Polymorphism issue

[Alexandros Stefanidis]

Hi I have following code:

abstract class MyTester {
    public static void main(String[] args) {
        Person p1 = new Person("Kings", "Paul", 22, true, 1200f);
        MarriedPerson mp1 = new MarriedPerson("Tront", "Betty", 31, 980.5f, 3);
        p1.printInfo();
        mp1.printInfo();
    }
}

public class Person
{
    // instance variables - replace the example below with your own
    private String lastname;
    private String firstname;
    private int age;
    private boolean married;
    private float salary;

    /**
     * Constructor for objects of class Person
     */
    public Person(String lastname, String firstname, int age, boolean married, float salary)
    {
        // initialise instance variables
        this.lastname = new String(lastname);
        this.firstname = new String(firstname);
        this.age = age;
        this.married = married;
        this.salary = salary;
    }

    public String getLastName() {return lastname;}
    public String getFirstName() {return firstname;}
    public int getAge() {return age;}
    public boolean isMarried() {return married;}
    public float getSalary() {return salary;}
    
    public void printInfo(){
        System.out.print(getFirstName()+" "+getLastName()+" is "
       +getAge()+" years old, gets a "+getSalary()
       +" Euros salary and is");
       if (isMarried() == false)
          System.out.print(" not");
       System.out.println(" married."); 
    }
}

public class MarriedPerson extends Person
{
    // instance variables - replace the example below with your own
    private int children;

    /**
     * Constructor for objects of class MarriedPerson
     */
    public MarriedPerson(String lastname, String firstname, int age, float salary, int children)
    {
        // initialise instance variables
        super(lastname, firstname, age, true, salary);
        this.children = children;
    }
    
    @Override
    public void printInfo()
    {
        System.out.print(getFirstName()+" "+getLastName()
                +" is " +getAge()+ " years old, gets a " + getSalary()
                +" Euros salary and is" + " married with ");
        if (getNoOfChildren() > 0)
            System.out.print(getNoOfChildren());
        else System.out.print("no");
        System.out.println(" children.");
    }

    public int getNoOfChildren()
    {
        // put your code here
        return children;
    }
}

The code compiles and runs successfully.
How can I use the super keyword in printInfo() of MarriedPerson class to replace existing code and do the same job?

[Carey Brown]

public class Person
{
    public String toString(){
        String s = getFirstName()+" "+getLastName()+" is "
       +getAge()+" years old, gets a "+getSalary()
       +" Euros salary and is");
       if (isMarried() == false)
          s += (" not");
       s += (" married."); 
       return s;
    }
}

public class MarriedPerson extends Person
{
    @Override
    public String toString()
    {
        String s = super.toString();
        if (getNoOfChildren() > 0)
            s += (getNoOfChildren());
        else s += ("no");
        s += (" children.");
        return s;
    }
}

I suggest using toString(), and then printing the string elsewhere.

[Campbell Ritchie]

Welcome to the Ranch

I like to put that sort of thing into one statement if I can:-

@Override
public String toString()
{
   return super.toString() + " " +
            (getNoOfChildren() > 0 ? getNoOfChildren() : "no") + " children.";
}

You can use the String#format() method to create such a String as an alternative.
Avoid += on Strings because it can lead to slow performance. Avoid == false, too. Use the bang sign ! for not.

[Paul Clapham]

I like to use a ChoiceFormat for that, because not only does it simplify that code, it takes care of an issue you missed. Have a look at the tutorial: https://docs.oracle.com/javase/tutorial/i18n/format/choiceFormat.html

[Campbell Ritchie]

I never knew about choice format. Thank you

[Alexandros Stefanidis]

I want to keep the printInfo() so I have written the following:

In Person:

public void printInfo(){
        System.out.print(getFirstName()+" "+getLastName()+" is "
       +getAge()+" years old, gets a "+getSalary()
       +" Euros salary and is");
       if (isMarried() == false)
          System.out.print(" not");
       System.out.println(" married."); 
    }

In MarriedPerson:

