我需要一些帮助来了解实例/引用

Question

Just want to know if what I'm trying to teach myself is right.

I'll end up with something like this in one of my programs:

public class Test {
    int array[];

    public static void main(String[] args){
        Test test = new Test();
        test.array = new int[10];
        test.fillArray();

        for(int i=0;i<test.array.length;i++)
            System.out.println(test.array[i]);
    }

    public void fillArray(){
        Test test = new Test();
        for(int i=0;i<test.array.length;i++)
            test.array[i]=i;
    }
}

But I get a null pointer exceptions. I seem to run into these types of issues a decent amount.. Would proper planning of my programs help with this?

The null Pointer was because when I say new it creates a separate object that only exists inside that method correct?

Are there any other ways to fix this other than making the array static or giving it a parameter (or is it argument..?) like I did below?

public class Test {
    int array[];

    public static void main(String[] args){
        Test test = new Test();
        test.array = new int[10];
        test.fillArray(test.array);

        for(int i=0;i<test.array.length;i++)
            System.out.println(test.array[i]);
    }

    public void fillArray(int a[]){
        for(int i=0;i<a.length;i++)
            a[i]=i;
    }
}

Answer 1

Your first fillArray has a shadowing problem in that you are creating a new test instance and initializing it - instead initialize the field array within the current (or this ) instance,

public void fillArray() {
  // Test test = new Test();
  for (int i = 0; i < this.array.length; i++) {
    this.array[i] = i; // <-- this.array or just array[i]
  }
}