https://openjdk.java.net/ 
/*
 * Copyright (c) 1994, 2021, 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.Enumeration;
import java.util.Objects;
import java.util.Vector;


A SequenceInputStream represents the logical concatenation of other input streams. It starts out with an ordered collection of input streams and reads from the first one until end of file is reached, whereupon it reads from the second one, and so on, until end of file is reached on the last of the contained input streams. 
Author: Author van Hoff
Since:  1.0
/**
 * A {@code SequenceInputStream} represents
 * the logical concatenation of other input
 * streams. It starts out with an ordered
 * collection of input streams and reads from
 * the first one until end of file is reached,
 * whereupon it reads from the second one,
 * and so on, until end of file is reached
 * on the last of the contained input streams.
 *
 * @author  Author van Hoff
 * @since   1.0
 */
public class SequenceInputStream extends InputStream {
    Enumeration<? extends InputStream> e;
    InputStream in;

    
Initializes a newly created SequenceInputStream by remembering the argument, which must be an Enumeration that produces objects whose run-time type is InputStream. The input streams that are produced by the enumeration will be read, in order, to provide the bytes to be read from this SequenceInputStream. After each input stream from the enumeration is exhausted, it is closed by calling its close method. 
Params:
  • e –   an enumeration of input streams.
See Also:
/**
     * Initializes a newly created {@code SequenceInputStream}
     * by remembering the argument, which must
     * be an {@code Enumeration}  that produces
     * objects whose run-time type is {@code InputStream}.
     * The input streams that are  produced by
     * the enumeration will be read, in order,
     * to provide the bytes to be read  from this
     * {@code SequenceInputStream}. After
     * each input stream from the enumeration
     * is exhausted, it is closed by calling its
     * {@code close} method.
     *
     * @param   e   an enumeration of input streams.
     * @see     java.util.Enumeration
     */
    public SequenceInputStream(Enumeration<? extends InputStream> e) {
        this.e = e;
        peekNextStream();
    }

    
Initializes a newly created SequenceInputStream by remembering the two arguments, which will be read in order, first s1 and then s2, to provide the bytes to be read from this SequenceInputStream. 
Params:
  • s1 –   the first input stream to read.
  • s2 –   the second input stream to read.
/**
     * Initializes a newly
     * created {@code SequenceInputStream}
     * by remembering the two arguments, which
     * will be read in order, first {@code s1}
     * and then {@code s2}, to provide the
     * bytes to be read from this {@code SequenceInputStream}.
     *
     * @param   s1   the first input stream to read.
     * @param   s2   the second input stream to read.
     */
    public SequenceInputStream(InputStream s1, InputStream s2) {
        Vector<InputStream> v = new Vector<>(2);
        v.addElement(s1);
        v.addElement(s2);
        e = v.elements();
        peekNextStream();
    }

    
Continues reading in the next stream if an EOF is reached.

/**
     * Continues reading in the next stream if an EOF is reached.
     */
    final void nextStream() throws IOException {
        if (in != null) {
            in.close();
        }
        peekNextStream();
    }

    private void peekNextStream() {
        if (e.hasMoreElements()) {
            in = e.nextElement();
            if (in == null)
                throw new NullPointerException();
        } else {
            in = null;
        }
    }

    
Returns an estimate of the number of bytes that can be read (or
skipped over) from the current underlying input stream without
blocking by the next invocation of a method for the current
underlying input stream. The next invocation might be
the same thread or another thread.  A single read or skip of this
many bytes will not block, but may read or skip fewer bytes.

 This method simply calls available of the current underlying input stream and returns the result. 
Throws:
Returns: an estimate of the number of bytes that can be read (or skipped over) from the current underlying input stream without blocking or 0 if this input stream has been closed by invoking its close() method
Since:   1.1
/**
     * Returns an estimate of the number of bytes that can be read (or
     * skipped over) from the current underlying input stream without
     * blocking by the next invocation of a method for the current
     * underlying input stream. The next invocation might be
     * the same thread or another thread.  A single read or skip of this
     * many bytes will not block, but may read or skip fewer bytes.
     * <p>
     * This method simply calls {@code available} of the current underlying
     * input stream and returns the result.
     *
     * @return   an estimate of the number of bytes that can be read (or
     *           skipped over) from the current underlying input stream
     *           without blocking or {@code 0} if this input stream
     *           has been closed by invoking its {@link #close()} method
     * @throws   IOException  if an I/O error occurs.
     *
     * @since    1.1
     */
    public int available() throws IOException {
        if (in == null) {
            return 0; // no way to signal EOF from available()
        }
        return in.available();
    }

