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How can I change a superclass variable from a subclass?

In C ++, I could use 1 class in multiple files, and when I change a value with a function from one file, it changes that value globally.

In Java, every file must have its own class; when i change a value using a method from one file it doesn't change that value globally.

For example, in C ++, files might look like this:

someClass.h

class someClass{
private:
    int x;
public:
    //Constructor
    someClass(){
        x = 5;
    }
    void changeVariable(); //Declaring function for later
}

main.cpp

int main(){
    someClass cClass;
    cClass.changeVariable(); //Call function
    cout << x; //outputs 6
}

fileA.cpp

void someClass::changeVariable(){
    x = 6; //x is changed to 6 globally.
}

In Java:

someClass.java

public class someClass {
    int x;

    //Constructor
    public someClass() {
        x = 5;
    }

main.java

public class main {
    public static void main() {
        someClass cClass = new someClass();
        subClass cSub = new subClass();
        cSub.changeVariable();
        System.out.print(x); //outputs 5
    }
}

fileA.java

public class fileA extends someClass {
    void changeVariable() {
        x = 6; //x is changed to 6 only for fileA
    }
}

My question is how to change a variable from a subclass so that the variable changes globally (for Java). Sorry if the question is still confusing.

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3 answers


Try the following:

public class someClass {
    static int x;

    //Constructor
    public someClass() {
        x = 5;
    }

That variable, where static means that its value is common to all objects of the same class. The fact that only one variable is created for everyone x

, and not one for each object.



Read this answer if you want it to explain well what static means:

What does the static keyword do in a class?

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No need for a static variable, this is the most equivalent thing you can do in Java:

public abstract class SomeClass {
  protected int x = 5;
  public abstract void changeVariable();
}

public class FileA extends SomeClass {
  @Override
  public void changeVariable() {
    x = 6;
  }
}


SomeClass

of course it doesn't have to be abstract

, but you'll have to implement it changeVariable

.

It also x

can't be private

, it must be protected

, so subclasses can be accessed.

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Expose a function. setXyz in parent

public class someClass {
    int x;
 //Constructor
    public someClass() {
        x = 5;
    }
    public void setX(int n){
         this.x = n;
     }
  } 

   public class fileA extends someClass {
    void changeVariable() {
      setX(6); 
   }
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