static object array

Question

public class Array                                                               
{
    static String[] a = new String[] {"red", "green", "blue"};
    static Point[] p = new Point[] {new Point(1, 2), "3,4"};

    public static void main(String[] args)
    {
        System.out.println("hello");
    }

    class Point
    {
        int x;
        int y;

        Point(int x, int y)
        {
            this.x = x;
            this.y = y;
        }

        Point(String s)
        {
            String[] a = s.split(",");
            x = a[0].parseInt();
            y = a[1].parseInt();
        }
    }
}

In the above program, the static Point array initialization fails, reporting error:

Array.java:4: non-static variable this cannot be referenced from a static context
    static Point[] p = new Point[] {new Point(1, 2), "3,4"};  

But, the static String array succeeds. What's the difference between them?

I really need a static object array, because it's easy to refer to without instantiate the outer class.

Thanks

Answer 1

You have to do three things to make your code to work. I will explain them . First see the working version.

public class Array {
    static String[] a = new String[]{"red", "green", "blue"};
    static Point[] p = new Point[]{new Point(1, 2), new Point("3,4")};

    public static void main(String[] args) {
        System.out.println("hello");
    }

    static class Point {
        int x;
        int y;

        Point(int x, int y) {
            this.x = x;
            this.y = y;
        }

        Point(String s) {
            String[] a = s.split(",");
            x = Integer.parseInt(a[0]);
            y = Integer.parseInt(a[1]);
        }
    }
}

The following are the three changes you have to make.

1. change "3,4" to new Point("3,4") or new Point(3,4)

We know that an array can hold items of similar types. Here you are declaring an array named p of type Point. That means it can contain item of type Point only(or its subtypes). But the second element "3,4" is of type String and you have a mismatch. So you must specify either new Point("3,4") or new Point(3,4) to get items of type Point.

2. You need to make your Point class static

From Java Tutorial:

An instance of InnerClass can exist only within an instance of OuterClass 
and has direct access to the methods and fields of its enclosing instance.

Here your Point class is an inner class and it must have given access to all the members of Array class. For that, each object of Point class must associated to an object of Array class. But, the array p you are creating is in static context. So, you have to either make the Point class a static one or make the array p a non-static one.

3. parseInt is not a method of String class

parseInt is a static method of Integer class and not of String class. So you must call it like Integer.parseInt(stringValue).

Hope this helps :)

Answer 2

You'd need to make Point a static nested class, like this:

static class Point {

Answer 3

Point[] must be array or Point "3,4" is not instance of Point and make Point class static for this error