Develop Reference

Find object created inside a method in other class

For example, 2 class: Ticket and Customer
public class Ticket{
private String cstName;
public Ticket(String name){
this.cstName = name;
}
}
public class Customer{
private String name;
public void book(){
Ticket t = new Ticket(t);
}
}
How can I find and use t object elsewhere ???

What you ask for is completely impossible. An object is made, the object is assigned to a local variable, and the method ends.
As the method ends, all local variables (and t is a local variable), immediately go into the bin and there is nothing in java that lets you 'plug into' this process or that lets you stop this process. The variable is just gone.
The object is still on the heap somewhere, but no longer accessible. Eventually it will be garbage collected. There's nothing you can do about that, either. Java does not have a 'list all objects in the heap' method and never will.
You can mess with reference queues which is an extremely advanced topic that in no way is suitable given the way this question is stated, and wouldn't work for arbitrary methods like this.
If you control the code of Ticket itself you can save the reference as part of the constructor, which would be extremely bad design, and would have nothing at all to do with the notion of t, or that the book method made it.
What you presumably want, is a field:
public class Customer {
private String name;
private Ticket ticket;
public void book() {
this.ticket = new Ticket(t);
}
public Ticket getTicket() {
return this.ticket;
}
}
and now you could do:
Customer c = new Customer();
c.book();
Ticket t = c.getTicket();
or perhaps do:
public class Customer {
private String name;
private Ticket ticket;
public Ticket book() {
this.ticket = new Ticket(t);
return this.ticket;
}
}
and now you could do:
Customer c = new Customer();
Ticket t = c.book();

Why do we need to use the builder design pattern when we can do the same thing with setters? [duplicate]

This question already has answers here:
When would you use the Builder Pattern? [closed]
(13 answers)
Closed 3 years ago.
public class Employee {
private String name;
private String address;
private int id;
public Employee() {
// TODO Auto-generated constructor stub
}
#Override
public String toString() {
return "Employee [name=" + name + ", address=" + address + ", id=" + id + "]";
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
}
public class Main {
public static void main(String[] args) {
Employee e = new Employee();
e.setName("Priyanka");
Employee e1 = new Employee();
e1.setName("Rahul");
e1.setAddress("Delhi");
System.out.println("Value of e :"+ e);
System.out.println("Value of e1:"+ e1);
}
}

The builder pattern can be useful to:
apply some check on the data used to initialize the object. For example if you need a double check between variables
create immutable objects. You can't change an object once initialized, so you can't use setters
add readability of code.
reduce the code used to initialize the object
have the instance in a valid state. Using setters the object instance can be in a not valid state before all the setters are called.
Note on using the builder to create immutable objects.
When you work in a multithread environment an immutable object can be shared between threads without explicit synchronization. Because the object can't change during the time is not possible to have a race condition accessing and modifying it by two threads at the same time.

There is no need to use any pattern. You can even avoid setters with making the variables public. However,
the intent of the Builder design pattern is to separate the
construction of a complex object from its representation
Source: https://en.wikipedia.org/wiki/Builder_pattern

Using a builder pattern has a few advantages:
Unlike with setters (which make your class mutable), a builder can be used to contruct immutable objects. In many cases immutable objects are preferred over mutable objects, because they are easier to understand and maintain, and because they avoid the need for locking in multithreaded environments.
A builder can make sure that the object satisfies some invariants even directly after construction. For example, if your class has a name field which must never be null, the builder can check this condition and fail to construct the object when not satisfied.
Both things you can also accomplish by using a constructor which takes all the class contents as parameters, but that will be quite unreadable when your class has more than a few fields to initialize.

Creating a superclass and subclass with constructors - Java

I am new to Java. I have a problem to solve, but I don't quite understand how constructors work. I understand how to create a superclass and a subclass but I don't understand the constuctors within them (or how they actually work - I have done rediculous amounts of research on constructors, but it's just not making much sense).
I am trying to write a program that creates a superclass called Employees. This Employee class has instance variables employeeId (which is an integer) and employeeName (which is a String).
The subclass is called Manager. The Manager subclass has an instance variable called employeeTitle (which is a String). It also has a method with the name of managerDetails(). ManagerDetails() is supposed to display the employeeId, employeeName, and the employeeTitle.
This is what I have so far:
package tryingoutjava;
public class TryingOutJava {
class Employee {
int employeeId;
String employeeName;
void Employee() {
}
}
class Manager extends Employee {
String employeeTitle;
void managerDetails() {
}
}
public static void main(String[] args) {
}
}
I am very confused on how to set up the constructors for the superclass and the subclass, or even what a constructor really looks like. I've seen examples all over the internet, but no one actually highlights the actual part that is the constructor, or how everything is linked visually, which is what helps me learn.
I guess I'm also having issues with understanding how to set up a method that calls on an object. If anyone has the time to help, it would greatly be appreciated. Thanks!

