Java.Streams.Is it possible to redirect standard input/output streams?

Java allows you to redirect standard input, output, and error streams quite easily using the System class. It’s super useful for logging, testing, piping data between tools, and even simulating a console in GUI apps.

🔁 Standard I/O Streams in Java

Stream	Field	Default Destination
Standard input	System.in	Keyboard (console input)
Standard output	System.out	Console
Standard error	System.err	Console (often red-colored)

🔧 How to Redirect Them

🔹 1. Redirect System.out and System.err

PrintStream fileOut = new PrintStream("output.txt");
System.setOut(fileOut);     // Now System.out goes to the file
System.setErr(fileOut);     // So does System.err

System.out.println("This goes to output.txt");
System.err.println("So does this error message");

🔹 2. Redirect System.in

String simulatedInput = "42\nhello\n";
InputStream fakeInput = new ByteArrayInputStream(simulatedInput.getBytes());
System.setIn(fakeInput);

Scanner scanner = new Scanner(System.in);
int number = scanner.nextInt();     // Reads 42
String word = scanner.next();       // Reads "hello"

System.out.println("Read: " + number + " and " + word);

💡 Real-World Use Cases

Use Case	Description
Testing	Simulate user input or capture console output
Logging	Redirect System.out / System.err to a log file
GUI apps	Capture console output into a GUI text area
Scripting tools	Connect Java with other command-line tools (like in a shell)

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⚠️ Caveats

• Redirection affects the entire JVM, so be cautious in multi-threaded apps.
• Always restore original streams if you’re redirecting temporarily.

PrintStream originalOut = System.out;
// ... redirect ...
System.setOut(originalOut); // Restore

🧵 TL;DR

Operation	Code
Redirect stdout	System.setOut(new PrintStream("file.txt"))
Redirect stderr	System.setErr(new PrintStream("errors.log"))
Redirect stdin	System.setIn(new ByteArrayInputStream("input".getBytes()))