An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

In the context of Core Java, an I/O filter typically refers to an object that is used to perform filtering on data as it is read from or written to a stream. Input/Output (I/O) streams in Java are used for reading from or writing to various sources, such as files, network connections, or in-memory data.

An I/O filter is an additional layer that can be added to an existing stream to modify or manipulate the data as it passes through. Filters can be chained together to create a sequence of transformations. They are often used for tasks such as encryption, compression, or custom data processing.

In Java, the FilterInputStream and FilterOutputStream classes are base classes for building custom input and output stream filters. You can create your own filter classes by extending these classes and overriding the necessary methods to provide the desired behavior. This allows you to customize the behavior of I/O operations without directly modifying the underlying streams.

Here’s a simple example of using an I/O filter in Java:

java

import java.io.*;

class UppercaseFilterInputStream extends FilterInputStream {
protected UppercaseFilterInputStream(InputStream in) {
super(in);
}

@Override
public int read() throws IOException {
int data = super.read();
if (data != –1) {
// Convert lowercase to uppercase
data = Character.toUpperCase((char) data);
}
return data;
}
}

public class Example {
public static void main(String[] args) {
try (FileInputStream fileInputStream = new FileInputStream(“example.txt”);
UppercaseFilterInputStream uppercaseFilter = new UppercaseFilterInputStream(fileInputStream)) {

int data;
while ((data = uppercaseFilter.read()) != –1) {
System.out.print((char) data);
}
} catch (IOException e) {
e.printStackTrace();
}
}
}

In this example, UppercaseFilterInputStream is a custom filter that converts lowercase characters to uppercase as data is read from the underlying input stream. The main method demonstrates how to use this filter with a FileInputStream.