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Compilation error in BlueJ- if else with return statement

I am using BlueJ IDE to write java programs.

I have a method with String return type. I have put the return statements within if-else, such that if the boolean variable "flag" has true value, then one value is returned, while if the value is false, another value is returned.

Now, the problem is that BlueJ asks for another return statement even after this, as shown below.

enter image description here

If I give another return after if-else, it works.

Why is this happening? I had learnt that there can be no statements after the return statement. So, why is the compiler asking for another return statement?

If someone wants the code for cut-paste purposes, here it is. This code is meant to convert binary numbers to their decimal equivalents, including fractions, but no negative numbers.

public class Conversions{
    protected String Binary_Decimal(String str){
        int a = str.indexOf('.');
        boolean flag = false;
        if (a == -1){
            str += ".0";
            a = str.indexOf('.');
            flag = true;
        }
        String bd = str.substring(0, a);
        String ad = str.substring(a + 1);
        a = 0;
        double num = 0;
        for (int i = bd.length() - 1; i >= 0; i--){
             num += Math.pow(2, a) * Integer.parseInt(Character.toString(str.charAt(i)));
            a++;
        }
        if (flag == true){
            return Integer.toString((int) num);
        }
        else if (flag == true) {
            a = -1;
            for (int i = 0; i < ad.length(); i++){
                num += Math.pow(2, a) * Integer.parseInt(Character.toString(str.charAt(i)));
                a--;
            }
            return String.valueOf(num);
        }
        return String.valueOf(num); //<-- WHY DOESN'T IT RUN WITHOUT THIS EXTRA return?
    }
}

Here, str is the string that is input by the user using a different method Input().

Solution

  • The issue is that you wrote an if - else as an if - else if. The compiler does not understand or care that the two conditions you have are mutually exclusive and therefore cover all cases. Given how you wrote the branches, you need an explicit else or a catchall return for the compiler to be assured that the function always returns a String.

    This is one example of why it is a bad idea to explicitly spell out the else when you have a set of conditions. The more important reason being that your if will often contain something much more complex and you might not negate it properly.