Can't get my integer to increase constantly in method-CodePudding

I am fairly new to java (and most programming languages as a whole) and am trying to get some unfamiliar concepts down, one of which is recursion. To test it out I tried making it so that a number would constantly increase and get displayed.

public static void recursionTest() {
    int numb = 0;
    System.out.println(numb);
    numb  = 1;
    recursionTest();
}

I tried writing it out like this, but it will only print the number 0 constantly with no increase. I then tried putting before println, but it only produced the number 1. I then tried replacing with a while loop.

public static void recursionTest() {
    int numb = 0;
    System.out.println(numb);
    while (numb != -1) {
        numb =  1;
    }
    recursionTest();
}

This ended up printing out just a single 0, I then tried moving it above println like I did before, but it then didn't display anything. Is there something I'm missing? Sorry if my question is stupid.

CodePudding user response：

As you have defined it, numb is a local variable scoped within the method. It's created with the instruction int numb = 0 at the beginning of each call of the method, and destroyed at the end of it. Each time you call the method (even if the call is recursive) is a different variable. With the same name in the code, but it's a different one.

In order to achieve a counter you should either define numb as a static field of the class or pass as parameter to the method. You can do something like the following:

Option 1 (static field):

public class TheClass {
    static int numb = 0;
    public static void recursionTest() {
        System.out.println(numb);
        numb  = 1;
        recursionTest();
    }
}

Option 2 (pass as parameter):

public class TheClass {
    public static void recursionTest(int numb) {
        System.out.println(numb);
        numb  = 1;
        recursionTest(numb);
    }
}

In the second attempt you are doing the System.out.println outside the while loop, so you are printing the value only once. Moreover, the recursive call is never being done due to the infinite while loop, but in the case you put a limit to the loop, the new call to the same method will result in the counter being reset because the same reason as before.

CodePudding user response：

You should read up about the scope and lifespan of variables in Java. You declare a local variable inside the function, which is "created" and initiated (to 0) when you write:

int numb = 0;

This variable is only visible inside this current function call. After your function is executed the variable and its value is invalidated and destroyed. When calling the function again, the variable is created and initiated (to 0) again.

If your variable needs to "survive" multiple calls of your function consider to declare it outside of the function:

private static int numb = 0;

public static void recursionTest() {
   System.out.println(numb);
   while (numb != -1) {
      numb =  1;
   }
   recursionTest();
}

Or pass it as a parameter:

public static void main(String[] args) {
   recursionTest(0);
}

public static void recursionTest(int numb) {
   System.out.println(numb);
   while (numb != -1) {
      numb =  1;
   }
   recursionTest(numb);
}