What does System.in.read actually return?

What does :

System.in.read()

return ? The documentation says :

Returns: the next byte of data, or -1 if the end of the stream is reached.

But for example if I enter : 10 I get back 49 . Why is that ?

49 is the ASCII value of the char 1. It is the value of the first byte.

The stream of bytes that is produced when you enter 10Enter on your console or terminal contains the three bytes {49,48,10} (on my Mac, may end with 10,12 or 12 instead of 10, depending on your System).

So the output of the simple snippet

int b = System.in.read();
while (b != -1) {
    System.out.println(b);
    b = System.in.read();
}

after entering a 10 and hitting enter, is (on my machine)

49
48
10

System.in.read() reads just one byte.

49 is the Unicode point value for 1.

Try to print:

System.out.println((char)49);

This will help you to understand it more.