@Override
    public void printInfo()
    {
        super.printInfo();
        String s = ("with ");
        if (getNoOfChildren() > 0)
            s += (getNoOfChildren());
        else s += ("no");
        s += (" children.");
        System.out.print(s);
    }

I get in my output:

Paul Kings is 22 years old, gets a 1200.0 Euros salary and is married.
Betty Tront is 31 years old, gets a 980.5 Euros salary and is married.
with 3 children

How can I get in my output the following:
Paul Kings is 22 years old, gets a 1200.0 Euros salary and is married.
Betty Tront is 31 years old, gets a 980.5 Euros salary and is married with 3 children.

[Campbell Ritchie]

I suggest you don't write a printInfo method, but override the toString() method.

@Override
public String toString()
{
   return ...
}

Whatever you had in the printInfo class, move it to toString but no print calls. Everything to be returned. In the subclass, use one of that cheat Campbell Ritchie's naughty tricks:-

@Override
public String toString()
{
   return super.toString() + "\b, ..." ...;
}

Try the backspace character which will probably allow you to overwrite the full stop
If you want a printInfo method, you don't need to override it in the subclass. It is much simpler than you think now:-

public void printInfo()
{
   System.out.println(this);
}

If you go looking for System.out.println, after about 6‑8 clicks you find out that the toString method is called, so you now know what is printed out.

[Alexandros Stefanidis]

Hi! First I have pressed button that this topic is resolved. But is not yet. Is there a way to undo that.
Second I have expanded the code:

public class Person
{
    // instance variables - replace the example below with your own
    public static final int MALE = 0;
    public static final int FEMALE = 1;
    
    private String lastname;
    private String firstname;
    private int age;
    private boolean married;
    private float salary;
    private int sex;
    
    /**
     * Constructor for objects of class Person
     */
    public Person(String lastname, String firstname, int age, boolean married, float salary,
            int sex)
    {
        // initialise instance variables
        setLastName(lastname);
        setFirstName(firstname);
        setAge(age);
        setMarried(married);
        setSalary(salary);
        setSex(sex);
    }

    public String getLastName() {return lastname;}
    public String getFirstName() {return firstname;}
    public int getAge() {return age;}
    public int getSex() {return sex;}
    public boolean isMarried() {return married;}
    public float getSalary() {return salary;}
    
    public void setFirstName(String firstname)
    {
        this.firstname = new String(firstname);
    }
    
    public void setLastName(String lastname)
    {
        this.lastname = new String(lastname);
    }
    
    public void setAge(int age)
    {
        this.age = age;
    }
    
    public void setSex(int sex)
    {
        this.sex = sex;
    }
    
    public void setSalary(float salary)
    {
        this.salary = salary;
    }
    
    public void setMarried(boolean married)
    {
        this.married = married == true ? true : false;
    }
    
    public void printInfo(){
        System.out.print(getFirstName()+" "+getLastName()+" is " +getAge()+" years old, ");
        if(getSex() == 0)
            System.out.print("male");
        else if(getSex() == 1)
            System.out.print("female");
        System.out.print(", gets a "+getSalary()+" Euros salary and is");
        if (isMarried() == false)
            System.out.print(" not");
        System.out.println(" married."); 
    }
}

public class MarriedPerson extends Person
{
    // instance variables - replace the example below with your own
    private int children;

    /**
     * Constructor for objects of class MarriedPerson
     */
    public MarriedPerson(String lastname, String firstname, int age, float salary, int sex,
            int children)
    {
        // initialise instance variables
        super(lastname, firstname, age, true, salary, sex);
        setNoOfChildren(children);
    }
    
