Java Program to Convert Decimal to Octal Number

In Java, convert a decimal number to octal by using Integer.toOctalString(decimalNumber) or by repeatedly dividing the number by 8 and collecting remainders.

Examples

Input8

Output10

Input65

Output101

Input0

Output0

How to Think About It

To convert decimal to octal, think of dividing the decimal number by 8 repeatedly. Each division gives a remainder between 0 and 7, which forms the octal digits from right to left. Collect these remainders until the number becomes zero.

Algorithm

1

Get the decimal number as input.

2

While the number is greater than zero, divide it by 8 and record the remainder.

3

Store each remainder as an octal digit starting from the right.

4

Reverse the collected digits to get the octal number.

5

If the input is zero, the octal is also zero.

6

Return or print the octal number.

Code

java

public class DecimalToOctal {
    public static void main(String[] args) {
        int decimalNumber = 65;
        String octalNumber = Integer.toOctalString(decimalNumber);
        System.out.println("Octal of " + decimalNumber + " is: " + octalNumber);
    }
}

Output

Octal of 65 is: 101

Dry Run

Let's trace converting decimal 65 to octal using repeated division.

1

Divide 65 by 8

65 ÷ 8 = 8 remainder 1 (rightmost octal digit)

2

Divide 8 by 8

8 ÷ 8 = 1 remainder 0 (next octal digit)

3

Divide 1 by 8

1 ÷ 8 = 0 remainder 1 (leftmost octal digit)

Decimal Number	Divide by 8	Quotient	Remainder (Octal Digit)
65	65 ÷ 8	8	1
8	8 ÷ 8	1	0
1	1 ÷ 8	0	1

Why This Works

Step 1: Using Integer.toOctalString

Java's built-in method Integer.toOctalString() converts decimal to octal by handling the division and remainder internally.

Step 2: Manual division method

Dividing the decimal number by 8 repeatedly and collecting remainders builds the octal number from right to left.

Step 3: Reversing remainders

Since the first remainder is the least significant digit, reversing the collected digits gives the correct octal representation.

Alternative Approaches

Manual conversion using loop and StringBuilder

java

public class DecimalToOctalManual {
    public static void main(String[] args) {
        int decimal = 65;
        StringBuilder octal = new StringBuilder();
        int number = decimal;
        if (number == 0) {
            octal.append('0');
        }
        while (number > 0) {
            int remainder = number % 8;
            octal.insert(0, remainder);
            number /= 8;
        }
        System.out.println("Octal of " + decimal + " is: " + octal);
    }
}

This method shows the step-by-step process but is longer than using built-in methods.

Using Integer.toString with radix 8

java

public class DecimalToOctalRadix {
    public static void main(String[] args) {
        int decimal = 65;
        String octal = Integer.toString(decimal, 8);
        System.out.println("Octal of " + decimal + " is: " + octal);
    }
}

This is a concise alternative using <code>Integer.toString()</code> with base 8.

Complexity: O(log n) time, O(log n) space

Time Complexity

The conversion requires dividing the number by 8 repeatedly until it reaches zero, which takes about log base 8 of n steps.

Space Complexity

Storing the octal digits requires space proportional to the number of digits, which is also about log base 8 of n.

Which Approach is Fastest?

Using built-in methods like Integer.toOctalString() or Integer.toString(number, 8) is fastest and less error-prone compared to manual division.

Approach	Time	Space	Best For
Integer.toOctalString()	O(log n)	O(log n)	Quick and simple conversion
Manual division with loop	O(log n)	O(log n)	Learning and understanding conversion process
Integer.toString(number, 8)	O(log n)	O(log n)	Concise alternative to toOctalString