Java Program to Convert Octal to Decimal

The octal numbers are numbers with 8 base and use digits from 0-7. This system is a base 8 number system. The decimal numbers are the numbers with 10 as their base and use digits from 0-9 to represent the decimal number. They also require dot to represent decimal fractions.

We have to convert a number which is in Octal Number System to Decimal Number System.

The base in Octal Number is 8, which means that an Octal Number will have digits ranging from 0 to 7.

For Example:

In Octal: 167

In Decimal:(7 * 8⁰) + (6 * 8¹) +(1 * 8²)=119

The below diagram explains how to convert an octal number (123) to an equivalent decimal value:

Approach 1: Using Integer.parseInt() method

To convert any string form to decimal, we can use type.parseType() method. For example, here we need to convert from octal to decimal, and octal form is an integer, so we can use Integer.parseInt() to convert it.

Java

// Java program to convert
// octal number to decimal using
// Integer.parseInt()

public class GFG {

public static void main(String args[])
    {
        // octal value
        String onum = "157";

        // octal to decimal using Integer.parseInt()
        int num = Integer.parseInt(onum, 8);

        System.out.println(
            "Decimal equivalent of Octal value 157 is: "
            + num);
    }
}

Output

Decimal equivalent of Octal value 157 is: 111

Approach 2: 

Algorithm:

1. Start and take the octal input from the user.
2. Create a result variable with an initial value of 0 to store the resultant Decimal number.
3. Create a loop for getting every digit in the Input.
4. Multiply each digit in the number with 8^n-1, where n is the digit’s position.
5. Then add it to the result.
6. Store the value in Step(5) to result variable.
7. Print the result variable.

Java

// Java program to convert octal
// to decimal number using custom code

import java.util.Scanner;
import java.lang.Math;
public class Main {

    public static void main(String[] args)
    {

        int a = 167;

        // Initialize result variable to 0.
        int result = 0;

        // Take a copy of input
        int copy_a = a;

        for (int i = 0; copy_a > 0; i++) {

            // Take the last digit
            int temp = copy_a % 10;

            // Appropriate power on 8 suitable to
            // its position.
            double p = Math.pow(8, i);

            // Multiply the digits to the  into the Input
            // and
            //  then add it to result
            result += (temp * p);
            copy_a = copy_a / 10;
        }

        System.out.print("Decimal of Octal Number (" + a
                         + ") : " + result);
    }
}

Output

Decimal of Octal Number (167) : 119