Convert Decimal to Octal in Java

Topic: Convert Decimal to Octal

In this program, we’ll convert a number from decimal (base 10) to octal (base 8). Decimal numbers use digits 0–9, while octal numbers use digits 0–7.

For example, the decimal number 10 becomes 12 in octal. This type of conversion is common in computer science, especially in low-level programming and memory address calculations.

We’ll use a simple loop that repeatedly divides the number by 8 and stores the remainders, which together form the octal number.

Examples

Example 1:
Input: 10
Output: 12
Explanation: 10 ÷ 8 = 1 remainder 2 → 1 ÷ 8 = 0 remainder 1 → Octal = 12

Example 2:
Input: 64
Output: 100
Explanation: 64 ÷ 8 = 8 rem 0 → 8 ÷ 8 = 1 rem 0 → 1 ÷ 8 = 0 rem 1 → Octal = 100

Example 3 (Edge Case):
Input: 0
Output: 0
Explanation: 0 in decimal is 0 in octal.

Interviewer Expectations

The interviewer wants to see if you can:

• Understand base conversions (decimal to octal)
• Use loops and arithmetic operators
• Build the result by processing remainders in reverse order
• Write clean and readable code

Approach

To convert a decimal number to octal:

1. Initialize an empty string or use an array to store remainders.
2. Keep dividing the number by 8 until it becomes 0.
3. Store the remainders (in reverse order) to form the octal number.

Dry Run:

Decimal: 10

10 ÷ 8 = 1 remainder 2  
1 ÷ 8 = 0 remainder 1  
Octal = 12 (read remainders from bottom to top)

Java Program

public class DecimalToOctal {
  public static void main(String[] args) {
    int decimal = 10;
    String octal = "";

    while (decimal > 0) {
      int remainder = decimal % 8;
      octal = remainder + octal;
      decimal = decimal / 8;
    }

    System.out.println("Octal representation: " + octal);
  }
}

Octal representation: 12

Convert Decimal to Octal

Possible Followup Questions with Answers

Q1: Can I use built-in methods for this conversion?

Yes. Java provides Integer.toOctalString() which converts a decimal to octal directly:

int decimal = 10;
System.out.println(Integer.toOctalString(decimal)); // Output: 12

Q2: How would you convert octal back to decimal?

You can use Integer.parseInt(octalString, 8) to convert an octal string to decimal:

String octal = "12";
int decimal = Integer.parseInt(octal, 8); // Output: 10

Q3: What if the input is 0?

The output will also be 0. You can handle this with a special case at the beginning of the program:

if (decimal == 0) {
  System.out.println("0");
  return;
}