Java Creating instance of subclass error

Question

Hi guys Im having trouble creating an instance of a subclass:

    package room;

    public class Room {

    String roomNumber;  
    int capacity;
    boolean projection;

    public Room(String rm, int n, boolean p) { 
        roomNumber = rm; 
        capacity = n;
        projection = p;
    }

    public String getRoomNumber() {
        return roomNumber;
    }

    public int getCapacity() {
        return capacity;
    }

    public boolean hasProjector() { 
        return projection;
    }


    public class ComputerLab extends Room {

    private String os;

    public ComputerLab(String rm, int n, boolean p, String os) {
        super(rm, n, p);
        this.os = os;
}

    public String getOS() {
        return os;
    }

    public void setOS(String update) {
        os = update;
    }

}

    public static void main(String[] args) {
        Room r;
        r = new ComputerLab("G102", 20, true, "WindowsXP"); 
        System.out.println(r.getCapacity());
    }
}

In the line where I create the ComputerLab object in my main function, i am getting the following error:

Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - non-static variable this cannot be referenced from a static context
    at room.Room.main(Room.java:48)

Can anyone help explain why this error is occuring and how to fix it? Many thanks

First thing to do: don't try to run code which doesn't compile. You've got a compilation error, so you should spot that in the list of errors before you try to run the code. — Jon Skeet
– Jon Skeet, Commented Mar 17, 2014 at 16:03

amit · Accepted Answer · 2014-03-17 16:03:45Z

2

Your class ComputerLab is an inner class. In java, an inner class require an instance of the enclosing class - but you don't have one in main() (it is a static method).

Try changing your inner class to be static - that means it does not require an instance of the outer class, by changing the definition from:

public class ComputerLab extends Room

to

public static class ComputerLab extends Room

answered Mar 17, 2014 at 16:03

amit

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Comments

Tristan · Accepted Answer · 2014-03-17 16:06:51Z

Because your inner class is not static, it requires an instance of the surrounding class. While there is a way around this, I believe the behavior you are expecting would be from code more like this.

public class Room {

    protected String roomNumber;
    protected int capacity;
    protected boolean projection;

    public Room(String rm, int n, boolean p) {
        roomNumber = rm;
        capacity = n;
        projection = p;
    }

    public String getRoomNumber() {
        return roomNumber;
    }

    public int getCapacity() {
        return capacity;
    }

    public boolean hasProjector() {
        return projection;
    }

    public static void main(String[] args) {
        Room r = new ComputerLab("G102", 20, true, "WindowsXP");
        System.out.println(r.getCapacity());
    }
}

class ComputerLab extends Room {

    private String os;

    public ComputerLab(String rm, int n, boolean p, String os) {
        super(rm, n, p);
        this.os = os;
    }

    public String getOS() {
        return os;
    }

    public void setOS(String update) {
        os = update;
    }

}

Grim · Accepted Answer · 2014-03-17 16:03:31Z

0

Try

new Room(...).new ComputerLab(...)

answered Mar 17, 2014 at 16:03

Grim

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Comments

Brian · Accepted Answer · 2014-03-17 16:07:06Z

0

Your inner class should be static to use it like this. A good use of Nested classes are as as helper classes for a given nesting class.

I'm a personal fan of otherwise separating your classes into separate files.

answered Mar 17, 2014 at 16:07

Brian

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Comments

Snake51 · Accepted Answer · 2014-03-17 16:12:02Z

0

You can add static in the declaration of the ComputerLab class :

public static class ComputerLab extends Room ...

answered Mar 17, 2014 at 16:12

Snake51

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