    
Reads the next byte of data from this input stream. The byte is returned as an int in the range 0 to 255. If no byte is available because the end of the stream has been reached, the value -1 is returned. This method blocks until input data is available, the end of the stream is detected, or an exception is thrown. 
 This method tries to read one character from the current substream. If it reaches the end of the stream, it calls the close method of the current substream and begins reading from the next substream. 
Throws:
Returns: the next byte of data, or -1 if the end of the stream is reached.
/**
     * Reads the next byte of data from this input stream. The byte is
     * returned as an {@code int} in the range {@code 0} to
     * {@code 255}. If no byte is available because the end of the
     * stream has been reached, the value {@code -1} is returned.
     * This method blocks until input data is available, the end of the
     * stream is detected, or an exception is thrown.
     * <p>
     * This method
     * tries to read one character from the current substream. If it
     * reaches the end of the stream, it calls the {@code close}
     * method of the current substream and begins reading from the next
     * substream.
     *
     * @return     the next byte of data, or {@code -1} if the end of the
     *             stream is reached.
     * @throws     IOException  if an I/O error occurs.
     */
    public int read() throws IOException {
        while (in != null) {
            int c = in.read();
            if (c != -1) {
                return c;
            }
            nextStream();
        }
        return -1;
    }

    
Reads up to len bytes of data from this input stream into an array of bytes. If len is not zero, the method blocks until at least 1 byte of input is available; otherwise, no bytes are read and 0 is returned. 
 The read method of SequenceInputStream tries to read the data from the current substream. If it fails to read any characters because the substream has reached the end of the stream, it calls the close method of the current substream and begins reading from the next substream. 
Params:
  • b –     the buffer into which the data is read.
  • off –  the start offset in array b at which the data is written.
  • len –   the maximum number of bytes read.
Throws:
Returns:    int   the number of bytes read.
/**
     * Reads up to {@code len} bytes of data from this input stream
     * into an array of bytes.  If {@code len} is not zero, the method
     * blocks until at least 1 byte of input is available; otherwise, no
     * bytes are read and {@code 0} is returned.
     * <p>
     * The {@code read} method of {@code SequenceInputStream}
     * tries to read the data from the current substream. If it fails to
     * read any characters because the substream has reached the end of
     * the stream, it calls the {@code close} method of the current
     * substream and begins reading from the next substream.
     *
     * @param      b     the buffer into which the data is read.
     * @param      off   the start offset in array {@code b}
     *                   at which the data is written.
     * @param      len   the maximum number of bytes read.
     * @return     int   the number of bytes read.
     * @throws     NullPointerException If {@code b} is {@code null}.
     * @throws     IndexOutOfBoundsException If {@code off} is negative,
     *             {@code len} is negative, or {@code len} is
     *             greater than {@code b.length - off}
     * @throws     IOException  if an I/O error occurs.
     */
    public int read(byte[] b, int off, int len) throws IOException {
        if (in == null) {
            return -1;
        } else if (b == null) {
            throw new NullPointerException();
        }
        Objects.checkFromIndexSize(off, len, b.length);
        if (len == 0) {
            return 0;
        }
        do {
            int n = in.read(b, off, len);
            if (n > 0) {
                return n;
            }
            nextStream();
        } while (in != null);
        return -1;
    }

    
Closes this input stream and releases any system resources associated with the stream. A closed SequenceInputStream cannot perform input operations and cannot be reopened. 
 If this stream was created from an enumeration, all remaining elements are requested from the enumeration and closed before the close method returns. 
Throws:
/**
     * Closes this input stream and releases any system resources
     * associated with the stream.
     * A closed {@code SequenceInputStream}
     * cannot  perform input operations and cannot
     * be reopened.
     * <p>
     * If this stream was created
     * from an enumeration, all remaining elements
     * are requested from the enumeration and closed
     * before the {@code close} method returns.
     *
     * @throws     IOException  if an I/O error occurs.
     */
    public void close() throws IOException {
        IOException ioe = null;
        while (in != null) {
            try {
                in.close();
            } catch (IOException e) {
                if (ioe == null) {
                    ioe = e;
                } else {
                    ioe.addSuppressed(e);
                }
            }
            peekNextStream();
        }
        if (ioe != null) {
            throw ioe;
        }
    }
}