Java: Convert a octal number to a decimal number

Last update on May 12 2025 13:19:55 (UTC/GMT +8 hours)

Octal to Decimal Converter

Write a Java program to convert a octal number to a decimal number.

Octal number: The octal numeral system is the base-8 number system, and uses the digits 0 to 7.

Decimal number: The decimal numeral system is the standard system for denoting integer and non-integer numbers. It is also called base-ten positional numeral system.

Test Data:
Input any octal number: 10

Pictorial Presentation: Octal to Decimal number

Java: Convert a octal number to a decimal number

Sample Solution:

Java Code:

 import java.util.Scanner;

public class Exercise25 {
    public static void main(String[] args) {
        // Create a Scanner object to read input from the user
        Scanner in = new Scanner(System.in);
        
        // Declare variables to store octal and decimal numbers, and an index counter
        long octal_num, decimal_num = 0;
        int i = 0;
        
        // Prompt the user to input an octal number
        System.out.print("Input any octal number: ");
        octal_num = in.nextLong();
        
        // Convert the octal number to decimal
        while (octal_num != 0) {
            decimal_num = (long)(decimal_num + (octal_num % 10) * Math.pow(8, i++));
            octal_num = octal_num / 10;
        }
        
        // Display the equivalent decimal number
        System.out.print("Equivalent decimal number: " + decimal_num + "\n");
    }
}

Explanation:

In the exercise above -

First, it takes an octal number as input from the user using the "Scanner" class and stores it in the variable 'octal_num'.
It initializes a variable 'decimal_num' to store the decimal equivalent and another variable 'i' to keep track of the position of each octal digit.
It enters a loop to perform the octal-to-decimal conversion:

In each iteration, it calculates the remainder when 'octal_num' is divided by 10 (which gives the least significant octal digit).
It multiplies this remainder by 8 raised to the power of 'i' and adds it to 'decimal_num' to accumulate the decimal value.
To move to the next octal digit, it increments i.
It updates 'octal_num' by removing the least significant digit (rightmost) by dividing it by 10.
The loop continues until 'octal_num' becomes zero, effectively converting the entire octal number to decimal.

After the loop, it prints the decimal representation of the original octal number stored in the ‘decimal_num’ variable.

Sample Output:

Input any octal number: 10                                                                                    
Equivalent decimal number: 8

Flowchart:

Flowchart: Java exercises: Convert a octal number to a decimal number

For more Practice: Solve these Related Problems:

Convert an octal number with more than five digits to decimal.
Modify the program to handle octal numbers with leading zeros.
Write a program that validates the user input before conversion.
Implement octal-to-decimal conversion using only multiplication and addition.

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