Java mistakes with numbers

Question

I am trying to fix the following Java code,

I cannot figure out why the printout is still 5.

public class simpleMath
{
    public static void main(String[] args) 
    {
        int number=5;
        half(number);

        System.out.println("5 divided by 2 is: " + number);

    }

    private static double half(int number) {
         number =  number/2;
         return number;
    }
}

Answer 1

Because you're not re-assigning the returned value.

int number = 5;
number = half(number);

Answer 2

When you call the function, you're discarding its return value:

half(number);

You probably meant to write:

number = half(number);

Also, in Java, arguments are passed by value. This means that, even though you change number inside the function, the change does not propagate back to the caller.

There are several further problems:

Problem 1: The suggested change will store the result in number, which is an integer variable. Thus, the result of half() -- which is of type double -- will be truncated to an integer. To avoid the loss of precision, you either have to change number to be a floating-point variable, or store the result in a different variable of the appropriate type.

Problem 2: The following uses integer division:

number =  number/2;

The result is truncated to an integer, i.e. 5 / 2 is 2. The latter is then converted to a double (2.0), which is what the function returns.

To fix, change the function like so:

private static double half(int number) {
     return number / 2.0;
}

P.S. Floating-point numbers have a lot of properties that can be unintuitive. I recommend having a look at What Every Computer Scientist Should Know About Floating-Point Arithmetic.

Answer 3

You passing the primitive data type which is done by value. You need to give SOP in method half() EDIT: Need to use the result returned by method half() by either assigning it to number or calling this method in SOP itself.

Answer 4

Why half(number) doesn't modify number declared in main() function? It is because you will pass the value of number to half() function to evaluate, i.e. you give a copy of value in number to half() function. Therefore, whatever half() function does to number will not get reflected back to number variable declared in main(). You need to assign the return value of half() to number in main() if you want to update its value.

There are other cases, such as variable shadowing, that I'm not going to talk in details, since it may confuses you.