What's my mistake?

Question about the task

Under discussion

Why I'm not able to pass the vaildation? This is the comment from my mentor but I'm not able to see my mistake. Be sure that the toDecimal(String) method returns 0 if it receives an empty string or null as input. Some help, please?

The public static toBinary(int) method must convert the integer received as an input parameter from the decimal numeral system to binary and return its string representation. And conversely, the public static toDecimal(String) method converts from the string representation of a binary number to a decimal number.

The methods only work with positive numbers and non-empty strings.
If the input parameter is less than or equal to 0, the toBinary(int) method returns an empty string.
If the input parameter is an empty string or null, then the toDecimal(String) method returns 0.

Your task is to implement these methods.

One algorithm for converting a decimal number to the string representation of a binary number is as follows:

while (the decimal number is not 0) {
the binary representation = remainder of the division of the decimal number by 2 + the binary representation
the decimal number = the decimal number / 2
}

One algorithm for converting the string representation of a binary number to a decimal number is as follows:

for (int i = 0; i < length of the binary representation; i++) {
the decimal number = the decimal number + digit in the binary representation * 2 to the power of i
}

Initially, the rightmost number is taken from the binary representation.
With each iteration of the loop, we take the next number closer to the beginning of the binary representation.

Hint: You can use the Math.pow(number, degree) method to raise a number to a power.
The main() method is not involved in testing.

Requirements:

The toBinary(int) method needs to be implemented as outlined in the task conditions.
The toDecimal(String) method needs to be implemented as outlined in the task conditions.
The Integer.toBinaryString(int) and Long.toBinaryString(int) methods cannot be used.
The Integer.parseInt(String, int) and Long.parseLong(String, int) methods cannot be used.
The Integer.toString(int, int) and Long.toString(long, int) methods cannot be used.
You cannot use a BigInteger object.
You cannot use a BigDecimal object.

package en.codegym.task.pro.task09.task0906; /* Binary converter */ public class Solution { public static void main(String[] args) { int decimalNumber = Integer.MAX_VALUE; System.out.println("Decimal number " + decimalNumber + " is equal to binary number " + toBinary(decimalNumber)); String binaryNumber = "1111111111111111111111111111111"; System.out.println("Binary number " + binaryNumber + " is equal to decimal number " + toDecimal(binaryNumber)); } public static String toBinary(int decimalNumber) { //write your code here if(decimalNumber < 0) return ""; String binaryRep = ""; while(decimalNumber !=0){ binaryRep = (decimalNumber%2) + binaryRep; decimalNumber = decimalNumber/2; } return binaryRep; } public static int toDecimal(String binaryNumber) { //write your code here if(binaryNumber.isEmpty() || binaryNumber.equals(null)) return(int) 0; long decimalNumber = 0; for(int i = 0; i < binaryNumber.length(); i++){ char digit = binaryNumber.charAt(binaryNumber.length() - i - 1); int digito = (digit=='0')?0:1; decimalNumber = (long) (decimalNumber + Math.pow((digito*2),i)); } return (int) decimalNumber; } }

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Dawn( #11020889)

Level 37 , Toronto, Canada

31 July 2022, 04:31

public static String toBinary(int decimalNumber) { if(decimalNumber > 0){ String binaryNumber = ""; while (decimalNumber != 0){ binaryNumber = binaryNumber + decimalNumber % 2; decimalNumber = decimalNumber / 2; } return binaryNumber; } else { return ""; } } public static int toDecimal(String binaryNumber) { if(binaryNumber != null){ int decimalNumber = 0; for (int i = 0; i < binaryNumber.length(); i++) { decimalNumber = (int)(decimalNumber + binaryNumber.charAt(i) * Math.pow(2, i)); } return decimalNumber; }else { return 0; } That's my code and after running, everything is ok, but i got failed. Why? Anyone can help? Please!

LuisRC

Level 39 , Gijón, Spain

14 January 2022, 05:49

Ups. 😱 Thanks.

Lisa

Level 41

13 January 2022, 18:45

Your empty and null check does not work the way you may expect it to work. Any instance method you call on null throws a NullPointerException. So if null is passed to your toDecimal() method (try it) not 0 is return but the exception is thrown (cause the first thing you do is calling a method on that null, that will look for the compiler similar to null.isEmpty()) Solution: the check for null is the first you need to do and only if the object does not reference null you proceed. Your null check (using equals) doesn't work either. equals is an instance method and if the object is null and you call equals on that null reference you get a NullPointerException again. You have to check for null using references like binaryNumber == null

if (binaryNumber == null || binaryNumber.isEmpty())
    return 0;

and here another possibility to get the int value from a char

decimalNumber += (binaryNumber.charAt(binaryNumber.length() - i - 1) - '0') * Math.pow(2, i);

LuisRC

Level 39 , Gijón, Spain

13 January 2022, 21:09

Hi Lisa, First thanks about empty and null checks. Second I tried what you've said about getting the int value from a char and I am a bit stuck with this code. I don't know many times I've tried different ways to solve it and I get always the same failed validation: The toDecimal(String) method needs to be implemented as outlined in the task conditions.

public static int toDecimal(String binaryNumber) {
        //write your code here
        if (binaryNumber == null || binaryNumber.isEmpty()) return 0;

        int decimalNumber = 0;

        for (int i = 0; i < binaryNumber.length() - 1; i++) {
            decimalNumber += (binaryNumber.charAt(binaryNumber.length() - i - 1) - '0') * Math.pow(2,i);
        }
        return decimalNumber;
    }

Thanks in advance

Lisa

Level 41

13 January 2022, 23:03

The only problem I can spot (in your latest version, not in the original) is a loop that does not iterate over all chars (abort condition needs to be i < binaryNumber.length() )