I guess you want something like this. Be noted, that it is a good idea to separate classes one-per-file in this case, as they are separate entities here. It is a good idea to limit data access to entity fields, as such using encapsulation.
Employee.java:
package tryingoutjava;
public class Employee {
// Protected access because we want it in Manager
protected int employeeId;
protected String employeeName;
public Employee(int employeeId, String employeeName) {
this.employeeId = employeeId;
this.employeeName = employeeName;
}
}
Manager.java:
package tryingoutjava;
public class Manager extends Employee {
private String employeeTitle;
public Manager(String employeeTitle, int employeeId, String employeeName) {
// Use super to invoke Employee constructor
super(employeeId, employeeName);
this.employeeTitle = employeeTitle;
}
// Just create a simple string describing manager
#Override
public String toString() {
return "Manager{" +
"employeeTitle='" + employeeTitle +
"employeeId=" + employeeId +
", employeeName='" + employeeName + '\'' +
'}';
}
}
Application.java:
package tryingoutjava;
public class Application {
// Example of construction plus printing of Manager data
public static void main(String[] args) {
Employee davie = new Employee(1, "Dave The Cable Guy");
Manager tom = new Manager("CFO", 2, "Tomas");
System.out.println(tom.toString());
}
}
Constructors (most often than not) just delegate construction of parent through super invocation. While there are other techniques, like Builder pattern, this is the most basic and understandable approach. There are several other ways to do this, but this should get you started, hope it helps!

Purpose of Constructor
constructor is a method like other method but it is called when instantiate (or create a object from your class) for initialize your object for first use or later use. for example a class like Student must created (instantiated) when we give it name and family name for example. Without them, create a Student is not good because maybe we forget to give it proper name and use it incorrectly. constructor forces us to provide minimum things needed for instantiating objects from classes.
Constructor implementation in inheritance
About inheritance, it is different. When you want to create a Student which is a Human (extends Human) you must first create Human inside your Student and set special feature for your Student like ID which is not for Human (Human has name and etc). so when you create a Student with constructor, the super constructor (for Human) is called too.
What do we do in constructor
as I mentioned, we provide default value for our properties which must set them before creating and using object. (for using them properly) every subclass call super class constructor implicitly with super() but if super class doesn't have any default constructor (constructor with no argument) you must explicitly say super(...) at the first lien of subclass constructor (otherwise compile error)
What is the program steps when using constructor (Advanced)
super class static constructor and static variable (read by self if you want to know more about things I say here)
subclass class static constructor and static variable
super class variable and block constructor
super class constructors
sub class variable and block constructor
sub class constructors
I only mentioned 4 & 6.
I try to explain completely. My English is not good. I'm sorry.

If you know how a method works, then you know how a constructor works. The constructor is simply a special method that allows you to execute some code before the object is created.
Person p = new Person("Bob", 25); // Calls constructor Person(String name, int age)
Then in the constructor you can do things like assign initial values to any instance variables.
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
If the class is a subclass you need to call a constructor of the parent class before the object is created unless the parent class has a constructor with no parameter in which case java can call it for you if you don't specify anything. Here Worker extends Person.
private String occupation;
public Worker(String name, int age, String occupation) {
super(name, age) // Calls constructor Person(String name, int age)
this.occupation = occupation;
}