    @Override
    public void printInfo()
    {
        System.out.print(getFirstName()+" "+getLastName()+" is "+getAge()+" years old, ");
        if(getSex() == 0)
            System.out.print("male");
        else if(getSex() == 1)
            System.out.print("female");    
        System.out.print(", gets a "+getSalary()+" Euros salary and is" + " married with ");
        if (getNoOfChildren() > 0)
            System.out.print(getNoOfChildren());
        else System.out.print("no");
            System.out.println(" children.");
    }

    public void setNoOfChildren(int children)
    {
        this.children = children;
    }
    
    public int getNoOfChildren()
    {
        // put your code here
        return children;
    }
}

abstract class MyTester {
    public static void main(String[] args) {
        Person p1 = new Person("Kings", "Paul", 22, true, 1200f, Person.MALE);
        MarriedPerson mp1 = new MarriedPerson("Tront", "Betty", 31, 980.5f, Person.FEMALE, 3);
        p1.setMarried(false);
        mp1.setMarried(false);
        p1.printInfo();
        mp1.printInfo();
        System.out.println("Is mp1 married?: "+mp1.isMarried());
    }
}

My output is:
Paul Kings is 22 years old, male, gets a 1200.0 Euros salary and is not married.
Betty Tront is 31 years old, female, gets a 980.5 Euros salary and is married with 3 children.
Is mp1 married?: false

It seems that in second line that married variable is true.
In third line married variable is false.

How can I keep variable married unchanged when called via object mp1? Do I have to create a setMarried method in MarriedPerson class?

[Carey Brown]

MarriedPerson Line 25 you've hard coded the outcome of 'married'.

[Campbell Ritchie]

Well, all married people are married. That isn't the real problem, I think. I think the real problem is having a married person class in the first place because people can change from one category (single) to the other and back (divorce, widowhood). It i;s very difficult to model such things in an inheritance hierarchy, and simply having a married field is probably better. Having a Condition (that is the official term) enum is better still:-
public enum Condition{ SINGLE, MARRIED, DIVORCED, WIDOWED; }

[Winston Gutkowski]

Alexandros Stefanidis wrote:How can I get in my output the following:
Betty Tront is 31 years old, gets a 980.5 Euros salary and is married with 3 children.

It's possibly worth mentioning that you're buying yourself a world of work (and possible mistakes) by requiring these "English" sentences. If you simply output attributes as a "list", there is far less to go wrong, viz:

  Betty Tront - age: 31; salary: 980.5 Euros; status: married; children: 3.

However, I strongly suspect some PHB (or tutor) asked you to do it, so it's probably not your fault.

Winston

[Alexandros Stefanidis]

Here is changed code:

public class MarriedPerson extends Person
{
    // instance variables - replace the example below with your own
    private int children;

    /**
     * Constructor for objects of class MarriedPerson
     */
    public MarriedPerson(String lastname, String firstname, int age, float salary, int sex,
            int children)
    {
        // initialise instance variables
        super(lastname, firstname, age, true, salary, sex);
        setNoOfChildren(children);
    }
    
    @Override
    public void printInfo()
    {
        System.out.print(getFirstName()+" "+getLastName()+" is "+getAge()+" years old, ");
        if(getSex() == 0)
            System.out.print("male");
        else if(getSex() == 1)
            System.out.print("female");    
        System.out.print(", gets a "+getSalary()+" Euros salary and is");
        if (isMarried() == false)
            System.out.print(" not married.");
        else System.out.println(" married with ");
        if (getNoOfChildren() > 0)
            System.out.print(getNoOfChildren());
        else System.out.print("no");
            System.out.println(" children.");
    }
    
    @Override
    public void setMarried(boolean married)
    {
        this.married = true;
    }

    public void setNoOfChildren(int children)
    {
        this.children = children;
    }
    
    public int getNoOfChildren()
    {
        // put your code here
        return children;
    }
}

public class Person
{
    // instance variables - replace the example below with your own
    public static final int MALE = 0;
    public static final int FEMALE = 1;
    
    private String lastname;
    private String firstname;
    private int age;
    boolean married;
    private float salary;
    private int sex;
    
