Tag: association

Introduction The association describes the relationships or connections between various objects. It represents a general relationship between objects without any strong dependency. The association has two forms: Composition and Aggregation. Objects associated with each other communicate using references to use their properties and characteristics. Let us look into the types of Association and their role in object-oriented programming. One-to-One Association One object is associated with exactly one object. Example: A person is linked with exactly one social security number and vice versa. Here is an example of how a person can be linked with exactly one social security number and vice versa in Java: public class Person { private String name; private SocialSecurityNumber ssn; public Person(String name, SocialSecurityNumber ssn) { this.name = name; this.ssn = ssn; } // Other methods and attributes // Getter and setter for name and ssn } public class SocialSecurityNumber { private String number; private Person person; public SocialSecurityNumber(String number, Person person) { this.number = number; this.person = person; } // Other methods and attributes // Getter and setter for number and person } In this example, the Person class has a reference to a SocialSecurityNumber object, and the SocialSecurityNumber class has a reference to a Person object. This represents a one-to-one association between a person and their social security number. One-to-Many Association One-to-Many: One object can be associated with many objects. Example: A person can have multiple bank accounts, but a single account is linked to a single person. import java.util.ArrayList; import java.util.List; public class Person { private String name; private List bankAccounts; public Person(String name) { this.name = name; this.bankAccounts = new ArrayList<>(); } public void addBankAccount(BankAccount account) { bankAccounts.add(account); } public void removeBankAccount(BankAccount account) { bankAccounts.remove(account); } // Other methods and attributes // Getter and setter for name and bankAccounts } public class BankAccount { private String accountNumber; private Person owner; public BankAccount(String accountNumber, Person owner) { this.accountNumber = accountNumber; this.owner = owner; } // Other methods and attributes // Getter and setter for accountNumber and owner } In this example, the Person class has a list of BankAccount objects to represent the multiple bank accounts associated with a person. Each BankAccount object has a reference to the owner, which is a Person object. This represents a one-to-many association between a person and their bank accounts. Many-to-Many Association A variable number of objects belonging to a class may be associated with a variable number of objects belonging to another class. Example: A person can have multiple home addresses, and a home address may be linked to multiple people. import java.util.ArrayList; import java.util.List; public class Person { private String name; private List homeAddresses; public Person(String name) { this.name = name; this.homeAddresses = new ArrayList<>(); } public void addHomeAddress(HomeAddress address) { homeAddresses.add(address); address.addPerson(this); } public void removeHomeAddress(HomeAddress address) { homeAddresses.remove(address); address.removePerson(this); } // Other methods and attributes // Getter and setter for name and homeAddresses } public class HomeAddress { private String address; private List residents; public HomeAddress(String address) { this.address = address; this.residents = new ArrayList<>(); } public void addPerson(Person person) { residents.add(person); } public void removePerson(Person person) { residents.remove(person); } // Other methods and attributes // Getter and setter for address and residents } In this example, the Person class has a list of HomeAddress objects to represent the multiple home addresses associated with a person. Each HomeAddress object has a list of residents, which are Person objects. By adding a person to a home address and vice versa, we establish a many-to-many association between people and home addresses. Many-to-One Association The opposite of a one-to-many association is a many-to-one association. Here, one object from one class is linked to several objects from another class. Example: Consider a school with multiple classes, and in each class, many students are studying together. This is a Many-to-one association. import java.util.ArrayList; import java.util.List; public class School { private String name; private List classrooms; public School(String name) { this.name = name; this.classrooms = new ArrayList<>(); } public void addClassroom(Classroom classroom) { classrooms.add(classroom); classroom.setSchool(this); } public void removeClassroom(Classroom classroom) { classrooms.remove(classroom); classroom.setSchool(null); } // Other methods and attributes // Getter and setter for name and classrooms } public class Classroom { private String name; private School school; private List students; public Classroom(String name) { this.name = name; this.students = new ArrayList<>(); } public void addStudent(Student student) { students.add(student); student.setClassroom(this); } public void removeStudent(Student student) { students.remove(student); student.setClassroom(null); } public void setSchool(School school) { this.school = school; } // Other methods and attributes // Getter and setter for name, school and students } public class Student { private String name; private Classroom classroom; public Student(String name) { this.name = name; } public void setClassroom(Classroom classroom) { this.classroom = classroom; } // Other methods and attributes // Getter and setter for name and classroom } In this example, we have three classes: School, Classroom, and Student. The School class represents a school and has a list of Classroom objects associated with it. The Classroom class represents a classroom and has a reference to the School it belongs to, as well as a list of Student objects associated with it. The Student class represents a student and has a reference to the Classroom they belong to. To establish the many-to-one association: When adding a classroom to the school, we call the setSchool() method of the classroom to set the reference to the school. When adding a student to a classroom, we call the setClassroom() method of the student to set the reference to the classroom. This way, we can have multiple classrooms in a school, and each classroom can have many students studying together.

Tag: association

Association

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