What is System.in in Java?

System.in is an InputStream which is typically connected to keyboard input of console programs.

Here's how you can use it to read stuff from your keyboard:

var keyboardInput = new Scanner(System.in);

while (keyboardInput.hasNext()) {
  System.out.printf("You have typed: %s%n", keyboardInput.next());

Run it, then type something to the console and hit Enter.

Every time your input will be echoed back to you.