如何在JAVA中将构造函数中的值传递给另一个类中的setter？

Question

Im learning object oriented programming in school right now, and there are some aspects of it I don't quite understand yet. I have a program that creates a database of users with their names, and birthdates. So I have 3 classes: person, PersonProgram(the main), and Date. The Person class has the constructor, setter, and getters for the names and the birth date. The Date class has error checking for proper dates and leap years etc. In the main program I create 5 People, and then give menu options to change and modify the names and dates. So for example, if the user wants to change the name my code looks like this:

System.out.println("Enter new first name:");
people[choice-1].setFirstName(input.next());

and that works and makes sense to me. But I want to know how I can change the date properly? The Date constructor takes 3 integers for the day, year, and month, so in the main program I prompt the user to input the 3 new dateswhich are stored in day, month, year integers. So my understanding is from there I would pass those 3 integers to the Date constructor:

new Date(month, day, year);

What I am confused on is where to go from there. The Date constructor gets the new Date call, and passes it to the setters. How can this newly created date object be passed back to the Person program, so the setter in Person for the birthdate can update the corresponding Person object? If I am not clear on my question please let me know, I figured I could articulate what I am trying to ask without posting all my code.

Answer 1

In your Person class you should have something like this:

public class Person {

    private Date birthDate;

    public Date getBirthDate() {
        return birthDate;
    }

    public void setBirthDate(Date birthDate){
        this.birthDate = birthDate;
    }
}

And then in you would set the birthDate like:

person[choice-1].setBirthDate(new Date(month,day,year));

Taking into consideration that you are starting with OOP there is an important concept here, Encapsulation, the Person class restricts the free access to its fields, like birthDate , and sets the rules for the interaction with them. As an example you could check if the date is null before assigning it.

public void setBirthDate(Date birthDate){
    if(birthDate != null) {
        this.birthDate = birthDate;
    } else {
        //Whatever you wanna do here (throw an Exception, etc, etc)
    }
}

Comment Question

Although it would be better to create another question :

Do I have to create an instance of the Date class in my Person class? Or anywhere for that matter?

No, the property/field birthDate is a reference to a Date object which will be stored in memory until no references are left. And it's up to you where to create them, nonetheless there are Creational Patterns , a familiy of Design Patterns that help you with this matter.

Answer 2

is it the birth date in person class of type 'Date'?

if so, you should create an instance of your class Date , do the control that you need and pass it to the constructor or setter of the birth date in the persson instance .

Date birthDate = new Date(month, day, year);
// Some controls
people[choice-1].setBirthDate(birthDate);

Answer 3

Date the_birth_date = new Date(mounth, day, year);
people[choice - 1].setBirthDate(the_birth_date);

Answer 4

You can set it like this: people[choice-1].setBirthday(new Date(month, day, year)); . You would have to give the option to select the Person first.