Suppose I create an object in Java like this
Student std1 = new Student("Tom", "male");
I need to prevent another object(such: std2, std3, etc) to have same value as std1. How to do it?
[edit] The example of what I want is:
disallowed: Student std2 = new Student("Tom", "male");
allowed: Student std2 = new Student("Not Tom", "male"); or Student std2 = new Student("Tom", "female"); or Student std2 = new Student("John", "male");, etc
Thanks
You can create a class that will control creation of the object - basically use a flyweight pattern.
Step 1. Make the constructor of Student package-private
public class Student {
Student(String name, String gender) {
...
Step 2. Implement equals method in Student
public boolean equals (Object other) {
if (!(other instanceof Student)) return false;
Student otherStudent = (Student) other;
return Objects.equals(otherStudent.getName(), this.getName()) && Objects.equals(otherStudent.getGender(), this.getGender());
}
Step 3. Implement a pool of Student objects in the same package
public class StudentPool {
private List<Student> students = new ArrayList<>();
public Student getOrCreate(String name, String gender) {
Student student = new Student(name, gender);
return students.stream()
.filter(s -> Objects.equals(s, student))
.findFirst()
.orElseGet(() -> {
students.add(student);
return student;
});
}
public List<Student> get(String name) {
return students.stream()
.filter(student -> Objects.equals(student.getName(), name))
.collect(Collectors.toList());
}
}
Assuming that by "value" you are referring to the students´ name, I´d suggest that you write a little function that will return true if the name is already taken. To do this, you can store all of your "students" in an ArrayList:
ArrayList<Student> students = new ArrayList<Student>();
Now you can add students to this ArrayList like the following:
private boolean nameIsAlreadyTaken(ArrayList<Student> students, String newName){
for (int i = 0; i < student.size(); i++){
if(students.get(i).getName().equals(newName)){
return true;
}
}
return false;
}
(You need to define a getName()-function in the student-class for this to work.)
then you can do something like this:
String newName = "Michael"; String newGender = "male";
if (!nameIsAlreadyTaken(students, newName)){
students.add(new Student(newName, newGender));
} else{
//something you want to to if the name is already taken
}
You can do this without passing the students-ArrayList to the function, but this is up to you.
You can create a static String HashSet by adding a concatenated name,gender to it.
class YourClass {
public static Set<String> studentSet = new HashSet<>();
public static void addStudent(String name, String gender) {
YourClass.studentSet.add(name + "," + gender);
}
public static Boolean studentExists(String name, String gender) {
return YourClass.studentSet.constains(name + "," + gender);
}
So the data in your HashSet will look like Tom,male , Tom,female, John,male.
class Student {
...
public Student(String name, String gender) {
this.name = name;
this.gender = gender;
YourClass.addStudent(name, gender);
}
}
And while creating an instance of Student you can check the HashSet if that doesn't have Name,Gender in it, and then instantiate and object.
if(!YourClass.studentExists(name, gender)) {
Student student = new Student(name, gender);
}
I hope this solves your problem
I'd say you need to override Student's equals and hashCode methods, and then check in constructor if such a student exists. Good answer on this point: link.
import java.util.*;
public class Student {
private static final Set<Student> REGISTERED_STUDENTS = new HashSet<>();
private String name;
private String gender;
public static Collection<Student> getRegisteredStudents() {
return Collections.unmodifiableCollection(REGISTERED_STUDENTS);
}
public Student(final String name, final String gender) {
this.name = name;
this.gender = gender;
if (REGISTERED_STUDENTS.contains(this))
throw DuplicateStudentException();
REGISTERED_STUDENTS.add(this);
}
@Override
public int hashCode() {
return Objects.hash(name, gender);
}
@Override
public boolean equals(final Object o) {
if (this == o) return true;
if (!(o instanceof Student)) return false;
final Student student = (Student) o;
return Objects.equals(name, student.name) && Objects.equals(gender, student.gender);
}
}
Please, note that this example is not thread-safe and uses arguable solution of throwing exception right in the constructor. Probably you will want a factory method and ConcurrentSkipListSet or another thread-safe collection, like this:
class Student {
private static final Set<Student> REGISTERED_STUDENTS = new ConcurrentSkipListSet<>(Comparator.comparing(Student::getName).thenComparing(Student::getGender));
private String name;
private String gender;
public static Set<Student> getRegisteredStudents() {
return Collections.unmodifiableSet(REGISTERED_STUDENTS);
}
public static void addStudent(final String name, final String gender) {
Student probablyExists = new Student(name, gender);
REGISTERED_STUDENTS.add(probablyExists);
}
private Student(final String name, final String gender) {
this.name = name;
this.gender = gender;
}
public String getName() {
return name;
}
public String getGender() {
return gender;
}
@Override
public int hashCode() {
return Objects.hash(name, gender);
}
@Override
public boolean equals(final Object o) {
if (this == o) return true;
if (!(o instanceof Student)) return false;
final Student student = (Student) o;
return Objects.equals(name, student.name) && Objects.equals(gender, student.gender);
}
}
