Why "010" equals 8?

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).

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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.

Now you should understand why 010 is 8.

A leading 0 denotes an octal numeric value so the value 010 can be decoded thus: 010 = 1 * 81 + 0 * 80 = 8