Java I/O Streams
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What is a Stream?
A stream in Java represents a continuous flow of data, either coming into a program from a source like a file or network connection, or going out of a program to a destination, and every file operation is built on top of an input or output stream.
Example: What is a Stream?
import java.io.FileWriter;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
FileWriter writer = new FileWriter("data.txt"); // stream: flow of data out
writer.write("Hello");
writer.close();
System.out.println("Written");
}
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Byte Streams vs Character Streams
Java splits streams into two families: byte streams like FileInputStream and FileOutputStream handle raw binary data one byte at a time, while character streams like FileReader and FileWriter handle text and automatically manage character encoding.
Example: Byte Streams vs Character Streams
import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
FileOutputStream byteStream = new FileOutputStream("bytes.dat"); // raw binary
byteStream.write(65);
byteStream.close();
FileWriter charStream = new FileWriter("text.txt"); // text
charStream.write("Hello");
charStream.close();
}
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Input Streams and Output Streams
An InputStream provides data flowing into a program through methods like read(), which returns -1 once the end of the data is reached, while an OutputStream provides a write() method for sending data out to its destination.
Example: Input Streams and Output Streams
import java.io.FileWriter;
import java.io.FileReader;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
FileWriter writer = new FileWriter("data.txt");
writer.write("Hi");
writer.close();
FileReader reader = new FileReader("data.txt");
int c;
while ((c = reader.read()) != -1) { // -1 marks end of data
System.out.print((char) c);
}
reader.close();
}
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Stream Buffering
Reading or writing one byte or character at a time is slow because each call can trigger a separate disk operation, so buffered streams like BufferedReader and BufferedWriter batch data in memory to reduce the number of actual I/O operations.
Example: Stream Buffering
import java.io.FileWriter;
import java.io.BufferedWriter;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
BufferedWriter writer = new BufferedWriter(new FileWriter("data.txt")); // batches writes
writer.write("Hello");
writer.close();
System.out.println("Written with buffering");
}
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Closing Streams Properly
Every stream must eventually be closed to release the underlying file handle or system resource, and Java's try-with-resources statement does this automatically and safely even when an exception interrupts the read or write.
Example: Closing Streams Properly
import java.io.FileWriter;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
try (FileWriter writer = new FileWriter("data.txt")) { // try-with-resources auto-closes
writer.write("Hello");
}
System.out.println("Closed automatically");
}
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