    /**
     * Constructor for objects of class Person
     */
    public Person(String lastname, String firstname, int age, boolean married, float salary,
            int sex)
    {
        // initialise instance variables
        setLastName(lastname);
        setFirstName(firstname);
        setAge(age);
        setMarried(married);
        setSalary(salary);
        setSex(sex);
    }

    public String getLastName() {return lastname;}
    public String getFirstName() {return firstname;}
    public int getAge() {return age;}
    public int getSex() {return sex;}
    public boolean isMarried() {return married;}
    public float getSalary() {return salary;}
    
    public void setFirstName(String firstname)
    {
        this.firstname = new String(firstname);
    }
    
    public void setLastName(String lastname)
    {
        this.lastname = new String(lastname);
    }
    
    public void setAge(int age)
    {
        this.age = age;
    }
    
    public void setSex(int sex)
    {
        this.sex = sex;
    }
    
    public void setSalary(float salary)
    {
        this.salary = salary;
    }
    
    public void setMarried(boolean married)
    {
        this.married = married == true ? true : false;
    }
    
    public void printInfo(){
        System.out.print(getFirstName()+" "+getLastName()+" is " +getAge()+" years old, ");
        if(getSex() == 0)
            System.out.print("male");
        else if(getSex() == 1)
            System.out.print("female");
        System.out.print(", gets a "+getSalary()+" Euros salary and is");
        if (isMarried() == false)
            System.out.print(" not");
        System.out.println(" married."); 
    }
}

abstract class MyTester {
    public static void main(String[] args) {
        Person p1 = new Person("Kings", "Paul", 22, true, 1200f, Person.MALE);
        MarriedPerson mp1 = new MarriedPerson("Tront", "Betty", 31, 980.5f, Person.FEMALE, 3);
        p1.setMarried(false);
        mp1.setMarried(false);
        p1.printInfo();
        mp1.printInfo();
        System.out.println("Is mp1 married?: "+mp1.isMarried());
    }
}

Here is output:
Paul Kings is 22 years old, male, gets a 1200.0 Euros salary and is not married.
Betty Tront is 31 years old, female, gets a 980.5 Euros salary and is married with
3 children.
Is mp1 married?: true

It works as expected. One question though- In person class line 10 married variable from private boolean was changed to boolean. Is this liable?

[Alexandros Stefanidis]

Hi!
to Winston Gutkowski - this is a Uni module excercise

Another question is to add a method setSalary in class MarriedPerson that will take as argument a MarriedPerson object named spouse and will do the following:

a) check spouse sex
b)If it is not the same with variable sex then in variable salary will be added the relevant variable of object spouse
c)if it the same nothing happens.

Also main have been changed to:

abstract class MyTester {
    public static void main(String[] args) {
        //Person p1 = new Person("Kings", "Paul", 22, true, 1200f, Person.MALE);
        MarriedPerson mp1 = new MarriedPerson("Tront", "Betty", 31, 980.5f, Person.FEMALE, 3);
        MarriedPerson mp2 = new MarriedPerson("Tront", "Kirk", 31, 2080f, Person.MALE, 2);
        MarriedPerson mp3 = new MarriedPerson("Tront", "Sonia", 31, 600f, Person.FEMALE, 0);
        mp1.printInfo();
        mp2.printInfo();
        mp3.printInfo();
        mp1.setSalary(mp2);
        mp1.printInfo();
        mp1.setSalary(mp3);
        mp1.printInfo();
        mp1.setSalary(mp2.getSalary());
        mp1.printInfo();
    }
}

So according to above requirements I have written following code:

public class Person
{
    // instance variables - replace the example below with your own
    public static final int MALE = 0;
    public static final int FEMALE = 1;
    
    private String lastname;
    private String firstname;
    private int age;
    boolean married;
    float salary;
    private int sex;
    
