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

Jomoos · Accepted Answer · 2012-02-22 09:13:47Z

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 :)

Björn Pollex · Accepted Answer · 2012-02-22 08:45:58Z

3

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

static class Point {

answered Feb 22, 2012 at 8:45

Björn Pollex

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Comments

Nirmal- thInk beYond · Accepted Answer · 2012-02-22 08:46:13Z

1

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

answered Feb 22, 2012 at 8:46

Nirmal- thInk beYond

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Comments

Simon · Accepted Answer · 2012-02-22 08:49:22Z

Here is the explanation from the Oracle Online Reference

Inner Classes

As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields. Also, because an inner class is associated with an instance, it cannot define any static members itself.

Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:

class OuterClass {
    ...
    class InnerClass {
        ...
    }
}

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.

Also it cannot be accessed from within static members of the enclosing type, as it requires a an instance to exist.

khachik · Accepted Answer · 2012-02-22 08:49:40Z

1

Declare your Point class as static:

...
static class Point
{
    ...
}

It is needed because Point is an inner class (String is not).

However, {new Point(1, 2), "3,4"}; is your next problem. I guess it should be {new Point(1, 2), Point(3, 4)};

answered Feb 22, 2012 at 8:49

khachik

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2 Comments

pengguang001 Over a year ago

Thanks. I want "3,4" to trigger the Point(String). It seems it's unable to do so.

khachik Over a year ago

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

Balaswamy Vaddeman · Accepted Answer · 2012-02-22 08:53:49Z

1

You might want to make Point Static nested class.

static class Point {

}

check this

answered Feb 22, 2012 at 8:53

Balaswamy Vaddeman

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