I guess you can achieve what you want in a single file via the code snippet below:
You can copy paste it in your code and it should work.
You can see how the constructor of parent class is being called by the help of super() and also the methods. Here I have used methods like getEmployeeTitle() which should help you get an overview on how to write methods. I have also overridden the toString() method so that you can understand how to override Object class' useful methods like toString().
Note : Although I have created all the classes in one code snippet for the sake of simplicity , but it is highly recommended that you create a separate file for each of these classes.
class Employee {
int employeeId;
String employeeName;
Employee(int employeeId, String employeeName) {
this.employeeId = employeeId;
this.employeeName = employeeName;
}
}
class Manager extends Employee {
private String employeeTitle;
Manager(int employeeId, String employeeName, String employeeTitle) {
super(employeeId, employeeName);
this.employeeTitle = employeeTitle;
}
public String getEmployeeTitle() {
return employeeTitle;
}
#Override
public String toString() {
return ("employeeId: " + employeeId + ", employeeName: " + employeeName + ", employeeTitle" + employeeTitle);
}
}
public class TryingOutJava {
public static void main(String[] args) {
Manager manager = new Manager(007, "John Doe", " Sr. Manager");
System.out.println(manager);
System.out.println(manager.getEmployeeTitle());
}
}

Implementation difference between Aggregation and Composition in Java

I'm aware of the conceptual differences between Aggregation and Composition. Can someone tell me the implementation difference in Java between them with examples?

Composition
final class Car {
private final Engine engine;
Car(EngineSpecs specs) {
engine = new Engine(specs);
}
void move() {
engine.work();
}
}
Aggregation
final class Car {
private Engine engine;
void setEngine(Engine engine) {
this.engine = engine;
}
void move() {
if (engine != null)
engine.work();
}
}
In the case of composition, the Engine is completely encapsulated by the Car. There is no way for the outside world to get a reference to the Engine. The Engine lives and dies with the car. With aggregation, the Car also performs its functions through an Engine, but the Engine is not always an internal part of the Car. Engines may be swapped, or even completely removed. Not only that, but the outside world can still have a reference to the Engine, and tinker with it regardless of whether it's in the Car.

I would use a nice UML example.
Take a university that has 1 to 20 different departments and each department has 1 to 5 professors.
There is a composition link between a University and its' departments.
There is an aggregation link between a department and its' professors.
Composition is just a STRONG aggregation, if the university is destroyed then the departments should also be destroyed. But we shouldn't kill the professors even if their respective departments disappear.
In java :
public class University {
private List<Department> departments;
public void destroy(){
//it's composition, when I destroy a university I also destroy the departments. they cant live outside my university instance
if(departments!=null)
for(Department d : departments) d.destroy();
departments.clean();
departments = null;
}
}
public class Department {
private List<Professor> professors;
private University university;
Department(University univ){
this.university = univ;
//check here univ not null throw whatever depending on your needs
}
public void destroy(){
//It's aggregation here, we just tell the professor they are fired but they can still keep living
for(Professor p:professors)
p.fire(this);
professors.clean();
professors = null;
}
}
public class Professor {
private String name;
private List<Department> attachedDepartments;
public void destroy(){
}
public void fire(Department d){
attachedDepartments.remove(d);
}
}
Something around this.
EDIT: an example as requested
public class Test
{
public static void main(String[] args)
{
University university = new University();
//the department only exists in the university
Department dep = university.createDepartment();
// the professor exists outside the university
Professor prof = new Professor("Raoul");
System.out.println(university.toString());
System.out.println(prof.toString());
dep.assign(prof);
System.out.println(university.toString());
System.out.println(prof.toString());
dep.destroy();
System.out.println(university.toString());
System.out.println(prof.toString());
}
}
University class
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
public class University {
private List<Department> departments = new ArrayList<>();
public Department createDepartment() {
final Department dep = new Department(this, "Math");
departments.add(dep);
return dep;
}
public void destroy() {
System.out.println("Destroying university");
//it's composition, when I destroy a university I also destroy the departments. they cant live outside my university instance
if (departments != null)
departments.forEach(Department::destroy);
departments = null;
}
#Override
public String toString() {
return "University{\n" +
"departments=\n" + departments.stream().map(Department::toString).collect(Collectors.joining("\n")) +
"\n}";
}
}
Department class
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
public class Department {
private final String name;
private List<Professor> professors = new ArrayList<>();
private final University university;
public Department(University univ, String name) {
this.university = univ;
this.name = name;
//check here univ not null throw whatever depending on your needs
}
public void assign(Professor p) {
//maybe use a Set here
System.out.println("Department hiring " + p.getName());
professors.add(p);
p.join(this);
}
public void fire(Professor p) {
//maybe use a Set here
System.out.println("Department firing " + p.getName());
professors.remove(p);
p.quit(this);
}
public void destroy() {
//It's aggregation here, we just tell the professor they are fired but they can still keep living
System.out.println("Destroying department");
professors.forEach(professor -> professor.quit(this));
professors = null;
}
#Override
public String toString() {
return professors == null
? "Department " + name + " doesn't exists anymore"
: "Department " + name + "{\n" +
"professors=" + professors.stream().map(Professor::toString).collect(Collectors.joining("\n")) +
"\n}";
}
}
Professor class
import java.util.ArrayList;
import java.util.List;
public class Professor {
private final String name;
private final List<Department> attachedDepartments = new ArrayList<>();
public Professor(String name) {
this.name = name;
}
public void destroy() {
}
public void join(Department d) {
attachedDepartments.add(d);
}
public void quit(Department d) {
attachedDepartments.remove(d);
}
public String getName() {
return name;
}
#Override
public String toString() {
return "Professor " + name + " working for " + attachedDepartments.size() + " department(s)\n";
}
}
The implementation is debatable as it depends on how you need to handle creation, hiring deletion etc. Unrelevant for the OP