    /**
     * Constructor for objects of class Person
     */
    public Person(String lastname, String firstname, int age, boolean married, float salary,
            int sex)
    {
        // initialise instance variables
        setLastName(lastname);
        setFirstName(firstname);
        setAge(age);
        setMarried(married);
        setSalary(salary);
        setSex(sex);
    }

    public String getLastName() {return lastname;}
    public String getFirstName() {return firstname;}
    public int getAge() {return age;}
    public int getSex() {return sex;}
    public boolean isMarried() {return married;}
    public float getSalary() {return salary;}
    
    public void setFirstName(String firstname)
    {
        this.firstname = new String(firstname);
    }
    
    public void setLastName(String lastname)
    {
        this.lastname = new String(lastname);
    }
    
    public void setAge(int age)
    {
        this.age = age;
    }
    
    public void setSex(int sex)
    {
        this.sex = sex;
    }
    
    public void setSalary(float salary)
    {
        this.salary = salary;
    }
    
    public void setMarried(boolean married)
    {
        this.married = married == true ? true : false;
    }
    
    public void printInfo(){
        System.out.print(getFirstName()+" "+getLastName()+" is " +getAge()+" years old, ");
        if(getSex() == 0)
            System.out.print("male");
        else if(getSex() == 1)
            System.out.print("female");
        System.out.print(", gets a "+getSalary()+" Euros salary and is");
        if (isMarried() == false)
            System.out.print(" not");
        System.out.println(" married."); 
    }
}

public class MarriedPerson extends Person
{
    // instance variables - replace the example below with your own
    private int children;

    /**
     * Constructor for objects of class MarriedPerson
     */
    public MarriedPerson(String lastname, String firstname, int age, float salary, int sex,
            int children)
    {
        // initialise instance variables
        super(lastname, firstname, age, true, salary, sex);
        setNoOfChildren(children);
    }
    
    @Override
    public void printInfo()
    {
        System.out.print(getFirstName()+" "+getLastName()+" is "+getAge()+" years old, ");
        if(getSex() == 0)
            System.out.print("male");
        else if(getSex() == 1)
            System.out.print("female");    
        System.out.print(", gets a "+getSalary()+" Euros salary and is");
        if (isMarried() == false)
            System.out.print(" not married.");
        else System.out.println(" married with ");
        if (getNoOfChildren() > 0)
            System.out.print(getNoOfChildren());
        else System.out.print("no");
            System.out.println(" children.");
    }
    
    @Override
    public void setMarried(boolean married)
    {
        this.married = true;
    }
    
    public void  setSalary(MarriedPerson spouse)
    {
        if (spouse.getSex() != this.getSex())
            this.salary += spouse.getSalary();             
    }
    
    public void setNoOfChildren(int children)
    {
        this.children = children;
    }
    
    public int getNoOfChildren()
    {
        // put your code here
        return children;
    }
}

Basically I have added at line 41 of class MarriedPerson the method setSalary(MarriedPerson spouse)
and in line 11 of class Person I have removed the private access to salary.

Here is my output:

Betty Tront is 31 years old, female, gets a 980.5 Euros salary and is married with
3 children.
Kirk Tront is 31 years old, male, gets a 2080.0 Euros salary and is married with
2 children.
Sonia Tront is 31 years old, female, gets a 600.0 Euros salary and is married with
no children.
Betty Tront is 31 years old, female, gets a 3060.5 Euros salary and is married with
3 children.
Betty Tront is 31 years old, female, gets a 3060.5 Euros salary and is married with
3 children.
Betty Tront is 31 years old, female, gets a 2080.0 Euros salary and is married with
3 children.


I am not sure about the last output phrase.

[Campbell Ritchie]

Alexandros Stefanidis wrote:. . . Another question is to add a method setSalary in class MarriedPerson that will take as argument a MarriedPerson object named spouse and will do the following:

a) check spouse sex
b)If it is not the same with variable sex then in variable salary will be added the relevant variable of object spouse
c)if it the same nothing happens.
. . .
and in line 11 of class Person I have removed the private access to salary.
. . .

What a strange requirement, combining the salaries. What happens if you add a second spouse? Do you divorce the first one?
Removing private access looks like a serious mistake to me.

