what's the problem?

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
}

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.