Why does this output 8?

Question

public class Test {  
    public static void main(String... args) {

        int i=010;

        System.out.print(i);
    }
}

output:

8

Why? What is the logic?

Answer 1

0 is the prefix for octal numbers, just like 0x is the prefix for hexadecimal numbers (and 0b is the prefix for binary numbers, since Java 7).

So 010 means 1 * 81 + 0 * 80, which is 8.

Answer 2

Have a look at the Java Language Specification, Chapter 3.10.1 Integer Literals

An integer literal may be expressed in decimal (base 10), hexadecimal (base 16), octal (base 8), or binary (base 2).

An octal numeral consists of an ASCII digit 0 followed by one or more of the ASCII digits 0 through 7 interspersed with underscores, and can represent a positive, zero, or negative integer.

This is why 010 = 8.