[Alexandros Stefanidis]

If I keep the salary variable as private then I get the following error:

error: salary has private access in Person
this.salary += spouse.getSalary(); //line 44 in class MarriedPerson


thanks

[Campbell Ritchie]

Alexandros Stefanidis wrote:If I keep the salary variable as private then I get the following error:

error: salary has private access in Person
this.salary += spouse.getSalary(); //line 44 in class MarriedPerson


thanks

That is because you are not accessing the salary field correctly. You should be using getXXX and setXXX methods. Those will permit the changes without removing private access. You can also validate the input in a setXXX method

public final void setSalary(someType newSalary)
{
   if (newSalary < salary)
   {
      throw new LossOfSalaryException();
   }
   salary = newSalary;
}

You can also write an incrementSalary method which is rather similar.

[Alexandros Stefanidis]

to cambell Ritcie

Hi. I am doing this excersice sheet which has a question 5 that says
a) construct in every class for every method get or is a relevant method set so that you could change the values of variable instances.
b) Change the body of main in MyTester with following code:

abstract class MyTester {
    public static void main(String[] args) {
        Person p1 = new Person("Kings", "Paul", 22, true, 1200f, Person.MALE);
        MarriedPerson mp1 = new MarriedPerson("Tront", "Betty", 31, 980.5f, Person.FEMALE, 3);
        //MarriedPerson mp2 = new MarriedPerson("Tront", "Kirk", 31, 2080f, Person.MALE, 2);
        //MarriedPerson mp3 = new MarriedPerson("Tront", "Sonia", 31, 600f, Person.FEMALE, 0);
        p1.setMarried(false);
        mp1.setMarried(false);
        p1.printInfo();
        mp1.printInfo();
        System.out.println("Is mp1 married?: "+mp1.isMarried());
    }
}

Run and note the output. What do you observe?

c) Warning! Variable married for instances of class MarriedPerson should not change. Override method setMarried in class MarriedPErson so this doesn't happen.

So from above I have made the code:

abstract class MyTester {
    public static void main(String[] args) {
        Person p1 = new Person("Kings", "Paul", 22, true, 1200f, Person.MALE);
        MarriedPerson mp1 = new MarriedPerson("Tront", "Betty", 31, 980.5f, Person.FEMALE, 3);
        //MarriedPerson mp2 = new MarriedPerson("Tront", "Kirk", 31, 2080f, Person.MALE, 2);
        //MarriedPerson mp3 = new MarriedPerson("Tront", "Sonia", 31, 600f, Person.FEMALE, 0);
        p1.setMarried(false);
        mp1.setMarried(false);
        p1.printInfo();
        mp1.printInfo();
        System.out.println("Is mp1 married?: "+mp1.isMarried());
    }
}

public class Person
{
    // instance variables - replace the example below with your own
    public static final int MALE = 0;
    public static final int FEMALE = 1;
    
    private String lastname;
    private String firstname;
    private int age;
    private boolean married;
    private float salary;
    private int sex;
    
    /**
     * Constructor for objects of class Person
     */
    public Person(String lastname, String firstname, int age, boolean married, float salary,
            int sex)
    {
        // initialise instance variables
        setLastName(lastname);
        setFirstName(firstname);
        setAge(age);
        setMarried(married);
        setSalary(salary);
        setSex(sex);
    }

    public String getLastName() {return lastname;}
    public String getFirstName() {return firstname;}
    public int getAge() {return age;}
    public int getSex() {return sex;}
    public boolean isMarried() {return married;}
    public float getSalary() {return salary;}
    
    public void setFirstName(String firstname)
    {
        this.firstname = new String(firstname);
    }
    
    public void setLastName(String lastname)
    {
        this.lastname = new String(lastname);
    }
    
    public void setAge(int age)
    {
        this.age = age;
    }
    
    public void setSex(int sex)
    {
        this.sex = sex;
    }
    
    public void setSalary(float salary)
    {
        this.salary = salary;
    }
    
    public void setMarried(boolean married)
    {
        this.married = married == true ? true : false;
    }
    
    public void printInfo(){
        System.out.print(getFirstName()+" "+getLastName()+" is " +getAge()+" years old, ");
        if(getSex() == 0)
            System.out.print("male");
        else if(getSex() == 1)
            System.out.print("female");
        System.out.print(", gets a "+getSalary()+" Euros salary and is");
        if (isMarried() == false)
            System.out.print(" not");
        System.out.println(" married."); 
    }
}

public class MarriedPerson extends Person
{
    // instance variables - replace the example below with your own
    private int children;

