java initializing a field which is a class

Question

sorry, c++ programmer new to java strikes again

i have this code

public class MainView extends View {

    static final int DRAW_LIST_SIZE=100;
    class EventDrawStuff {
        int         a;
        int         b;
        int         c;
    }
    static EventDrawStuff   m_drawList[] = new EventDrawStuff[DRAW_LIST_SIZE];

    class DrumEventDrawStuff {
        int     x;
        int     y;
    } 
    static DrumEventDrawStuff m_eventDrawStuff = new DrumEventDrawStuff();

the declaration of m_drawList seems to work ok, the declaration of m_eventDrawStuff doesn't compile. what's the difference, can t just be that m_drawList is an array? i notice that if i say

static DrumEventDrawStuff[] m_eventDrawStuff = new DrumEventDrawStuff[1];

that is ok but i don't really want it to be an array of one, since its only a single thing. i realise the way to fix the original code is to initialize m_eventDrawStuff in the constructor but that seem cumbersome and unnecessary.

perhaps i've got the wrong idea altogether, please enlighten me, thanks

Subhrajyoti Majumder · Accepted Answer · 2013-01-11 08:07:28Z

1

You can do it in two way -

Make your inner class static
Create DrumEventDrawStuff object with the help of MainView object.

static DrumEventDrawStuff m_eventDrawStuff = new MainView().new DrumEventDrawStuff();

edited Jan 11, 2013 at 8:07

answered Jan 11, 2013 at 7:18

Subhrajyoti Majumder

41.3k13 gold badges84 silver badges103 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

steveh Over a year ago

thanks, i made the inner class static and it fixed the problem i asked about. however, i have another issue with the first declaration. i made EventDrawStuff static too. and the initialization code i have i guess really creates 100 pointers right, not 100 EventDrawStuff data structures as it would in c++. do i now need to go though and initialize each element in m_drawList with a new EventDrawStuff() ?

Subhrajyoti Majumder Over a year ago

@steveh There is no pointer in java :). It will create 100 indexes array to hold 100 references of EventDrawStuff.

steveh Over a year ago

thanks, i realize there is nothing called pointer in java but in reality m_drawList[] in my example is an array of pointers (or references if you prefer). in c/c++ i could have created an array of data structures but i guess that's not possible here.

Subhrajyoti Majumder Over a year ago

you can. EventDrawStuff[] is custom data-structure's array

Cameron Skinner · Accepted Answer · 2013-01-11 07:22:54Z

The problem is that you are trying to instantiate a DrumEventDrawStuff in a static context. DrumEventDrawStuff is an inner class of MainView, which means that each instance of DrumEventDrawStuff has an implicit reference to the instance of MainView that holds it (the "outer this").

If you make DrumEventDrawStuff a static class then you'll be OK because that will remove the implicit outer this:

static class DrumEventDrawStuff {
    ...
}

At this point you're probably wondering why the non-static EventDrawStuff class can be used in a static context.

The answer is that you are not actually creating any instances of EventDrawStuff when you create the array. Unlike C++, Java does not instantiate any objects when you create a new array. Thus, it's perfectly OK to statically declare and create the array of EventDrawStuff because it will be filled with null values.

snieke · Accepted Answer · 2013-01-11 07:21:01Z

Since DrumEventDrawStuff is a non-static inner class here, it needs a "surrounding" instance of MainView. Without that, it cannot be instantiated.

Your array, m_drawList is only the array without instances of EventDrawStuff, otherwise you had the same problem.

If you really want to have those static fields, the classes must be static so they don't need a surrounding instance:

public class MainView extends View {

static final int DRAW_LIST_SIZE=100;
static class EventDrawStuff {
    int         a;
    int         b;
    int         c;
}
static EventDrawStuff   m_drawList[] = new EventDrawStuff[DRAW_LIST_SIZE];

static class DrumEventDrawStuff {
    int     x;
    int     y;
} 
static DrumEventDrawStuff m_eventDrawStuff = new DrumEventDrawStuff();

Collectives™ on Stack Overflow

java initializing a field which is a class

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related