从一种方法调用变量到另一种方法

Question

Alright, this is driving me crazy! I have two methods and two classes. I am trying to call a variable from method A to method B. Method A resides in Class A, however it extends Class B. Method B is in Class B. I am using private variables with get/set, but I cannot understand why, when the variable is update in method A it is not transferring over to method B, even though the variable resides in Class B. Here is the code:

public class A extends class B {
   public double method A () {
     setSales(userInput.nextDouble());
     double mySales = getSales;
System.out.print("my sales is " +mySales+);
    }
}

public class B {
    private double sales;

    void setSales(double sales) {
    this.sales=sales;
    }

    double getSales() {
    return sales;
    }

   public method B () {
      System.out.print(+sales+);
      //printout equals 0 with user input of any number
     }
}

Answer 1

Local Variables Have No Memory

When you exit a method, both the local variables and the parameter variables disappear (although, if you pass an object as a parameter, you usually have access to the object when you exit the method). That means if you make a call to testRightTriangle() and run the declarations, it creates new boxes and initializes them. When you exit, those local variables disappears.

When you call it a second time, it recreates the local variables, and reinitializes them.

Parameters behave somewhat like this. These variables gets initalized to the arguments.

Only instance variables maintain their value over method calls. That is, if some instance variable has value of 4 for height, and you change it to 10, then the next time you make a method call, it's still 10. It stays 10 until some method call changes the value.

Answer 2

 double mySales = getSales;

Needs to be

  double mySales = getSales();

since getSales is a method.

also

this.sales=sales;

needs to be switched if you are trying to use the parameter passed to the method to set the private variable in class B.

Answer 3

Class A is not Class B all they have in common are the methods and attributes. Because Class A extends Class B doesn't mean that they share the same memory thus the field they each hold are stored at different memory locations. That's why when you update attribute A in Class A it doesn't not transfer over to Class B. If you want to be able to modify the attribute in Class B you might want to add a parameter to method A.

Change your method A to be this instead:

public double method A (Class B instanceToModify) 
 {
     setSales(userInput.nextDouble());
     instanceToModify.mySales = getSales();
     System.out.print("my sales is " +mySales+);
 }