Java Program to Convert Decimal to Binary

In Java, you can convert a decimal number to binary using Integer.toBinaryString(decimalNumber) or by repeatedly dividing the number by 2 and collecting remainders.

Examples

Input5

Output101

Input10

Output1010

Input0

Output0

How to Think About It

To convert decimal to binary, think of dividing the decimal number by 2 repeatedly and noting the remainder each time. These remainders, read in reverse order, form the binary number. Alternatively, Java provides a built-in method to do this conversion easily.

Algorithm

1

Get the decimal number as input.

2

If the number is 0, return "0" as binary.

3

While the number is greater than 0, divide it by 2 and store the remainder.

4

Collect all remainders in reverse order to form the binary string.

5

Return the binary string.

Code

java

public class DecimalToBinary {
    public static void main(String[] args) {
        int decimal = 10;
        String binary = Integer.toBinaryString(decimal);
        System.out.println("Binary of " + decimal + " is: " + binary);
    }
}

Output

Binary of 10 is: 1010

Dry Run

Let's trace converting decimal 5 to binary using repeated division.

1

Divide 5 by 2

5 / 2 = 2 remainder 1

2

Divide 2 by 2

2 / 2 = 1 remainder 0

3

Divide 1 by 2

1 / 2 = 0 remainder 1

Quotient	Remainder
5	1
2	0
1	1

Why This Works

Step 1: Using Integer.toBinaryString

Java's Integer.toBinaryString method converts the decimal number directly to its binary string representation.

Step 2: Manual division method

Dividing the decimal number by 2 repeatedly and collecting remainders builds the binary digits from least significant to most significant.

Step 3: Reversing remainders

Reading the collected remainders in reverse order gives the correct binary number.

Alternative Approaches

Manual conversion using StringBuilder

java

public class DecimalToBinaryManual {
    public static void main(String[] args) {
        int decimal = 10;
        StringBuilder binary = new StringBuilder();
        int num = decimal;
        if (num == 0) {
            binary.append('0');
        }
        while (num > 0) {
            binary.append(num % 2);
            num /= 2;
        }
        System.out.println("Binary of " + decimal + " is: " + binary.reverse().toString());
    }
}

This method shows the step-by-step process but is longer and less concise than the built-in method.

Using Java 8 Streams (advanced)

java

import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class DecimalToBinaryStream {
    public static void main(String[] args) {
        int decimal = 10;
        String binary = IntStream.iterate(decimal, n -> n > 0, n -> n / 2)
            .map(n -> n % 2)
            .mapToObj(String::valueOf)
            .collect(Collectors.collectingAndThen(Collectors.toList(), lst -> {
                java.util.Collections.reverse(lst);
                return String.join("", lst);
            }));
        System.out.println("Binary of " + decimal + " is: " + binary);
    }
}

Uses streams for functional style but is more complex and less readable for beginners.

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

Time Complexity

The conversion requires dividing the number by 2 repeatedly until it reaches 0, which takes about log base 2 of n steps.

Space Complexity

Storing the binary digits requires space proportional to the number of bits, which is also about log base 2 of n.

Which Approach is Fastest?

Using Integer.toBinaryString is fastest and most efficient, while manual methods are educational but slower and more error-prone.

Approach	Time	Space	Best For
Integer.toBinaryString	O(log n)	O(log n)	Quick and simple conversion
Manual division with StringBuilder	O(log n)	O(log n)	Learning and understanding process
Java Streams method	O(log n)	O(log n)	Functional programming style