java using this word is optional or not? [duplicate]

Question

Is using the keyword this optional in Java? Or do I have to use it? And when it is not optional? In the following code it does not affect my application, regardless of how many instances of Employee I made. The print method prints Employee details without anything wrong even if details do not match.

public class Employee {
    String name;
    int Salary;
    int pension;
    String workPlace;
    String teleNo;
    int age;

    void printDetails(){
        System.out.println("Name is :  "+this.name );
        System.out.println("age is  :  "+this.age );
        System.out.println("WorkPlace is  :  "+this.workPlace );
        System.out.println("Salary is  :  "+Salary );
        System.out.println("Pension is  :  "+this.pension );
        System.out.println("Telephone No. is  :  "+this.teleNo );
        System.out.println("age is  :  "+Integer.toString(age) );
    }
}

public class Main extends Employee {

    /**
     * @param args
     */
    public static void main(String[] args) {
        Employee obj=new Employee();
        obj.age=25;
        obj.name="Yasser";
        obj.pension=100_000;
        obj.teleNo="xxx_xxxx";
        obj.workPlace="Egypt";
        obj.Salary=1000000;

        obj.printDetails();
        Employee obj1=new Employee();
        obj1.age=29;
        obj1.name="asser";
        obj1.pension=100_000;
        obj1.teleNo="xxx_xxxx";
        obj1.workPlace="rgypt";
        obj1.Salary=2000000;
        obj1.printDetails();
    }
}

Answer 1

It's always optional unless you are referencing a field which has the same name as a local variable. For example:

class Sample
{
    int value;

    void method(int value)
    {
        this.value = value; //this is required
    }
}

If this is not the case, then using this behaves identically to referencing the variable directly.

Answer 2

this in Java is optional, unless you have a local variable or parameter of the same name, in which case the variable name will be referring to the latter.

public class Employee {

    private String name = "one";

    public void printDetails() {
        String name = "two";
        System.out.println(this.name); // "one"
        System.out.println(name); // "two"
    }
}