Example 1: Program to Convert Decimal to Octal

public class DecimalOctal{

    public static void main(String[] args){
        int decimal = 78;
        int octal = convertDecimalToOctal(decimal);
        System.out.printf("%d in decimal = %d in octal", decimal, octal);
    }

    public static int convertDecimalToOctal(int decimal){
        int octalNumber = 0, i = 1;

        while (decimal != 0)
        {
            octalNumber += (decimal % 8) * i;
            decimal /= 8;
            i *= 10;
        }

        return octalNumber;
    }
}

Output

78 in decimal = 116 in octal

This conversion takes place as:

8 | 78
8 | 9 -- 6
8 | 1 -- 1
8 | 0 -- 1
(116)

Example 2: Program to Convert Octal to Decimal

public class OctalDecimal{

    public static void main(String[] args){
        int octal = 116;
        int decimal = convertOctalToDecimal(octal);
        System.out.printf("%d in octal = %d in decimal", octal, decimal);
    }

    public static int convertOctalToDecimal(int octal){
        int decimalNumber = 0, i = 0;

        while(octal != 0)
        {
            decimalNumber += (octal % 10) * Math.pow(8, i);
            ++i;
            octal/=10;
        }

        return decimalNumber;
    }
}

Output

116 in octal = 78 in decimal

This conversion takes place as:

1 * 82 + 1 * 81 + 6 * 80 = 78