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Generate an ID for each object in java [duplicate]

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Why does my ArrayList contain N copies of the last item added to the list?
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I have an array in the main class which holds Employee class objects. I'm trying to generate a unique ID for each object but it is printing the same ID for all the objects
Main class:
public static void main(String[] args) {
Employee employee1 = new Employee("Luke", 36);
Employee employee2 = new Employee("Martin", 49);
Employee employee3 = new Employee("Kevin", 21);
Employee employee4 = new Employee("Sam", 43);
Employee employee5 = new Employee("Nicole", 45);
Employee employee6 = new Employee("Linta", 21);
Employee[] allEmployees = { employee1, employee2, employee3, employee4, employee5, employee6 };
for (int i = 0; i < allEmployees.length; i++) {
System.out.println(allEmployees[i]);
}
}
Employee class
public class Employee {
private String name;
private int age;
private static String employeeID = "0";
Employee(String name, int age) {
this.name = name;
this.age = age;
employeeID = getNextUniqueID();
}
public String getName() {
return name;
}
public String setName(String name) {
this.name =name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public static String getNextUniqueID() {
int id = Integer.parseInt(employeeID);
++id;
return Integer.toString(id);
}
public String getEmployeeID() {
return employeeID;
}
public String toString() {
return getName() + " " + getAge() + " " + getEmployeeID();
}
}
I want the employeeID as string and I can't use java.util.UUID; for my project.

You need a static variable associated with the class to maintain the unique id and an instance variable to keep that particular employee's ID in the class.
private String employeeID; // instance member
private static String uniqueID = "0"; // static class variable
public static String getNextUniqueID() {
int id = Integer.parseInt(uniqueID); // get the static variable
++id; // increment it
uniqueID = Integer.toString(id); // update the static variable
return uniqueID; // return the value to use for the employee
}
Then in the Employee constructor, use the static member:
Employee(String name, int age) {
this.name = name;
this.age = age;
employeeID = Employee.getNextUniqueID();
}
updated Employee class:
public class Employee {
private String name;
private int age;
private String employeeID;
private static String uniqueID = "0";
Employee(String name, int age) {
this.name = name;
this.age = age;
employeeID = Employee.getNextUniqueID();
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public static String getNextUniqueID() {
int id = Integer.parseInt(uniqueID);
++id;
uniqueID = Integer.toString(id);
return uniqueID;
}
public String getEmployeeID() {
return employeeID;
}
public String toString() {
return getName() + " " + getAge() + " " + getEmployeeID();
}
}
Output:
Luke 36 1
Martin 49 2
Kevin 21 3
Sam 43 4
Nicole 45 5
Linta 21 6

You should store last generated id in static field but use non static for id of certain employee.
Also you should use AtomicInteger type for thread safety which you can convert to String. Check that:
import java.util.concurrent.atomic.AtomicInteger;
public class Employee {
private String employeeID;
private String name;
private int age;
private static AtomicInteger lastGeneratedId = new AtomicInteger(0);
Employee(String name, int age) {
this.name = name;
this.age = age;
employeeID = getNextUniqueID();
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public static String getNextUniqueID() {
return String.valueOf(lastGeneratedId.incrementAndGet());
}
public String getEmployeeID() {
return employeeID;
}
public String toString() {
return getName() + " " + getAge() + " " + getEmployeeID();
}
public static void main(String[] args) {
Employee employee1 = new Employee("Luke", 36);
Employee employee2 = new Employee("Martin", 49);
Employee employee3 = new Employee("Kevin", 21);
Employee employee4 = new Employee("Sam", 43);
Employee employee5 = new Employee("Nicole", 45);
Employee employee6 = new Employee("Linta", 21);
Employee[] allEmployees = { employee1, employee2, employee3, employee4, employee5, employee6 };
for (int i = 0; i < allEmployees.length; i++) {
System.out.println(allEmployees[i]);
}
}
}

Java - How can I create an array of a different class and iterate through it calling its toString method?

I have a Person class with the fields "name" and "phoneNumber" that are set through the constructor. I am trying to create a separate testing class that will create an array of Person and iterate through them by calling to their toString() method.
I am not sure how to do this, any help is appreciated.
Here is my first class which is all I have so far;
public class Person
{
private String name;
private String phoneNumber;
public Person(String name, String phoneNumber)
{
this.name = name;
this.phoneNumber = phoneNumber;
}
public String getName()
{
return name;
}
public String getNumber()
{
return phoneNumber;
}
public String getPerson()
{
return name + " " + phoneNumber;
}
#Override
public String toString()
{
return "["+getPerson()+"]";
}
}