A simple Composition program
public class Person {
private double salary;
private String name;
private Birthday bday;
public Person(int y,int m,int d,String name){
bday=new Birthday(y, m, d);
this.name=name;
}
public double getSalary() {
return salary;
}
public String getName() {
return name;
}
public Birthday getBday() {
return bday;
}
///////////////////////////////inner class///////////////////////
private class Birthday{
int year,month,day;
public Birthday(int y,int m,int d){
year=y;
month=m;
day=d;
}
public String toString(){
return String.format("%s-%s-%s", year,month,day);
}
}
//////////////////////////////////////////////////////////////////
}
public class CompositionTst {
public static void main(String[] args) {
// TODO code application logic here
Person person=new Person(2001, 11, 29, "Thilina");
System.out.println("Name : "+person.getName());
System.out.println("Birthday : "+person.getBday());
//The below object cannot be created. A bithday cannot exixts without a Person
//Birthday bday=new Birthday(1988,11,10);
}
}

In simple terms :
Both Composition and Aggregation are Associations.
Composition -> Strong Has-A relationship
Aggregation -> Weak Has-A relationship.

There is a great explanation in the given url below.
http://www.codeproject.com/Articles/330447/Understanding-Association-Aggregation-and-Composit
Please check!!!

First we must talk about what actually the difference between Aggregation and Composition is to be on the same page.
Aggregation is an association where the associated entity may exist independent of the association. For example, a Person may be associated to an Organisation but he/she may have independent existence in the system.
whereas
Composition refers to a situation when one of the associated entities is strongly related to the other and cannot exist without the other's existence. In fact the identity of that entity is always associated with the identity of the other object. For example, wheels in a car.
Now, aggregation can simply be achieved by holding a property of one entity in another as below:
class Person {
Organisation worksFor;
}
class Organisation {
String name;
}
class Main {
public static void main(String args[]) {
//Create Person object independently
Person p = new Person();
//Create the Organisation independently
Organisation o = new Organisation();
o.name = "XYZ Corporation";
/*
At this point both person and organisation
exist without any association
*/
p.worksFor = o;
}
}
For Composition it is necessary that the dependent object is always created with the identity of its associated object. You can use an inner class for the same.
class Car {
class Wheel {
Car associatedWith;
}
}
class Main {
public static void main() {
//Create Car object independently
Car car = new Car();
//Cannot create Wheel instance independently
//need a reference of a Car for the same.
Car.Wheel wheel = car.new Wheel();
}
}
Please note that the same use case may fall under aggregation/composition depending on the application scenario. For example, the Person-Organisation case may become composition if you are developing an application for people working in some organisation and the reference to organisation is must for sign up. Similarly, if you are maintaining inventory for parts of a Car, Car-Wheel relationship can be aggregation.

