class input{
    private double num1;
    private double num2;
    
    public void setNum1(double num1) {
        this.num1 = num1;
    }

    public double getNum1() {
        return this.num1;
    }
    
    public void setNum2(double num2) {
        this.num2 = num2;
    }

    public double getNum2() {
        return this.num2;
    }
}

class calculator extends input{
    double a = getNum1();
    double b = getNum2();
    
    double add() {
        return a b;
    }
}

class advcalc extends calculator{
    double less() {
        return a-b;
    }
}

public class Inheritence {
    public static void main(String[] args) {
    advcalc cal = new advcalc();
    cal.setNum1(10);
    cal.setNum2(10);
    
    System.out.println(cal.add());
    }
}

Here I'm practicing inheritance. The first class gets inputs, second class is used for addition and 3rd is used for subtraction. I have created an object of 3rd class. After passing the inputs by setNum1() && setNum1(), when I try to print the return value of the add, it prints 0.0. I couldn't figure out what's the issue. Someone help me out please.

CodePudding user response：

a = getNum() is being executed before assigning the value.

double a = getNum1();
double b = getNum2();
double add() {
  return a b;
}

instead of this you can assign the values inside add method.This way it will be assigned after you create an instance.

double a, b = 0;
double add() {
    this.a = getNum1();
    this.b = getNum2();
    return a   b;
}

There are various ways to achieve this . You can also pass the values in add method or create a parameterized constructor.

CodePudding user response：

In your code the variables a and b, defined in calculator will always be 0.0, because they are set during the initialization of you object and only once:

1. You call new advcalc()
2. During initialization double a = getNum1(); and double b = getNum2(); are called
3. You call setNum1 and change the value of num1 in input, but a stays unchanged
4. You call add and a and b are both 0.0

If you drop the variables a and b and call getNum1 and getNum2 in add it should work. You could also override setNum1 and setNum2 in calculator and set a and b there.

CodePudding user response：

I'm practicing inheritance.

Then are you aware that a and b (in class calculator) are member variables¹ and that they are hiding² member variables num1 and num2 in the parent class (i.e. class input). Hiding can lead to problems (as you have discovered). Hence you should make num1 and num2 protected³ and remove a and b.

Consider the following:
(Note that I changed the class names according to Java naming conventions.)

public class Inheritance {

    public static void main(String[] args) {
        AdvCalc cal = new AdvCalc();
        cal.setNum1(10);
        cal.setNum2(10);
        System.out.println(cal.add());
    }
}

class Input {
    protected double num1;
    protected double num2;

    public void setNum1(double num1) {
        this.num1 = num1;
    }

    public double getNum1() {
        return this.num1;
    }

    public void setNum2(double num2) {
        this.num2 = num2;
    }

    public double getNum2() {
        return this.num2;
    }
}

class Calculator extends Input {
    public double add() {
        return num1   num2;
    }
}

class AdvCalc extends Calculator {
    public double less() {
        return num1 - num2;
    }
}

¹ Refer to Declaring Member Variables (in Oracle's Java tutorials).
² Refer to Variable Shadowing and Hiding in Java
³ Refer to Controlling Access to Members of a Class