    /**
     * Constructor for objects of class MarriedPerson
     */
    public MarriedPerson(String lastname, String firstname, int age, float salary, int sex,
            int children)
    {
        // initialise instance variables
        super(lastname, firstname, age, true, salary, sex);
        setNoOfChildren(children);
    }
    
    @Override
    public void printInfo()
    {
        System.out.print(getFirstName()+" "+getLastName()+" is "+getAge()+" years old, ");
        if(getSex() == 0)
            System.out.print("male");
        else if(getSex() == 1)
            System.out.print("female");    
        System.out.print(", gets a "+getSalary()+" Euros salary and is");
        if (isMarried() == false)
            System.out.print(" not married.");
        else System.out.println(" married with ");
        if (getNoOfChildren() > 0)
            System.out.print(getNoOfChildren());
        else System.out.print("no");
            System.out.println(" children.");
    }
    
    /*@Override
    public boolean isMarried() {return married;}
    
    @Override
    public void setMarried(boolean married)
    {
        this.isMarried() = ;
    }*/
    
    
    
    /*public void  setSalary(MarriedPerson spouse)
    {
        if (spouse.getSex() != this.getSex())
            setSalary() += spouse.getSalary();             
    }*/
    
    public void setNoOfChildren(int children)
    {
        this.children = children;
    }
    
    public int getNoOfChildren()
    {
        // put your code here
        return children;
    }
}

My code is up to part b where I get the output:
Paul Kings is 22 years old, male, gets a 1200.0 Euros salary and is not married.
Betty Tront is 31 years old, female, gets a 980.5 Euros salary and is not married.3 children.
Is mp1 married?: false

I don't know how to do part c with get and set methods. I had removed keyword private from variable married which is not liable as you said.

[Campbell Ritchie]

I think there is something wrong about having a married person class in the first place, but I think we cannot sort that problem out now. You can have single people turn into married people (that is called a wedding) and married people can turn into single people (that is sometimes called divorce), but you cannot turn a SinglePerson object into a MarriedPerson object or vice versa.

The fields should all be in the superclass. So should the setXXX and getXXX methods. Should the subclasses need to gain access to those data, they should use those methods, and should not have their own copies of the fields. In fact, the subclasses can become very short because they often do not require new methods or fields. If you are lucky, you can get away with writing constructors in subclasses and nothing else. You have managed quite a lot of that (‍) but there is something wrong with a MarriedPerson class where it is possible for isMarried to return false.
Your setMarried method on line 63 is unnecessarily complicated. In fact, it would make no difference if you simply wrote this.married = married;
Never use == false or == true. They are poor style and error‑prone if you write = by mistake.

Why have you not written a toString method, as I suggested last week?

[Junilu Lacar]

I have to say that this exercise is very poorly thought out and constructed. This is not a good example for polymorphism and the reasons have already been alluded to by others, especially by what Campbell is saying about being able to change from one status to another over time. The class name "MarriedPerson" is smelly because it carries a temporal (time based) state of a Person and makes it permanent. If you take that to the extreme, you could say that you should also have DivorcedPerson, SinglePerson, SeparatedPerson, WidowedPerson, etc. Taking that even further, you could have TeenAgePerson, ToddlerPerson, InfantPerson, SeniorPerson, etc. These are all arguably very poor design choices.

Just look at this part of the code and really think through whether it makes sense or not:

   MarriedPerson mp1 = new MarriedPerson("Tront", "Betty", 31, 980.5f, Person.FEMALE, 3);
   ...
   mp1.setMarried(false);
   System.out.println("Is mp1 married?: "+mp1.isMarried()); // false

The above code declares mp1 as a MarriedPerson. Then it goes and sets its "married" attribute to false. So now, this MarriedPerson is no longer married. So why is it still a MarriedPerson?

