"non-static variable this cannot be referenced from a static context"?

I'm a Java newbie and I'm trying to deploy a fibonacci trail through recursive function and then calculate the run time. here is the code I have managed to write:

class nanoTime{
   开发者_运维百科 int fib(int n){
        if(n==0) return 0;
        if(n==1) return 1;
        return this.fib(n-1)+this.fib(n-2);
    }
    public static void main(String[] args){
        double beginTime,endTime,runTime;
        int n=10;
        beginTime = System.nanoTime();
        n = this.fib(n);
        endTime = System.nanoTime();
        runTime = endTime-beginTime;
        System.out.println("Run Time:" + runTime);
    }
}

The problem is when I'm trying to turn it into Byte-code I get the following error:

nanoTime.java:11: non-static variable this cannot be referenced from a static context

I'm wondering what is the problem?!

Change

n = this.fib(n);

to

n = fib(n);

and make the method fib static.

Alternatively, change

n = this.fib(n);

to

n = new nanoTime().fib(n);

You need to instantiate a nanoTime to call an instance method, or make the fib method static as well.

The problem is just what the message says. Your main method is static, which means it is not linked to an instance of the nanoTime class, so this doesn't mean anything. You need to make your fib method static as well, and then use nanoTime.fib(n).

There is no reason to use this in your code.

Steps to take:

1. Rename your class to anything that starts with an upper case letter
2. Remove all this from your code
3. Add the static keyword before int fib(int n){
4. Finally get a good Java book! ;)

# Name the class something else to avoid confusion between System.nanoTime and the name of your class.

class nanoTime1{
    int fib(int n){
        if(n==0) return 0;
        if(n==1) return 1;
        return this.fib(n-1)+this.fib(n-2);
    }

    public static void main(String[] args){
        double beginTime,endTime,runTime;
        int n=10;
        beginTime = System.nanoTime();


        # Instantiate an object of your class before calling any non-static member function

        nanoTime1 s = new nanoTime1();
        n = s.fib(n);
        endTime = System.nanoTime();
        runTime = endTime-beginTime;
        System.out.println("Run Time:" + runTime);
    }
}

Be careful ! In Java the main has to be in a class definition, but it's only the entry point of the program and absolutely not a method of the object/class.

Change this.fib(n) to :

nano obj = new nano();   
n = obj.fib(n);   

this is associated with the instance of the class. A static method does not run with a class instance but with the class itself. So either change the fib method to static or replace the line where you call fib to the above two lines.