为什么在此布尔数组上得到NullPointerException？

Question

In my class' constructor, I initialize a boolean array with boolean[] list = new boolean[n] where n is the only parameter for the constructor, and I assign every index of list to true with Arrays.fill(list, true) . EDIT: list is first created outside the constructor with private boolean[] list

Then in a method I do this:

//n still refers to the parameter in the constructor
for(int i = 2; i < n; i++){
    if(list[i]){
        for(int j = i; j < n; j*=i){
            list[j] = false;
        }
    }
}

And if(list[i]) throws the NullPointerException, even though I initialized all of list with the Arrays.fill(list, true) . I originally had a loop that individually set everything in list to true, and that gave the same error, so now I'm stumped.

EDIT: Here's the full constructor.

public Seive(int n){

        //create an array of booleans of length n
        list = new boolean[n];
        this.n = n;

        //set all booleans in the array to true
        Arrays.fill(list, true);

        //set 0 and 1 to false so that the algorithm can ignore them 
        //and they won't be put into the list of primes
        list[0] = false;
        list[1] = false;

}

I left one thing out that I just realized was important: I create list outside of the constructor with private boolean[] list , so the method that the exception is thrown in should be able to access the array. I also made the change that Eran suggested before posting this block of code.

Answer 1

Since you have this - boolean[] list = new boolean[n]; - in your constructor, this array is declared and initialized locally in the constructor. The method accesses a different array with the same name (probably the member you declared in your class) that is not initialized.

Change the initialization in your constructor to :

list = new boolean[n];