java thread a small problem

The next, the novice, would like to ask why the following code what the results are not right ?

  import java.util.concurrent.*;
  
  public class Fibonacci implements Runnable{
  	private int n = 0;
  	private int[] fibo;
  	
  	public Fibonacci(){}
  	public Fibonacci(int n){
  		this.n = n;
  		fibo = new int[n];
  	}
  	
  	public String toString(){
  		String str = "";
  		for(int i = 0;i < n; i ++){
  			str += fibo[i] + " ";
  		}
  		
  		return str;
  	}
  	
  	public void run(){
  		fibo[0] = 0;
  		fibo[1] = 1;
  		n -= 2;
  		
  		int i = 2;
  		while(n-- > 0){
  			fibo[i] = fibo[i - 1] + fibo [i - 2];
  			i ++;
  		}
  		System.out.println(toString());
  		Thread.yield();
  	}
  	
  	public static void main(String[] arg){
  		ExecutorService exec = Executors.newCachedThreadPool();
  		for(int i = 0;i < 5;i ++)
  			exec.execute(new Fibonacci(20));
  		
  		exec.shutdown();
  	}
  }
  
  

In addition, in ExecutorService see Future return type declaration , will explain how the first ?
Thank you ! !
------ Solution ---------------------------------------- ----
I said you did not result gnaw it, so I tried for a long time . . .
toString you used when n, has the run () which is - to -1 it.
becomes
for (int i = 0; i <-1; i + +) {
str + = fibo [i] + "";
}

------ Solution ------------------------------------ --------

first is a generic function declaration , there is no practical significance.
------ For reference only -------------------------------------- -

ah. . Yes, yes , thank you ! !