how to set value of octal in java?

// Decimal declaration and possible chars are [0-9]
int decimal    =  495;        

// HexaDecimal declaration starts with 0X or 0x and possible chars are [0-9A-Fa-f]
int hexa       =  0X1EF; 

// Octal declaration starts with 0 and possible chars are [0-7] 
int octal      =  0757;  

// Binary representation starts with 0B or 0b and possible chars are [0-1]  
int binary     =  0b111101111; 

If the number is string format then you can convert it into int using the below

String text = "0b111101111";
int value = text.toLowerCase().startsWith("0b") ? Integer.parseInt(text.substring(2), 2)
                                  : Integer.decode(text);

Octal is base-8 number system, so it means digit can be from 0 to 7, you can't use digit 8 (and 9 too) in octal number system.

why i cant give 018 and 019 to variable.

Because an integer literal prefixed with 0 is treated as octal, and '8' and '9' aren't valid octal digits.

From section 3.10.1 of the JLS:

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.

Trying to use '8' in an octal number is like trying to use 'G' in hex... it's simple not part of the set of symbols used in that base.