In this program, you’ll learn to convert binary number to a decimal number and vice-versa using functions in Java.

Convert Binary Number to Decimal Number

Binary numbers are numbers consisting only of 2 digits: 0 and 1. They can be expressed in the base 2 numeral system. For example,

10 (2), 1000 (8), 11001 (25)

Decimal numbers are numbers consisting of 10 digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. They can be expressed in the base 10 numeral system.

18, 222, 987

Here, we will be writing a Java program that will convert a binary number into decimal and vice versa using built-in methods and custom methods.

Example 1: Binary to Decimal Conversion Using Custom Method

class Main {
    
  public static void main(String[] args) {

    // binary number
    long num = 110110111;

    // call method by passing the binary number
    int decimal = convertBinaryToDecimal(num);

    System.out.println("Binary to Decimal");
    System.out.println(num + " = " + decimal);
  }

  public static int convertBinaryToDecimal(long num) {
    int decimalNumber = 0, i = 0;
    long remainder;
    
    while (num != 0) {
      remainder = num % 10;
      num /= 10;
      decimalNumber += remainder * Math.pow(2, i);
      ++i;
    }
    
    return decimalNumber;
  }
}

Output

110110111 in binary = 439 in decimal

Here’s how the above program works:

Example 2: Binary to Decimal Conversion Using parseInt()

class Main {
  public static void main(String[] args) {
    
    // binary number
    String binary = "01011011";
    
    // convert to decimal
    int decimal = Integer.parseInt(binary, 2);
    System.out.println(binary + " in binary = " + decimal + " in decimal.");
  }
}

Run Code

Output

01011011 in binary = 91 in decimal.

Here, we have used the parseInt() method of the Integer class to convert a binary number into decimal.

Example 3: Decimal to Binary Conversion using Custom Method

class Main {

  public static void main(String[] args) {

    // decimal number
    int num = 19;
    System.out.println("Decimal to Binary");

    // call method to convert to binary
    long binary = convertDecimalToBinary(num);

    System.out.println("\n" + num + " = " + binary);
    }

  public static long convertDecimalToBinary(int n) {

    long binaryNumber = 0;
    int remainder, i = 1, step = 1;

    while (n!=0) {
      remainder = n % 2;
        System.out.println("Step " + step++ + ": " + n + "/2");

        System.out.println("Quotient = " + n/2 + ", Remainder = " + remainder);
        n /= 2;

        binaryNumber += remainder * i;
        i *= 10;
    }
    
    return binaryNumber;
    }
}

Run Code

Output

Decimal to Binary                                                                                                                                                                    
Step 1: 19/2                                                                                                                                                                         
Quotient = 9, Remainder = 1                                                                                                                                                          
Step 2: 9/2                                                                                                                                                                          
Quotient = 4, Remainder = 1                                                                                                                                                          
Step 3: 4/2                                                                                                                                                                          
Quotient = 2, Remainder = 0                                                                                                                                                          
Step 4: 2/2                                                                                                                                                                          
Quotient = 1, Remainder = 0                                                                                                                                                          
Step 5: 1/2                                                                                                                                                                          
Quotient = 0, Remainder = 1                                                                                                                                                          
                                                                                                                                                                                     
19 = 10011

Here’s how the program works:

Example 4: Decimal to Binary Conversion using toBinaryString()

We can also use the toBinaryString() method of the Integer class to convert a decimal number into binary.

class Main {
  public static void main(String[] args) {
    
    // decimal number 
    int decimal = 91;
    
    // convert decimal to binary
    String binary = Integer.toBinaryString(decimal);
    System.out.println(decimal + " in decimal = " + binary + " in binary.");
  }
}

Run Code

Output

91 in decimal = 1011011 in binary.

Here, the toBinaryString() method takes an integer argument and returns the string representation of the number in base 2 (binary).

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