Question

What purpose does it serve?

Just read an example in a book where the author has done so.

int numOfGuesses=0;

Answer 1

The automatic assignment to zero only applies to members, not to local variables. If it is a local variable and the = 0 is omitted then that variable has no value, not even zero. Attempting to use the value before it is assigned will result in a compile error. For example this code attempts to use an uninitialized local variable:

public class Program
{   
    public static void main(String[] args)
    {
        int numOfGuesses;   // local variable
        System.out.println(numOfGuesses);
    }
}

and produces this compile error:

Program.java:6: variable numOfGuesses might not have been initialized
        System.out.println(numOfGuesses);

Whereas this code using a member works and outputs zero:

public class Program
{   
    int numOfGuesses; // member variable

    public void run()
    {
        System.out.println(numOfGuesses);
    }

    public static void main(String[] args)
    {
        new Program().run();
    }
}

For members I tend to assign to zero explicilty if my code uses the fact that the initial zalue is zero, and omit the assignment if my code doesn’t use the initial value (for example if it the value is assigned in the constructor or elsewhere). The result is the same either way, so it’s just a style issue.