As the meme goes: Y U No Makey Sense?!

I have a book by Peter Coad, Eric Lefebvre, and Jeff De Luca called Java Modeling in Color with UML (Prentice Hall, 1999). I don't refer to it often but there's one useful concept from it that I like to employ in cases like this and that's the idea of a "Moment-Interval" archetype. This applies to things that occur at specific moments or intervals in time like a Sale, a Rental, or a Loan. An address can be a Moment-Interval as well. For example, say you lived in a condo for a year, moved away for a few years, then moved back to the same condo. For certain application domains, you could have three addresses which are modeled as separate Moment-Intervals. Other domains might allow you to model that situation as just two separate addresses/moment-intervals. It all depends on the context whether one model or the other is appropriate to use.

As for MaritalStatus, I would say that is a good candidate for a Moment-Interval as well, since it applies to an interval of time in a Person's lifetime.

My advice to OP, since this is supposed to be for "beginner" levels, is to follow the instructions you have been given by your teacher but keep in mind that this is NOT a good example of using polymorphism. Using polymorphism like that to print out different messages is not a very good design. You should not do this kind of thing in a real-world program.

[Liutauras Vilda]

Alexandros Stefanidis,

Also doesn't look right to keep MyTester class as an abstract. Maybe you meant to make it final and with private constructor so no one could instantiate it. Revise that part.
That could look similar to:

public final MyTester {

    private MyTester(){}

    public static void main(String[] args) {
        new MyTester().run();
    }

    private final void run() {
        // everything from your current main method
        ...
    }
}

Also you omitted some curly braces around if statements in printInfo method. I know it is allowed when there is only one statement in it, but probably better not to omit them even then.
Next, same method:

if (isMarried() == false)
    System.out.print(" not");
System.out.println(" married.");

I see what you did here, not sure if it is a good approach. Might someone else more experienced will advice on this, but for less confusion i'd go in simple way:

if (isMarried()) {
    print "Married"
}
else {
    print "Not married"
}

MORE better probably is to write:

System.out.println(isMarried() ? "Married" : "Not married");

[Campbell Ritchie]

Agree about the ?: operator, though many beginners find that operator hard to understand.
i am not convinced I like the multiple print calls; each takes an appreciable time. I don't know how long, and I don't know whether other code can be executed in parallel with a print instruction, but i do know that I can watch hundreds of lines being printed to System.out, so I suspect each print call takes several milliseconds, as against creating Strings inside the Java® runtime, which probably takes a few μs. That is one reason I suggested a toString method; you can do all the String building in one place and print the result with a single print call. I have seen ?: used like this:-
"... %s ...", ..., isMarried() ? "married" : "not married", ...
You would use the %s tag inside method calls like myFormatter.format(), String#format, System.out.format, System.out.printf, etc. They all work quite similarly to one another.

Another reason for using toString is that it is the “Java® way” to do things. The toString() method is inherited from the Object class, and is therefore in every object you create. Also method calls like System.out.println() assume that toString has been overridden in the objects pass to them. You can write System.out.println(myObject) and get a useful printout if you override toString.

[Junilu Lacar]

Campbell Ritchie wrote:Another reason for using toString is that it is the “Java® way” to do things. The toString() method is inherited from the Object class, and is therefore in every object you create. Also method calls like System.out.println() assume that toString has been overridden in the objects pass to them. You can write System.out.println(myObject) and get a useful printout if you override toString.

If you're going to design for inheritance anyway, I might consider doing something like this:

// in base class

// override this in subclasses that need to 
// provide additional information for toString()
protected String moreToString() { return ""; }

@Override
public /* final */ String toString() {
   StringBuilder sb = new StringBuilder()
     .append(super.toString())
     .append("[")
     .append(...)
     .append(...);
     .append(moreToString());
     .append("]");

   return sb.toString();
}

If you want to prevent subclasses from totally changing the behavior of the base toString(), you would make it final in the base class. That way, subclasses would be forced to just use the moreToString() method to extend the behavior of toString(). This is basically the Template pattern.