java.io package

This lesson covers the Java platform classes used for basic I/O. It first focuses on I/O Streams, a powerful concept that greatly simplifies I/O operations. The lesson also looks at serialization, which lets a program write whole objects out to streams and read them back again. Then the lesson looks at file I/O and file system operations, including random access files.
Most of the classes covered in the I/O Streams section are in the java.io package.
Most of the classes covered in the File I/O section are in the java.nio.file package.

I/O Streams

• Byte Streams handle I/O of raw binary data.
• Character Streams handle I/O of character data, automatically handling translation to and from the local character set.
• Buffered Streams optimize input and output by reducing the number of calls to the native API.
• Scanning and Formatting allows a program to read and write formatted text.
• I/O from the Command Line describes the Standard Streams and the Console object.
• Data Streams handle binary I/O of primitive data type and String values.
• Object Streams handle binary I/O of objects.

I/O Streams

An I/O Stream represents an input source or an output destination.
A stream can represent many different kinds of sources and destinations, including disk files, devices, other programs, and memory arrays.
Streams support many different kinds of data, including simple bytes, primitive data types, localized characters, and objects.
Some streams simply pass on data; others manipulate and transform the data in useful ways.
No matter how they work internally, all streams present the same simple model to programs that use them:
A stream is a sequence of data.
A program uses an input stream to read data from a source, one item at a time:

Reading information into a program.

A program uses an output stream to write data to a destination, one item at time:

Writing information from a program.

In this lesson, we'll see streams that can handle all kinds of data, from primitive values to advanced objects.
The data source and data destination pictured above can be anything that holds, generates, or consumes data.
Obviously this includes disk files, but a source or destination can also be another program, a peripheral device, a network socket, or an array.
In the next section, we'll use the most basic kind of streams, byte streams, to demonstrate the common operations of Stream I/O. For sample input, we'll use the example file in.txt, which contains the following

Byte Streams

Programs use byte streams to perform input and output of 8-bit bytes. All byte stream classes are descended from InputStream and OutputStream.
There are many byte stream classes. To demonstrate how byte streams work, we'll focus on the file I/O byte streams, FileInputStream and FileOutputStream. Other kinds of byte streams are used in much the same way; they differ mainly in the way they are constructed.

Using Byte Streams

We'll explore FileInputStream and FileOutputStream by examining an example program named CopyBytes, which uses byte streams to copy xanadu.txt, one byte at a time.

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class CopyBytes {
    public static void main(String[] args) throws IOException {

        FileInputStream in = null;
        FileOutputStream out = null;

        try {
            in = new FileInputStream("xanadu.txt");
            out = new FileOutputStream("outagain.txt");
            int c;

            while ((c = in.read()) != -1) {
                out.write(c);
            }
        } finally {
            if (in != null) {
                in.close();
            }
            if (out != null) {
                out.close();
            }
        }
    }
}

CopyBytes spends most of its time in a simple loop that reads the input stream and writes the output stream, one byte at a time, as shown in the following figure.

Simple byte stream input and output.

Notice that read() returns an int value. If the input is a stream of bytes, why doesn't read() return a byte value? Using a int as a return type allows read() to use -1 to indicate that it has reached the end of the stream.

Always Close Streams

Closing a stream when it's no longer needed is very important — so important that CopyBytes uses a finally block to guarantee that both streams will be closed even if an error occurs. This practice helps avoid serious resource leaks.
One possible error is that CopyBytes was unable to open one or both files. When that happens, the stream variable corresponding to the file never changes from its initial null value. That's why CopyBytes makes sure that each stream variable contains an object reference before invoking close.

When Not to Use Byte Streams

CopyBytes seems like a normal program, but it actually represents a kind of low-level I/O that you should avoid. Since xanadu.txt contains character data, the best approach is to usecharacter streams, as discussed in the next section. There are also streams for more complicated data types. Byte streams should only be used for the most primitive I/O.
So why talk about byte streams? Because all other stream types are built on byte streams.

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