Question
Answer and Explanation
Input file redirection in Java typically involves reading data from a file rather than from standard input (the keyboard). This is commonly done using Java's I/O classes. Here’s how you can achieve this:
1. Using `FileInputStream` and `Scanner`:
- You can use a `FileInputStream` to create an input stream from a file, and then use a `Scanner` to parse the content of that stream.
- Example code:
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.Scanner;
public class FileInputRedirection {
public static void main(String[] args) {
try {
FileInputStream fileInputStream = new FileInputStream("input.txt");
Scanner scanner = new Scanner(fileInputStream);
while (scanner.hasNextLine()) {
String line = scanner.nextLine();
System.out.println(line); // Process each line from the file
}
scanner.close();
} catch (FileNotFoundException e) {
System.err.println("File not found: " + e.getMessage());
}
}
}
- In this example, "input.txt" is the file from which the input is read. Ensure that this file exists in the same directory as your Java program or provide the full path to the file.
2. Command Line Redirection:
- Another way to use file redirection is at the command line when running the Java program. Instead of modifying your Java code, you can redirect the standard input to a file during execution. This method is useful if you can't modify the Java source directly.
- Example command:
java YourProgram < input.txt
- Here, the `<` symbol redirects standard input to "input.txt". Your program should be designed to read from `System.in` (standard input), but the data will come from the specified file instead of the keyboard.
3. Reading with `BufferedReader`:
- If you need to read character by character or perform more advanced reading operations, you can use `FileInputStream` with `InputStreamReader` and `BufferedReader`.
- Example Code:
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
public class FileInputBuffer {
public static void main(String[] args) {
try {
FileInputStream fileInputStream = new FileInputStream("input.txt");
InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream);
BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
String line;
while ((line = bufferedReader.readLine()) != null) {
System.out.println(line);
}
bufferedReader.close();
} catch (FileNotFoundException e) {
System.err.println("File not found: " + e.getMessage());
} catch (IOException e) {
System.err.println("IO Error: " + e.getMessage());
}
}
}
-This method is more performant for reading files line by line.
In summary, you can either configure your Java code to directly use `FileInputStream` (or other input streams) to read data from a file or, when running your compiled Java program, use command-line redirection. Choose the method that best fits your use case, whether you have control over the code or only have the executable file.