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Java IO

References

  1. Generally speaking, BufferedReader/BufferedWrite are preferred over InputStreamReader/InputStreamWriter.

https://docs.oracle.com/javase/8/docs/api/java/io/BufferedWriter.html

https://stackoverflow.com/questions/6976893/in-java-what-is-the-advantage-of-using-bufferedwriter-to-append-to-a-file

public static Stream\<String> java.nio.file.Files.lines

public static List\<String> java.nio.file.Files.readAllLines

Read all lines from a file. Bytes from the file are decoded into characters using the UTF-8 charset. Notice that this method returns a List of Strings, which is different from the method java.nio.file.Files.lines (who returns a Stream of Strings).

public String BufferedRead.readLine

Reads a line of text. A line is considered to be terminated by any one of a line feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately by a linefeed.

public Stream\<String> BufferedRead.lines

Returns a Stream, the elements of which are lines read from this BufferedReader. The Stream is lazily populated, i.e., read only occurs during the terminal stream operation. The reader must not be operated on during the execution of the terminal stream operation. Otherwise, the result of the terminal stream operation is undefined.

After execution of the terminal stream operation there are no guarantees that the reader will be at a specific position from which to read the next character or line.

If an IOException is thrown when accessing the underlying BufferedReader, it is wrapped in an UncheckedIOException which will be thrown from the Stream method that caused the read to take place. This method will return a Stream if invoked on a BufferedReader that is closed. Any operation on that stream that requires reading from the BufferedReader after it is closed, will cause an UncheckedIOException to be thrown.

Scanner

In [ ]:
String file = "D:\\Dropbox\\Research\\Nettleton\\SPT\\Mean\\k.txt";
		Scanner scan = new Scanner(new File(file)).useDelimiter(" ");
		while(scan.hasNextDouble()){
			System.out.println(scan.nextDouble()+" ");
		}

PrintWriter

In [ ]:
PrintWriter out = new PrintWriter("java_out.txt");
		out.print(1.11111111+" ");
		out.print(2.2+" ");
		out.print(3.3+" ");
		out.close();

DataInputStream

In [ ]:
String file = "D:\\Dropbox\\Research\\Nettleton\\SPT\\Mean\\test-again.bin";
		//file = "D:\\Dropbox\\Common\\Matlab\\mtest.bin";
		DataInputStream in = new DataInputStream(new BufferedInputStream(new FileInputStream(file)));
		int count = 0;
		while(in.available()>0){
			System.out.println(in.readDouble()+" ");
			count++;
		}
		in.close();
		System.out.println(count+" items are read in.");

DataOutputStream

In [ ]:
String file = "java_out.bin";
		DataOutputStream out = new DataOutputStream(new FileOutputStream(file));
		out.writeDouble(1.1);
		out.writeDouble(2.2);
		out.writeDouble(3.3);
		out.close();
In [ ]:
Reader - Abstract class for reading character streams
    InputStream - Abstract class is the superclass of all classes representing an input stream of bytes.
And similary for Writer vs OutputStream

Scanner is used for parsing tokens from the contents of the stream while BufferedReader just reads the stream and does not do any special parsing.

In fact you can pass a BufferedReader to a scanner as the source of characters to parse.

As to the choice, use the Scanner if you want to parse the file, use the BufferedReader if you want to read the file line by line. Also see the introductory text of their aforelinked API documentations.



    A BufferedReader is a simple class meant to efficiently read from the underling stream. Generally, each read request made of a Reader like a FileReader causes a corresponding read request to be made to underlying stream. Each invocation of read() or readLine() could cause bytes to be read from the file, converted into characters, and then returned, which can be very inefficient. Efficiency is improved appreciably if a Reader is warped in a BufferedReader.

    BufferedReader is synchronized, so read operations on a BufferedReader can safely be done from multiple threads.

    A scanner on the other hand has a lot more cheese built into it; it can do all that a BufferedReader can do and at the same level of efficiency as well. However, in addition a Scanner can parse the underlying stream for primitive types and strings using regular expressions. It can also tokenize the underlying stream with the delimiter of your choice. It can also do forward scanning of the underlying stream disregarding the delimiter!

    A scanner however is not thread safe, it has to be externally synchronized.

    The choice of using a BufferedReader or a Scanner depends on the code you are writing, if you are writing a simple log reader Buffered reader is adequate. However if you are writing an XML parser Scanner is the more natural choice.

    Even while reading the input, if want to accept user input line by line and say just add it to a file, a BufferedReader is good enough. On the other hand if you want to accept user input as a command with multiple options, and then intend to perform different operations based on the command and options specified, a Scanner will suit better.


PrintWriter is the output equivalent of the Scanner class

```language
//PrintWriter is a class for writng text file.
package study.io;

import java.io.FileNotFoundException;
import java.io.PrintWriter;

public class TestPrintWriter {
	public static void main(String[] args) throws FileNotFoundException{
		PrintWriter out = new PrintWriter("java_out.txt");
		out.print(1.11111111+" ");
		out.print(2.2+" ");
		out.print(3.3+" ");
		out.close();
	}
}
```


## Input and Output

1. `System.out.println` is super fast. Generally speaking, you do not
have worry much about the overheading of printing information to the
console in Java.

## Reading and Writing Text File

1. `Scanner` is convenient for parsing text data.

2. `PrintWriter` is for writing text file.

## Reading and Writing Binary File

1. To read in binary data (through byte) from a file, you can use
`FileInputStream`; to write binary data into a file, you can use
`FileOutputStream`. To read in binary data into primitive types
from a file, you can wrap a FileInputStream in DataInputStream;
to write primitive types into a binary file directly, you can
wrap a FileOutputStream in DataOutputStream. Note that
DataInputStream and DataOutputStream is compatible, i.e., they
recognize the format of each other. However, DataInputStream is
not compatible with `writeBin` in R; neither is DataOutputStream
compatible with `readBin` in R. To communicate binary data
between Java and R, you can run Java code which do the right job
for you using `rJava`.