Hope this will help,
public class Test {
public static void main(String[] args) {
Person array[] = {new Person("Jason", "123456"), new Person("Karl", "78945"), new Person("Tom", "789456")};
for(int i = 0; i < array.length; i++){
array[i].toString();
//System.out.println(array[i].toString());
}
}
}
class Person
{
private String name;
private String phoneNumber;
public Person(String name, String phoneNumber)
{
this.name = name;
this.phoneNumber = phoneNumber;
}
public String getName()
{
return name;
}
public String getNumber()
{
return phoneNumber;
}
public String getPerson()
{
return name + " " + phoneNumber;
}
#Override
public String toString()
{
return "["+getPerson()+"]";
}
}
Save the file as Test.java

Firstly the toString method is for an INDIVIDUAL Person object, and cannot be applied to a whole set, you need to make the method static and have a whole static array defined in the class to be able to go through all instances of the Person class.
private static Person[] pArray = new Person[20];
//I picked 20 randomly, if you want any possible number use an arrayList<Person>
private static int count = 0;
In the constructor
pArray[count] = this;
count++;
Then your toString method:
String list = "[";
for(Person p : this.pArray)
list = list + p.getPerson() + " ,"
list = list + "]";
return list;

Java object scope between methods

I'm having issues with this code running, I'm trying to get the program to print the strings below by using input from the other classes. As you can see, the info put into the new Bride and Location objects are being put in to a Wedding Object and then I need to try and retrieve the details from the wedding object and display it on screen like so:
Wedding data:
Bride: Amy Cronos, age: 29
Location: South Rd, suburb: Tonsley
but I am instead met with 4 identical errors relating to the place.getName, place.getSuburb() etc. etc. that say
Main.java:6: error: cannot find symbol
System.out.println("Location"+place.getStreet()+", suburb:
"+place.getsuburb());
symbol: variable place
location: class Main
I'm pretty sure this has something to do with the scope, but cant work out what I need to do.
What is causing this error and how do I fix it?
Here is the code:
public class WeddingDetails {
public static void main(String[] args) {
Bride person = new Bride("Amy Cronos", 29);
Location place = new Location("Tonsley", "South Rd");
Wedding wed = new Wedding(person, place);
show(wed);
}
public static void show(Wedding wed) {
System.out.println("Wedding data:");
System.out.println("Bride: " + person.getName() + ", age: " + person.getAge());
System.out.println("Location: " + place.getStreet() + ", suburb: " + place.getSuburb());
}
public static class Location {
private String suburb;
private String street;
Location(String suburb, String street) {
this.suburb = suburb;
this.street = street;
}
public String getSuburb() {
return suburb;
}
public String getStreet() {
return street;
}
}
public static class Bride {
private String name;
private int age;
Bride(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
}
public static class Wedding {
private Bride person;
private Location place;
Wedding(Bride person, Location place) {
this.person = person;
this.place = place;
}
public Bride getBride() {
return person;
}
public Location getPlace() {
return place;
}
}
}

The issue here is your println statements are trying to access methods within objects, but by calling those methods on the wrong object. You should be accessing the Bride and Location objects with the Wedding class' getters (getBride() and getPlace(). The complete call would be wed.getBride().getName() and wed.getPlace().getStreet() so on.
Corrected code is below. NOTE: for the purposes of being able to compile all of the code inside one class, I added the static keyword to the Bride, Location and Wedding class declarations. You can just remove the static and copy and paste each class back into your .java files.
public class WeddingDetails {
public static void main(String[] args) {
Bride person = new Bride("Amy Cronos", 29);
Location place = new Location("Tonsley", "South Rd");
Wedding wed = new Wedding(person, place);
show(wed);
}
public static void show(Wedding wed) {
System.out.println("Wedding data:");
System.out.println("Bride: " + wed.getBride().getName() + ", age: " + wed.getBride().getAge());
System.out.println("Location: " + wed.getPlace().getStreet() + ", suburb: " + wed.getPlace().getSuburb());
}
public static class Location {
private String suburb;
private String street;
Location(String suburb, String street) {
this.suburb = suburb;
this.street = street;
}
public String getSuburb() {
return suburb;
}
public String getStreet() {
return street;
}
}
public static class Bride {
private String name;
private int age;
Bride(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
}
public static class Wedding {
private Bride person;
private Location place;
Wedding(Bride person, Location place) {
this.person = person;
this.place = place;
}
public Bride getBride() {
return person;
}
public Location getPlace() {
return place;
}
}
}

How to produce a functional LinkedList Class?

