Encapsulation and passing primitives to methods

Question

I have questions regarding encapsulation rules and difference between next two classes

class Phone { 
    public String model; 
    double weight; 
    public void setWeight(double w){
        weight = w;
    }
    public double getWeight(){
        return weight;
    }
}

class Home {
    public static void main(String[] args) {
        Phone ph = new Phone();
        ph.setWeight(12.23);
        System.out.println(ph.getWeight());
    }
}

In the book for Java for I OCA certification this is example of well encapsulated class and from set and get method we can access weight variable and pritns 12.23.

but what confused me is next class:

class Employee { 
    int age; 
    void modifyVal(int age) {  // even if parameter is d instead age prints the same
        age = age + 1;        // even if parameter is d instead age prints the same
        System.out.println(age); 
    }
}

class Office {
    public static void main(String args[]) { 
        Employee e = new Employee();
        System.out.println(e.age);
        e.modifyVal(e.age);
        System.out.println(e.age);
    }
}

It print: 010

meaning that method modifyVal cannot access age variable. Can somebody explain why variable havent change after applying the modufyVal method and what is the difference?

Answer 1

It's because of this line:

age = age + 1; //it's overwriting age parameter and not accessing age instance variable

If it was like that it would modify the age instance variable

this.age = age + 1;

Answer 2

Inside the modifyVal method, the operations on age are scoped to the age argument of the method, never to this.age of the class instance.

Therefore this.age is not incremented.

Answer 3

age = age + 1;

must be

this.age = age + 1;

When the instance variable and the method variable have the same name you have to tell java which one you are manipulating. In absence of this it will take the method variable hence you should use this.age.

Answer 4

 void modifyVal(int age) { 
 age = age + 1;            // not modifying age field of Employee class. local age field being passed to the function is being changed.
 System.out.println(age); 
 }


 System.out.println(e.age); // 0 : default value
 e.modifyVal(e.age);         // 1 : as  age = age + 1; i.e, 0+1
 System.out.println(e.age);  // 0 : default value