jdk/src/java.base/share/classes/java/io/IO.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * accompanied this code).
 *
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package java.io;

import jdk.internal.javac.PreviewFeature;

/**
 * A collection of static convenience methods that provide access to
 * {@linkplain System#console() system console} for implicitly declared classes.
 *
 * <p> Each of this class' methods throws {@link IOError} if the system console
 * is {@code null}; otherwise, the effect is as if a similarly-named method
 * had been called on that console.
 *
 * <p> Input and output from methods in this class use the character set of
 * the system console as specified by {@link Console#charset}.
 *
 * @since 23
 */
@PreviewFeature(feature = PreviewFeature.Feature.IMPLICIT_CLASSES)
public final class IO {

    private IO() {
        throw new Error("no instances");
    }

    /**
     * Writes a string representation of the specified object to the system
     * console, terminates the line and then flushes that console.
     *
     * <p> The effect is as if {@link Console#println(Object) println(obj)}
     * had been called on {@code System.console()}.
     *
     * @param obj the object to print, may be {@code null}
     *
     * @throws IOError if {@code System.console()} returns {@code null},
     *                 or if an I/O error occurs
     */
    public static void println(Object obj) {
        con().println(obj);
    }

    /**
     * Writes a string representation of the specified object to the system
     * console and then flushes that console.
     *
     * <p> The effect is as if {@link Console#print(Object) print(obj)}
     * had been called on {@code System.console()}.
     *
     * @param obj the object to print, may be {@code null}
     *
     * @throws IOError if {@code System.console()} returns {@code null},
     *                 or if an I/O error occurs
     */
    public static void print(Object obj) {
        con().print(obj);
    }

    /**
     * Writes a prompt as if by calling {@code print}, then reads a single line
     * of text from the system console.
     *
     * <p> The effect is as if {@link Console#readln(String) readln(prompt)}
     * had been called on {@code System.console()}.
     *
     * @param prompt the prompt string, may be {@code null}
     *
     * @return a string containing the line read from the system console, not
     * including any line-termination characters. Returns {@code null} if an
     * end of stream has been reached without having read any characters.
     *
     * @throws IOError if {@code System.console()} returns {@code null},
     *                 or if an I/O error occurs
     */
    public static String readln(String prompt) {
        return con().readln(prompt);
    }

    private static Console con() {
        var con = System.console();
        if (con != null) {
            return con;
        } else {
            throw new IOError(null);
        }
    }
}