Convert decimal to binary and vice-versa in Java

Question

I'm trying to learn Java, so I'm doing a few exercises from a programmer project idea book. This exercise requires that the program be able to convert a decimal number to binary and vice versa, like so:

Please enter the binary/decimal number to convert:9783569
100101010100100100010001

Please enter the binary/decimal number to convert:100101010100100100010001
9783569

Here's the code I came up with:

import java.util.Scanner;

public class BinaryToDecimalAndBackConverter{

    static String convertDecimalToBinary(String decimal){
            int integer = Integer.valueOf(decimal);
            String result = new String();

            while(integer > 0){
                result+=String.valueOf(integer%2);
                integer/=2;
            }

            result = new StringBuffer(result).reverse().toString();
            return result;
    }
    static int convertBinaryToDecimal(String binary){
            int result = 0;

            for(int reverseCounter = 0; reverseCounter < binary.length(); reverseCounter++){
                char currentChar = binary.charAt(binary.length() - reverseCounter - 1);
                int numericValue = Character.getNumericValue(currentChar);
                result+=Math.pow(2, reverseCounter) * numericValue;
//              System.out.println(Math.pow(2, reverseCounter) * Character.getNumericValue(binary.charAt(binary.length() - reverseCounter - 1)) + " reverseCounter " + reverseCounter);
            }

            return result;
    }
    static String isDecimalOrBinary(String input){
        for(int counter = 0; counter < input.length(); counter++){
            if(input.charAt(counter) != '0' && input.charAt(counter) != '1'){
                return "decimal";
            }
        }

        return "binary";
    }
    public static void main(String[] args){
        Scanner inputScanner = new Scanner(System.in);

        System.out.print("Please enter the binary/decimal number to convert:");
        String input = inputScanner.nextLine();

        switch(isDecimalOrBinary(input)){
            case "decimal":
                System.out.println(convertDecimalToBinary(input));
            break;
            case "binary":
                System.out.println(convertBinaryToDecimal(input));
            break;
        }
    }
}

Answer 1

In addition to @Francesco Pitzalis's answer...

String concatenation

String result = new String();

This is never required as Strings are immutable in Java. If you find yourself using it to append String values, then you should be using StringBuilder (the faster, non-synchronized version of StringBuffer, which is usually recommended unless you require the multi-threading safety of the latter):

StringBuilder result = new StringBuilder();
// ...
result.append(integer % 2); // add some spaces for readability
// ...
return result.reverse().toString();

try-with-resources

// don't forget to close the Scanner
inputScanner.close();

Since you are on Java 7 at least (from the use of String in switch), you should be using try-with-resources to safely and efficiently manage the underlying I/O resource used by your Scanner instance:

try (Scanner scanner = new Scanner(System.in)) {
    // ...
}

Choices and enums

On a related note to your isDecimalOrBinary() method (which @Francesco Pitzalis's answer made a very good point), the return type can also be modeled as an enum should you still choose to keep it:

enum ValueType {
    DECIMAL, BINARY;

    public static ValueType parse(String value) {
        // ...
    }
}

This eliminates the possibility of typos in your expected String return values, e.g. when you return "decimal" but are wrongly checking for "Decimal" from calling the method.

Answer 2

The best way to do it:

static String convertDecimalToBinary(String decimal) {
    return Integer.toString(Integer.parseInt(decimal, 10), 2);
}

static int convertBinaryToDecimal(String binary) {
    return Integer.parseInt(binary, 2);
}

By the way your code looks good except for at least two issues:

1. Don't work with negative numbers
2. The method isDecimalOrBinary is ambiguous: what's the result of the 100 input? It's binary of course, but if i meant one hundred? I suggest to delegate the user to specify it.

I would correct it in this way:

import java.util.Scanner;

public class BinaryToDecimalAndBackConverter {

    static String convertDecimalToBinary(String decimal) {
        final boolean isNegative = decimal.startsWith("-");
        if (isNegative) {
            decimal = decimal.substring(1);
        }

        int integer = Integer.valueOf(decimal);
        String result = new String();

        while (integer > 0) {
            result += String.valueOf(integer % 2);
            integer /= 2;
        }

        // use StringBuilder instead of StringBuffer, we are sure about
        // exclusive access, no need to use a thread safe class
        final StringBuilder resultBuilder = new StringBuilder(result);
        if (isNegative) {
            resultBuilder.append("-");
        }

        return resultBuilder.reverse().toString();
    }

    static int convertBinaryToDecimal(String binary) {
        final boolean isNegative = binary.startsWith("-");
        if (isNegative) {
            binary = binary.substring(1);
        }
        int result = 0;

        for (int reverseCounter = 0; reverseCounter < binary.length(); reverseCounter++) {
            final char currentChar = binary.charAt(binary.length() - reverseCounter - 1);
            final int numericValue = Character.getNumericValue(currentChar);
            result += Math.pow(2, reverseCounter) * numericValue;
            // System.out.println(Math.pow(2, reverseCounter) *
            // Character.getNumericValue(binary.charAt(binary.length() -
            // reverseCounter - 1)) + " reverseCounter " + reverseCounter);
        }

        if (isNegative) {
            result = -result;
        }

        return result;
    }

    public static void main(String[] args) {
        final Scanner inputScanner = new Scanner(System.in);

        System.out.print("Please enter the binary/decimal number to convert:");
        // expected input: NUM D|B
        final String[] splittedInput = inputScanner.nextLine().split(" ");
        if (splittedInput.length != 2) {
            // don't forget to close the Scanner
            inputScanner.close();
            throw new IllegalArgumentException("Wrong input format: expected NUM D|B");
        }
        final String input = splittedInput[0];

        switch (splittedInput[1]) {
        case "d":
        case "D":
            System.out.println(convertDecimalToBinary(input));
            break;
        case "b":
        case "B":
            System.out.println(convertBinaryToDecimal(input));
            break;
        default:
            // don't forget to close the Scanner
            inputScanner.close();
            throw new IllegalArgumentException(
                    "Wrong input format: expected NUM D|B, the second argument was not D nor B");
        }

        // don't forget to close the Scanner
        inputScanner.close();
    }
}