FileReader vs FileInputStream?
Or Reader vs InputStream
Writer vs OutputStream
DataInputStream/DataOutPutStream vs FileInputStream/FileOutputStream


The following is a simple yet very common way
```language
try(BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    StringBuilder sb = new StringBuilder();
    String line = br.readLine();

    while (line != null) {
        sb.append(line);
        sb.append(System.lineSeparator());
        line = br.readLine();
    }
    String everything = sb.toString();
}
```

---------------------------------------------------------------------------------------------------------------------------------------
In Java 7: similar to BufferedReader(FileInputStream...)
```language
Charset charset = Charset.forName("US-ASCII");
try (BufferedReader reader = Files.newBufferedReader(file, charset)) {
	String line = null;
	while ((line = reader.readLine()) != null) {
		System.out.println(line);
	}
} catch (IOException x) {
	System.err.format("IOException: %s%n", x);
}
```
--------------------------------------------------------------------------------------------------------------------------------------------
The difference between the two methods is what to use to construct a BufferedReader. Method 1 uses InputStreamReader and Method 2 uses FileReader. What's the difference between the two classes?

From Java Doc, "An InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified charset." InputStreamReader can handle other input streams than files, such as network connections, classpath resources, ZIP files, etc.

FileReader is "Convenience class for reading character files. The constructors of this class assume that the default character encoding and the default byte-buffer size are appropriate." FileReader does not allow you to specify an encoding other than the platform default encoding. Therefore, it is not a good idea to use it if the program will run on systems with different platform encoding.

In summary, InputStreamReader is always a safer choice than FileReader.


Scanner in = new Scanner(new FileReader("filename.txt"));

http://www.javapractices.com/topic/TopicAction.do?Id=42

http://howtodoinjava.com/2013/05/01/3-ways-to-read-files-using-java-nio/

http://www.programcreek.com/2011/03/java-read-a-file-line-by-line-code-example/

http://www.baeldung.com/java-read-lines-large-file
Faster file copy with mapped byte buffer

```language
package com.howtodoinjava.test.nio;
 
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
 
public class ReadFileWithMappedByteBuffer
{
	public static void main(String[] args) throws IOException
	{
		RandomAccessFile aFile = new RandomAccessFile
				("test.txt", "r");
		FileChannel inChannel = aFile.getChannel();
		MappedByteBuffer buffer = inChannel.map(FileChannel.MapMode.READ_ONLY, 0, inChannel.size());
		buffer.load(); 
		for (int i = 0; i < buffer.limit(); i++)
		{
			System.out.print((char) buffer.get());
		}
		buffer.clear(); // do something with the data and clear/compact it.
		inChannel.close();
		aFile.close();
	}
}
```







There are too many ways to read files from Java. Best practice?


FileReader vs FileInputStream?
Or Reader vs InputStream
Writer vs OutputStream
DataInputStream/DataOutPutStream vs FileInputStream/FileOutputStream


The following is a simple yet very common way

try(BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
		StringBuilder sb = new StringBuilder();
		String line = br.readLine();

		while (line != null) {
			sb.append(line);
			sb.append(System.lineSeparator());
			line = br.readLine();
		}
		String everything = sb.toString();
   }

---------------------------------------------------------------------------------------------------------------------------------------
In Java 7: similar to BufferedReader(FileInputStream...)
Charset charset = Charset.forName("US-ASCII");
try (BufferedReader reader = Files.newBufferedReader(file, charset)) {
	String line = null;
	while ((line = reader.readLine()) != null) {
		System.out.println(line);
	}
} catch (IOException x) {
	System.err.format("IOException: %s%n", x);
}
--------------------------------------------------------------------------------------------------------------------------------------------
The difference between the two methods is what to use to construct a BufferedReader. Method 1 uses InputStreamReader and Method 2 uses FileReader. What's the difference between the two classes?

From Java Doc, "An InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified charset." InputStreamReader can handle other input streams than files, such as network connections, classpath resources, ZIP files, etc.

FileReader is "Convenience class for reading character files. The constructors of this class assume that the default character encoding and the default byte-buffer size are appropriate." FileReader does not allow you to specify an encoding other than the platform default encoding. Therefore, it is not a good idea to use it if the program will run on systems with different platform encoding.

In summary, InputStreamReader is always a safer choice than FileReader.


Scanner in = new Scanner(new FileReader("filename.txt"));

http://www.javapractices.com/topic/TopicAction.do?Id=42

http://howtodoinjava.com/2013/05/01/3-ways-to-read-files-using-java-nio/

http://www.programcreek.com/2011/03/java-read-a-file-line-by-line-code-example/

http://www.baeldung.com/java-read-lines-large-file
Faster file copy with mapped byte buffer

package com.howtodoinjava.test.nio;
 
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
 
public class ReadFileWithMappedByteBuffer
{
	public static void main(String[] args) throws IOException
	{
		RandomAccessFile aFile = new RandomAccessFile
				("test.txt", "r");
		FileChannel inChannel = aFile.getChannel();
		MappedByteBuffer buffer = inChannel.map(FileChannel.MapMode.READ_ONLY, 0, inChannel.size());
		buffer.load(); 
		for (int i = 0; i < buffer.limit(); i++)
		{
			System.out.print((char) buffer.get());
		}
		buffer.clear(); // do something with the data and clear/compact it.
		inChannel.close();
		aFile.close();
	}
}

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