Java I/O stream

🔹 1. What is Java I/O?

I/O stands for Input and Output, and in Java it refers to how your program reads from or writes to:

• Keyboard (System.in)
• Console (System.out)
• Files (FileInputStream/FileWriter)
• Network sockets
• External devices

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🔹 2. Stream: The Core of Java I/O

🔸 What is a Stream?

A stream is a continuous flow of data.
In Java, streams abstract away the source or destination of data, and provide a simple way to process it byte by byte or character by character.

🔸 Two Main Stream Types

Type Description Examples

Byte Stream	For binary data (images, audio)	InputStream, OutputStream
Character Stream	For text (String, text files)	Reader, Writer

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🔹 3. Basic Java Stream Classes

📈 InputStream & OutputStream (Byte Stream)

📌 InputStream

• Abstract class
• Reads 1 byte at a time

Key methods:

int read();
int read(byte[] b);
void close();

Subclasses:

Class Description

FileInputStream	Reads bytes from a file
BufferedInputStream	Adds buffer for performance
ByteArrayInputStream	Reads from byte array in memory
ObjectInputStream	Reads serialized objects

📌 OutputStream

• Abstract class
• Writes 1 byte at a time

Key methods:

void write(int b);
void write(byte[] b);
void flush();
void close();

Subclasses:

Class Description

FileOutputStream	Writes bytes to a file
BufferedOutputStream	Adds buffer for performance
ByteArrayOutputStream	Writes to memory (byte array)
ObjectOutputStream	Writes serialized objects

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📈 Reader & Writer (Character Stream)

📌 Reader

• Abstract class
• Reads characters (2 bytes, UTF-16)

Key methods:

int read();
int read(char[] cbuf);
void close();

Subclasses:

Class Description

FileReader	Reads characters from a file
BufferedReader	Adds buffer; supports readLine()
InputStreamReader	Converts bytes to characters (with encoding)

📌 Writer

• Abstract class
• Writes characters

Key methods:

void write(int c);
void write(String str);
void write(char[] cbuf);
void flush();
void close();

Subclasses:

Class Description

FileWriter	Writes characters to a file
BufferedWriter	Adds buffer for performance
OutputStreamWriter	Converts characters to bytes (with encoding)

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📌 Stream Chaining (Connecting Streams)

Streams can be connected together in layers. This is called Stream Chaining.

BufferedReader br = new BufferedReader(
    new InputStreamReader(
        new FileInputStream("data.txt"), StandardCharsets.UTF_8));

This pipeline goes: file → byte → character → buffer.

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📄 Can InputStreamReader Be Used Alone?

No — InputStreamReader requires an existing InputStream as its source. You must provide it in the constructor. It cannot be used alone.

✅ Correct usage:

InputStreamReader reader = new InputStreamReader(System.in);
int ch = reader.read();
System.out.println((char) ch);
reader.close();

❌ Incorrect (won't compile):

InputStreamReader reader = new InputStreamReader(); // ❌ No default constructor

Always pass an InputStream, such as System.in, FileInputStream, or ByteArrayInputStream, when using InputStreamReader.

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🔹 4. File I/O in Java

🔸 Writing Text to File

FileWriter writer = new FileWriter("hello.txt");
writer.write("Hello, world!");
writer.close();

🔸 Reading Text from File

FileReader reader = new FileReader("hello.txt");
int data;
while ((data = reader.read()) != -1) {
    System.out.print((char) data);
}
reader.close();

🔸 Writing Binary File

FileOutputStream out = new FileOutputStream("image.png");
out.write(byteData);
out.close();

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🔹 5. Buffering for Performance

Reading or writing data one byte/character at a time is inefficient.
Buffered streams improve performance by reducing the number of actual I/O operations.

🔸 BufferedReader Example

BufferedReader reader = new BufferedReader(new FileReader("data.txt"));
String line;
while ((line = reader.readLine()) != null) {
    System.out.println(line);
}
reader.close();

🔸 BufferedWriter Example

BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"));
writer.write("This is fast and efficient!");
writer.newLine();
writer.write("Another line.");
writer.close();

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🔹 6. Handling Encoding Correctly

Use encoding explicitly when working with international text.

🔸 Writing with UTF-8

Writer writer = new OutputStreamWriter(new FileOutputStream("utf8.txt"), StandardCharsets.UTF_8);
writer.write("안녕하세요");
writer.close();

🔸 Reading with UTF-8

Reader reader = new InputStreamReader(new FileInputStream("utf8.txt"), StandardCharsets.UTF_8);
int data;
while ((data = reader.read()) != -1) {
    System.out.print((char) data);
}
reader.close();

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🔹 7. File Copy Example (Buffered)

try (
    BufferedInputStream in = new BufferedInputStream(new FileInputStream("source.jpg"));
    BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream("target.jpg"))
) {
    byte[] buffer = new byte[1024];
    int length;
    while ((length = in.read(buffer)) != -1) {
        out.write(buffer, 0, length);
    }
}

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🔹 8. Best Practices

• ✅ Always close streams (use try-with-resources)
• ✅ Use Buffered streams for performance
• ✅ Choose Writer/Reader for text, Stream for binary
• ✅ Handle encoding explicitly
• ✅ Match encoding when writing and reading text

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🔹 9. Summary Table

Text file write	FileWriter, BufferedWriter
Text file read	FileReader, BufferedReader
Binary file copy	BufferedInputStream/OutputStream
Encoding-safe writing	OutputStreamWriter with charset
Encoding-safe reading	InputStreamReader with charset