Java Program to Convert Binary to Decimal Number

In Java, convert a binary string to decimal using Integer.parseInt(binaryString, 2) or manually by processing each bit and summing powers of 2.

Examples

Input101

Output5

Input1101

Output13

Input0

Output0

How to Think About It

To convert binary to decimal, think of each binary digit as a power of 2 starting from the right (0th power). Multiply each bit by 2 raised to its position index and add all results to get the decimal number.

Algorithm

1

Get the binary string input.

2

Initialize a decimal result to 0.

3

For each character in the binary string from left to right:

4

Convert the character to a number (0 or 1).

5

Multiply it by 2 raised to the power of its position from the right.

6

Add this value to the decimal result.

7

Return the decimal result.

Code

java

public class BinaryToDecimal {
    public static void main(String[] args) {
        String binary = "1101";
        int decimal = Integer.parseInt(binary, 2);
        System.out.println("Decimal value: " + decimal);
    }
}

Output

Decimal value: 13

Dry Run

Let's trace the binary '1101' through the code.

1

Input binary string

binary = "1101"

2

Convert using Integer.parseInt

decimal = Integer.parseInt("1101", 2) = 13

3

Print result

Output: Decimal value: 13

Binary Digit	Position (from right)	Value (digit * 2^position)
1	3	1 * 8 = 8
1	2	1 * 4 = 4
0	1	0 * 2 = 0
1	0	1 * 1 = 1

Why This Works

Step 1: Parsing binary string

The method Integer.parseInt(binaryString, 2) reads the string as a base-2 number.

Step 2: Calculating decimal value

Each binary digit is multiplied by 2 raised to its position index and summed to get the decimal equivalent.

Step 3: Output the result

The decimal number is printed using System.out.println.

Alternative Approaches

Manual calculation with loop

java

public class BinaryToDecimalManual {
    public static void main(String[] args) {
        String binary = "1101";
        int decimal = 0;
        int length = binary.length();
        for (int i = 0; i < length; i++) {
            if (binary.charAt(length - 1 - i) == '1') {
                decimal += Math.pow(2, i);
            }
        }
        System.out.println("Decimal value: " + decimal);
    }
}

This method shows the step-by-step calculation but is less concise than using built-in parsing.

Using Integer.valueOf

java

public class BinaryToDecimalValueOf {
    public static void main(String[] args) {
        String binary = "1101";
        int decimal = Integer.valueOf(binary, 2);
        System.out.println("Decimal value: " + decimal);
    }
}

Similar to parseInt but returns an Integer object; useful if you need an object instead of primitive int.

Complexity: O(n) time, O(1) space

Time Complexity

The conversion processes each binary digit once, so time grows linearly with input length.

Space Complexity

Only a few variables are used, so space is constant regardless of input size.

Which Approach is Fastest?

Using Integer.parseInt is fastest and simplest; manual loops are educational but slower.

Approach	Time	Space	Best For
Integer.parseInt	O(n)	O(1)	Quick and simple conversion
Manual loop calculation	O(n)	O(1)	Learning and understanding process
Integer.valueOf	O(n)	O(1)	When Integer object needed