http://fastutil.di.unimi.it/: fastutil extends the Java Collections Framework by providing type-specific maps, sets, lists and priority queues with a small memory footprint and fast access and insertion; provides also big (64-bit) arrays, sets and lists, and fast, practical I/O classes for binary and text files.
  • Sebastiano Vigna 
package it.unimi.dsi.fastutil.io;

/*
 * Copyright (C) 2005-2019 Sebastiano Vigna
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


import java.io.IOException;


An stream that provides eager access to its length,
and keeps track of the current position (e.g., the number of bytes read so far, or the current
position of the file pointer).

This class has two methods, both specified as optional. This apparently bizarre behaviour is necessary because of wrapper classes which use reflection to support those methods (see, e.g., MeasurableInputStream, FastBufferedInputStream and FastBufferedOutputStream). 
Since:6.0.0
/** An stream that provides eager access to its length,
 * and keeps track of the current position (e.g., the number of bytes read so far, or the current
 * position of the file pointer).
 *
 * <p>This class has two methods, both specified as optional. This apparently bizarre
 * behaviour is necessary because of wrapper classes which use reflection
 * to support those methods (see, e.g., {@link MeasurableInputStream}, {@link FastBufferedInputStream} and {@link FastBufferedOutputStream}).
 *
 * @since 6.0.0
 */

public interface MeasurableStream  {

	
Returns the overall length of this stream (optional operation). In most cases, this will require the
 stream to perform some extra action, possibly changing the state of the input stream itself (typically, reading
 all the bytes up to the end, or flushing on output stream).
 Implementing classes should always document what state will the input stream be in
 after calling this method, and which kind of exception could be thrown.

/** Returns the overall length of this stream (optional operation). In most cases, this will require the
	 *  stream to perform some extra action, possibly changing the state of the input stream itself (typically, reading
	 *  all the bytes up to the end, or flushing on output stream).
	 *  Implementing classes should always document what state will the input stream be in
	 *  after calling this method, and which kind of exception could be thrown.
	 */
	long length() throws IOException;

	
Returns the current position in this stream (optional operation).

Usually, the position is just the number of bytes read or written since the stream was opened, but in the case of a RepositionableStream it represent the current position. 
/** Returns the current position in this stream (optional operation).
	 *
	 * <p>Usually, the position is just the number of bytes read or written
	 * since the stream was opened, but in the case of a
	 * {@link it.unimi.dsi.fastutil.io.RepositionableStream} it
	 * represent the current position.
	 */
	long position() throws IOException;
}