Java tutorial
/* * Copyright (c) 1996, 2018, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * 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 * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.io; import java.util.Objects; /** * Abstract class for writing to character streams. The only methods that a * subclass must implement are write(char[], int, int), flush(), and close(). * Most subclasses, however, will override some of the methods defined here in * order to provide higher efficiency, additional functionality, or both. * * @see BufferedWriter * @see CharArrayWriter * @see FilterWriter * @see OutputStreamWriter * @see FileWriter * @see PipedWriter * @see PrintWriter * @see StringWriter * @see Reader * * @author Mark Reinhold * @since 1.1 */ public abstract class Writer implements Appendable, Closeable, Flushable { /** * Temporary buffer used to hold writes of strings and single characters */ private char[] writeBuffer; /** * Size of writeBuffer, must be >= 1 */ private static final int WRITE_BUFFER_SIZE = 1024; /** * Returns a new {@code Writer} which discards all characters. The * returned stream is initially open. The stream is closed by calling * the {@code close()} method. Subsequent calls to {@code close()} have * no effect. * * <p> While the stream is open, the {@code append(char)}, {@code * append(CharSequence)}, {@code append(CharSequence, int, int)}, * {@code flush()}, {@code write(int)}, {@code write(char[])}, and * {@code write(char[], int, int)} methods do nothing. After the stream * has been closed, these methods all throw {@code IOException}. * * <p> The {@link #lock object} used to synchronize operations on the * returned {@code Writer} is not specified. * * @return a {@code Writer} which discards all characters * * @since 11 */ public static Writer nullWriter() { return new Writer() { private volatile boolean closed; private void ensureOpen() throws IOException { if (closed) { throw new IOException("Stream closed"); } } @Override public Writer append(char c) throws IOException { ensureOpen(); return this; } @Override public Writer append(CharSequence csq) throws IOException { ensureOpen(); return this; } @Override public Writer append(CharSequence csq, int start, int end) throws IOException { ensureOpen(); if (csq != null) { Objects.checkFromToIndex(start, end, csq.length()); } return this; } @Override public void write(int c) throws IOException { ensureOpen(); } @Override public void write(char[] cbuf, int off, int len) throws IOException { Objects.checkFromIndexSize(off, len, cbuf.length); ensureOpen(); } @Override public void write(String str) throws IOException { Objects.requireNonNull(str); ensureOpen(); } @Override public void write(String str, int off, int len) throws IOException { Objects.checkFromIndexSize(off, len, str.length()); ensureOpen(); } @Override public void flush() throws IOException { ensureOpen(); } @Override public void close() throws IOException { closed = true; } }; } /** * The object used to synchronize operations on this stream. For * efficiency, a character-stream object may use an object other than * itself to protect critical sections. A subclass should therefore use * the object in this field rather than {@code this} or a synchronized * method. */ protected Object lock; /** * Creates a new character-stream writer whose critical sections will * synchronize on the writer itself. */ protected Writer() { this.lock = this; } /** * Creates a new character-stream writer whose critical sections will * synchronize on the given object. * * @param lock * Object to synchronize on */ protected Writer(Object lock) { if (lock == null) { throw new NullPointerException(); } this.lock = lock; } /** * Writes a single character. The character to be written is contained in * the 16 low-order bits of the given integer value; the 16 high-order bits * are ignored. * * <p> Subclasses that intend to support efficient single-character output * should override this method. * * @param c * int specifying a character to be written * * @throws IOException * If an I/O error occurs */ public void write(int c) throws IOException { synchronized (lock) { if (writeBuffer == null) { writeBuffer = new char[WRITE_BUFFER_SIZE]; } writeBuffer[0] = (char) c; write(writeBuffer, 0, 1); } } /** * Writes an array of characters. * * @param cbuf * Array of characters to be written * * @throws IOException * If an I/O error occurs */ public void write(char cbuf[]) throws IOException { write(cbuf, 0, cbuf.length); } /** * Writes a portion of an array of characters. * * @param cbuf * Array of characters * * @param off * Offset from which to start writing characters * * @param len * Number of characters to write * * @throws IndexOutOfBoundsException * Implementations should throw this exception * if {@code off} is negative, or {@code len} is negative, * or {@code off + len} is negative or greater than the length * of the given array * * @throws IOException * If an I/O error occurs */ public abstract void write(char cbuf[], int off, int len) throws IOException; /** * Writes a string. * * @param str * String to be written * * @throws IOException * If an I/O error occurs */ public void write(String str) throws IOException { write(str, 0, str.length()); } /** * Writes a portion of a string. * * @implSpec * The implementation in this class throws an * {@code IndexOutOfBoundsException} for the indicated conditions; * overriding methods may choose to do otherwise. * * @param str * A String * * @param off * Offset from which to start writing characters * * @param len * Number of characters to write * * @throws IndexOutOfBoundsException * Implementations should throw this exception * if {@code off} is negative, or {@code len} is negative, * or {@code off + len} is negative or greater than the length * of the given string * * @throws IOException * If an I/O error occurs */ public void write(String str, int off, int len) throws IOException { synchronized (lock) { char cbuf[]; if (len <= WRITE_BUFFER_SIZE) { if (writeBuffer == null) { writeBuffer = new char[WRITE_BUFFER_SIZE]; } cbuf = writeBuffer; } else { // Don't permanently allocate very large buffers. cbuf = new char[len]; } str.getChars(off, (off + len), cbuf, 0); write(cbuf, 0, len); } } /** * Appends the specified character sequence to this writer. * * <p> An invocation of this method of the form {@code out.append(csq)} * behaves in exactly the same way as the invocation * * <pre> * out.write(csq.toString()) </pre> * * <p> Depending on the specification of {@code toString} for the * character sequence {@code csq}, the entire sequence may not be * appended. For instance, invoking the {@code toString} method of a * character buffer will return a subsequence whose content depends upon * the buffer's position and limit. * * @param csq * The character sequence to append. If {@code csq} is * {@code null}, then the four characters {@code "null"} are * appended to this writer. * * @return This writer * * @throws IOException * If an I/O error occurs * * @since 1.5 */ public Writer append(CharSequence csq) throws IOException { write(String.valueOf(csq)); return this; } /** * Appends a subsequence of the specified character sequence to this writer. * {@code Appendable}. * * <p> An invocation of this method of the form * {@code out.append(csq, start, end)} when {@code csq} * is not {@code null} behaves in exactly the * same way as the invocation * * <pre>{@code * out.write(csq.subSequence(start, end).toString()) * }</pre> * * @param csq * The character sequence from which a subsequence will be * appended. If {@code csq} is {@code null}, then characters * will be appended as if {@code csq} contained the four * characters {@code "null"}. * * @param start * The index of the first character in the subsequence * * @param end * The index of the character following the last character in the * subsequence * * @return This writer * * @throws IndexOutOfBoundsException * If {@code start} or {@code end} are negative, {@code start} * is greater than {@code end}, or {@code end} is greater than * {@code csq.length()} * * @throws IOException * If an I/O error occurs * * @since 1.5 */ public Writer append(CharSequence csq, int start, int end) throws IOException { if (csq == null) csq = "null"; return append(csq.subSequence(start, end)); } /** * Appends the specified character to this writer. * * <p> An invocation of this method of the form {@code out.append(c)} * behaves in exactly the same way as the invocation * * <pre> * out.write(c) </pre> * * @param c * The 16-bit character to append * * @return This writer * * @throws IOException * If an I/O error occurs * * @since 1.5 */ public Writer append(char c) throws IOException { write(c); return this; } /** * Flushes the stream. If the stream has saved any characters from the * various write() methods in a buffer, write them immediately to their * intended destination. Then, if that destination is another character or * byte stream, flush it. Thus one flush() invocation will flush all the * buffers in a chain of Writers and OutputStreams. * * <p> If the intended destination of this stream is an abstraction provided * by the underlying operating system, for example a file, then flushing the * stream guarantees only that bytes previously written to the stream are * passed to the operating system for writing; it does not guarantee that * they are actually written to a physical device such as a disk drive. * * @throws IOException * If an I/O error occurs */ public abstract void flush() throws IOException; /** * Closes the stream, flushing it first. Once the stream has been closed, * further write() or flush() invocations will cause an IOException to be * thrown. Closing a previously closed stream has no effect. * * @throws IOException * If an I/O error occurs */ public abstract void close() throws IOException; }