System.out.println的未知输出

Question

I am learning Java and while studying try catch loop, I encountered this weird behaviour.

Whatever number I supply to the below code, it returns something between 49 - 53.

public class demo{
    public static void main(String[] args) {
        System.out.println("Enter a number");
        try{
            int num = System.in.read();
            System.out.println(num);
        }catch(Exception e){
            e.printStackTrace();
        }       
    }
}

Answer 1

You must have entered numbers 1-5. The read() method of InputStream ( System.in is an InputStream ) returns the byte value as an int , but the value is the Unicode code. The characters '1' through '5' are represented by the codes 49-53. In fact, '0' through '9' are represented by the codes 48-57.

Don't call read directly on the InputStream . It's meant for low-level stream processing. Instead, wrap the InputStream in a Scanner and call nextInt() .

Answer 2

InputStream.read() returns an integer. You should do char c = (char) myInt; to convert it to a character if that's what you want

Answer 3

InputStream.read() reads only the next byte of data:

http://docs.oracle.com/javase/7/docs/api/java/io/InputStream.html#read()

If you'll need to read more than 1 byte of data at a time, try using a BufferedInputStream: http://docs.oracle.com/javase/7/docs/api/java/io/BufferedInputStream.html or a reader: https://docs.oracle.com/javase/7/docs/api/java/io/BufferedReader.html

Beware of simply casting an int produced by InputStream.read to char - it will only work if your character set is ISO-8859-1. Hence, a Reader with appropriate Charset is a good option.