可以在Java中将实例变量声明为静态变量

Question

I wrote the following code:

public class Count
{

    private static int[] array;

    public Count( int[] array )
    {
        this.array = array;
    }

    public int count()
    {
        int count = 0;
        for (int i = 0; i < array.length; i++) {
            int x = array[i];
            if (x < 0) {
                if (x == -1 && i > 0 && array[i - 1] == -1) {
                    break;
                }
            } else {
                count++;
            }
        }
        return count;
    }

}

and then in another class I wrote:

Count c = new Count(new int[]{1,-1,-1});

and checked the result of c.count() . I didn't get a compiler error,and I got 1 as I wanted. I wonder why because how can I assign a static variable to an instance variable? And also, what happens if I create another instance of class Count? Will they both have the same variable?or just their own copy of it? and what should happen if I try to access the variable by writing Count.array ? Thanks.

Answer 1

Classes can access their own static members.

All classes will see the same static members (ignoring thread contention complications).

If you create another instance of Count they will both be modifying the same variable.

That's what static means...

Answer 2

array is shared between all the instances of Count - they can all read from it and write to it. And as you stated, if you instance modifies it, all the other instances will see the newly modified value.

Answer 3

I'd recommend that you write the example you're asking about and see. There's no better authority than the JVM, and it'll tell you faster than SO will.

I think you should understand the meaning of class and instance variables and pick the one that matches your intent.

So it's either class:

public class Count
{
   // Shared by all 
   private static int[] array;
}

or instance:

public class Count
{
   // Owned by instance
   private int[] array;

   public Count(int [] v) {
       if (v == null) throw new IllegalArgumentException("array cannot be null");
       this.array = new int(v.length);
       System.arraycopy(v, this.array, 0, v.length);
   }
}

You should not just assign the array reference to the private data member. You need to allocate memory and copy the values into it. Otherwise the supplier of the input array will modify the private values if it updates using their reference.

Too many people don't realize that assigning mutable references to private members breaks encapsulation. Maybe you won't be one of them now.