The difference is that any composition is an aggregation and not vice versa.
Let's set the terms. The Aggregation is a metaterm in the UML standard, and means BOTH composition and shared aggregation, simply named shared. Too often it is named incorrectly "aggregation". It is BAD, for composition is an aggregation, too. As I understand, you mean "shared".
Further from UML standard:
composite - Indicates that the property is aggregated compositely,
i.e., the composite object has responsibility for the existence and
storage of the composed objects (parts).
So, University to cathedras association is a composition, because cathedra doesn't exist out of University (IMHO)
Precise semantics of shared aggregation varies by application area and
modeler.
I.e., all other associations can be drawn as shared aggregations, if you are only following to some principles of yours or of somebody else. Also look here.

Aggregation vs Composition
Aggregation implies a relationship where the child can exist independently of the parent. For example, Bank and Employee, delete the Bank and the Employee still exist.
whereas Composition implies a relationship where the child cannot exist independent of the parent. Example: Human and heart, heart don’t exist separate to a Human.
Aggregation relation is “has-a” and composition is “part-of” relation.
Composition is a strong Association whereas Aggregation is a weak Association.

Both types are of course associations, and not really mapped strictly to language elements like that. The difference is in the purpose, context, and how the system is modeled.
As a practical example, compare two different types of systems with similar entities:
A car registration system that primarily keep track of cars, and their owners, etc. Here we are not interested in the engine as a separate entity, but we may still have engine related attributes, like power, and type of fuel. Here the Engine may be a composite part of the car entity.
A car service shop management system that manages car parts, servicing cars, and replace parts, maybe complete engines. Here we may even have engines stocked and need to keep track of them and other parts separately and independent of the cars. Here the Engine may be an aggregated part of the car entity.
How you implement this in your language is of minor concern since at that level things like readability is much more important.

User types on library management system

I have to build a library management system and i've run into problems while trying to implement user types or profiles. I've already got a superclass user and two other subclasses of User, Student and Teacher, each with their own "characteristics". The thing is i have to implement 7 types of users (5 types of students and 2 types of clerks) based on the number of books they can borrow and the amount of time they can keep the books until they have to return them. Those are the only 2 differences between the classes.
How would you implement this? Inheritance? I'm looking for a clever way to implement this and i would love to hear your thoughts on this.
Thank you very much.

As a good rule of thumb, anywhere you see a noun in a project specification it's a good candidate for a class. If those nouns have relationships in the project spec, they probably aught to have one in your code too.
All of your people would fit in the category of a Userso perhaps this should be an interface they would all inherit. Down from this they appear to fit into two categories, Student and Staff perhaps these should also be abstract classes / interfaces. Then you have your 7 concrete classes. 2 inheriting Staff and 5 inheriting Student.
So you'd end up with something like this..
Of course, this design depends on what every User must do, what every Staff / Student must do but I'll leave the very specific details to you.

You have a "class" per person, which really limits your design; because, if you want to add a student or teacher, you need to start writing a new class.
Classes are templates, and each template is used to construct an "instance of the class" or more specifically an "instance". One template is typically used to construct more than one class (although it is not necessary for a class to be used more than once, using it once (or not using it at all) is fine).
So you could do
public class Student {
private String name;
public Student(String name) {
this.name = name;
}
public string getName() {
return this.name;
}
}
public class Staff {
private String name;
public Staff(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
}
As you can see, there is going to be a lot of duplication between staff and students. getName(), getAge(), getPhoneNumber(), getAddress(), etc can easily be applied to both, which under this structure means that you would have to duplicate those methods for both Student and Staff.
What does both a staff member and a student have in common? They are both People, and many of the common methods are common to all people.
public class Person {
private String name;
public Person(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
}
public Staff extends Person {
public void teachClass(Class class) {
...
}
}
public Student extends Person {
public void attendClass(Class class) {
...
}
}
This design also creates other issues, as it implies that a Staff member is not a Student, and a Student is not a Staff member. In the real world, sometimes the Staff enrolls for classes, and Students can take on teaching roles (think teacher's aide).
The most flexible method actually doesn't create a structural differentiation between a Student and Staff, it differentiates between the two by ability.
public class Person {
public Person(String name) {
...
}
public void canTeach(Course course) {
teaching.add(course);
}
public void attending(Course course) {
attending.add(course);
}
public boolean isStaff() {
return !teaching.isEmpty();
}
public boolean isStudent() {
return !attending.isEmpty();
}
}
However, this structure is radically different from the example you are being presented in class, and it side-steps the lessons you really are supposed to be learning about inheritance.