Java - 通过Class中的方法更改数组的值

Question

Here is my code:

class Myclass {

    private static int[] array;

    public static void main(String[] args) {
        Myclass m = new Myclass();

        for (int i = 0; i < 10; i++) {
            m.array[i] = i;
            System.out.println(m.array[i]);
        }
    }

    public Myclass() {
        int[] array = new int[10];
    }
}

It throws a java.lang.nullPointerException when trying to do this:

m.array[i] = i;

Can anybody help me please?

Answer 1

You have declared a local variable array in your constructor, so you're not actually initializing the array declared in Myclass .

You'll want to refer directly to array in the constructor. Instead of

int[] array = new int[10];

Use this

array = new int[10];

Additionally, you've declared array static in the scope of your Myclass class.

private static int[] array;

You only have one instance of Myclass here, so it doesn't matter, but normally this would not be static, if you're initializing it in a constructor. You should remove static :

private int[] array;

Answer 2

In your constructor you are making your assignment to a local variable names array, not the static class variable also named array. This is a scope problem.

I'm also guessing that since you access array via m.array, you want a member variable and not a static one. Here's the fix

class Myclass {

  private int[] array;

  public static void main(String[] args) {
    Myclass m = new Myclass();
    for (int i = 0; i < 10; i++) {
        m.array[i] = i;
        System.out.println(m.array[i]);
    }
  }

  public Myclass() {
        rray = new int[10];
  }

}

Answer 3

in MyClass() type this

this.array = new int [10];

instead of this

int[] array = new int[10];

Answer 4

Your code should be as below. In the constructor, instead of initializing the instance variable you created a new local variable and the instance variable was left uninitalized which caused the NullPointerException. Also the instance variable shouldn't be static.

class Myclass {

  private int[] array;

public static void main(String[] args) {
 Myclass m = new Myclass();
 for (int i = 0; i < 10; i++) {
    m.array[i] = i;
    System.out.println(m.array[i]);
 }
}

public Myclass() {
      array = new int[10];
}

}

Answer 5

First, if you plan to use array as a field of m (ie m.array ) don't declare it as static , but:

private int[] array;

Next thing you have to do is to initialize it. Best place to do that is in the constructor:

public MyClass() {
    array= new int[10]; //just array = new int[10]; don't put int[] in front of the array, because the variable already exists as a field.
}

The rest of the code should work.