hashCode notify notifyAll toString wait wait wait

Field Detail

br	back to summary
private static BufferedReader br The BufferedReader used by readln(). Initialized under a class lock by the reader() method. All access should be through the reader() method.

Constructor Detail

IO	back to summary
private IO()

Method Detail

print back to summary

print	back to summary
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:Object the object to print, may be `null`

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:Object: the object to print, may be null

println back to summary

println	back to summary
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:Object the object to print, may be `null`

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:Object: the object to print, may be null

println back to summary

println	back to summary
public static void println() Writes a line separator to the standard output. The effect is as if `println()` had been called on `System.out`.

public static void println()

Writes a line separator to the standard output.

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

reader	back to summary
pack-priv static synchronized BufferedReader reader() On the first call, creates an InputStreamReader to decode characters from System.in, wraps it in a BufferedReader, and returns the BufferedReader. These objects are cached and returned by subsequent calls. Returns:BufferedReader the internal BufferedReader instance

reader

back to summary

pack-priv static synchronized BufferedReader reader()

On the first call, creates an InputStreamReader to decode characters from System.in, wraps it in a BufferedReader, and returns the BufferedReader. These objects are cached and returned by subsequent calls.

Returns:BufferedReader: the internal BufferedReader instance

readln back to summary

readln	back to summary
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:String 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. Exceptions IOError: if an I/O error occurs

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:String

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.

Exceptions

IOError:: if an I/O error occurs

readln back to summary

readln	back to summary
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:String the prompt string, may be `null` Returns:String 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. Exceptions IOError: if an I/O error occurs

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:String: the prompt string, may be null

Returns:String

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.

Exceptions

IOError:: if an I/O error occurs

public final Class IO

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