As part of an assignment I am having to produce a LinkedList class called Registry. It is intended to be part of a simple student registration system with an interface.
Unfortately, I have literally hit a wall and have no idea on what to do next with what I am doing here. The Registry class is just intended to manage a linked list of students called studentList.
Below is the current, rather incomplete class I have made so far.
import java.util.*;
public class Registry
{
LinkedList<Student> studentList;
public Registry()
{
}
public void addStudent(Student aStudent)
{
studentList.add(aStudent);
}
public void deleteStudent(int studentID)
{
studentList.remove(studentID);
}
#Override
public String toString()
{
return getClass().getName() +
}
public String format()
{
System.out.format(studentList);
}
}
Now, my main worry is using Student. As part of the assignment, I have had to make another class called Student which create instances of Students, containing forenames, surnames, Student IDs and degree Schemes as strings.
How will I be able to use that sperate class to be added to the LinkedList instanted in Registry? And how can I get the Registry class to fully function?
I will try and provide any additional details on request. I am likely unclear, so if I am, let me know, and I will try and explain as best I can.
EDIT: This is the Student Class as requested:
public class Student
{
private String foreName;
private String surName;
private String studentID;
private String degreeScheme;
public Student()
{
}
public void setForeName(String foreName)
{
this.foreName = foreName;
}
public String getForeName()
{
return foreName;
}
public void setSurName(String surName)
{
this.surName = surName;
}
public String getSurName()
{
return surName;
}
public void setStudentID(String studentID)
{
this.studentID = studentID;
}
public String getStudentID()
{
return studentID;
}
public void setDegreeScheme(String degreeScheme)
{
this.degreeScheme = degreeScheme;
}
public String getDegreeScheme()
{
return degreeScheme;
}
#Override
public String toString()
{
return getClass().getName() + "[foreName = " + foreName + " surName "
+ surName + " studentID " + studentID + " degreeScheme "
+ degreeScheme + "]";
}
public void format()
{
System.out.format("%5s%20s%11s%20s", foreName, surName, studentID, degreeScheme);
}
}

import java.util.Iterator;
import java.util.LinkedList;
public class Tester {
public static void main(String[] args) {
Registry r = new Registry();
r.addStudent(new Student("13", "John", "Doe", "Physics")); // Add a student to the Registry
r.addStudent(new Student("212", "Jane", "Bow", "Chem")); // Add another Student
System.out.println(r); // Print the Student List
r.deleteStudent(212); // Deletes student with ID 212
System.out.println(r);
}
}
class Student {
private String studentID;
private String foreName;
private String surName;
private String degreeScheme;
public Student(String studentId, String foreName, String surName, String degreeScheme) {
this.studentID = studentId;
this.foreName = foreName;
this.surName = surName;
this.degreeScheme = degreeScheme;
}
public void setForeName(String foreName) {
this.foreName = foreName;
}
public String getForeName() {
return foreName;
}
public void setSurName(String surName) {
this.surName = surName;
}
public String getSurName() {
return surName;
}
public void setStudentID(String studentID) {
this.studentID = studentID;
}
public String getStudentID() {
return studentID;
}
public void setDegreeScheme(String degreeScheme) {
this.degreeScheme = degreeScheme;
}
public String getDegreeScheme() {
return degreeScheme;
}
#Override
public String toString() {
return getClass().getName() + "[foreName = " + foreName + " surName " + surName + " studentID "
+ studentID + " degreeScheme " + degreeScheme + "]";
}
public void format() {
System.out.format("%5s%20s%11s%20s", foreName, surName, studentID, degreeScheme);
}
}
class Registry {
LinkedList<Student> studentList;
public Registry() { // Creates studentList
studentList = new LinkedList<>();
}
public void addStudent(Student aStudent) {
studentList.add(aStudent);
}
public void deleteStudent(int studentID) {
int index = searchList(studentID); // Gets index of the student in the Registry
if (index == -1)
throw new IllegalArgumentException("Student not found");
// Since studentList is implemented as LinkedList, .remove removes element at specified position
studentList.remove(index);
}
// Search by studentID , if found, return position in the list
private int searchList(int studentID) {
Iterator<Student> it = studentList.iterator();
int count = -1;
while (it.hasNext()) {
count++;
Student temp;
temp = it.next();
if (Integer.parseInt(temp.getStudentID()) == studentID) {
return count;
}
}
return -1;
}
#Override
//Suggestions to improve the toString are welcome
public String toString() {
for (Student student : studentList) {
student.format();
System.out.println();
}
return "";
}
}

In any data structure there are three functions that almost always require implementation:
Insertion
Searching
Deletion
Let me begin by clarifying what the general Linked List structure consists of.
The linked list works by operating on nodes. Each node contains the actual data you want to store/modify/access.
Registry: This should be responsible for maintaining the structure and providing a way of inserting/searching for/deleting specific nodes in the list.
Student: Stores the data and controls how it is accessed and modified
So far, you have your Registry framework created appropriately. (Although you will want to implement a search method.)
For your student class, you simply create the member variables of the class that you need, as well as the appropriate getters/setters for them.
public class Student {
private String id;
private String forename;
private String surname;
private String degreeScheme;
private Student next; // Maintains a reference to the next node in the list
Student () {
//Default constructor values
}
Student (String id, String forename, String surname, String degreeScheme, Student next) {
this.id = id;
this.forename = forename;
this.surname = surname;
this.degreeScheme = degreeScheme;
this.next = next;
}
public void setID (String id) {
this.id = id;
}
public String getID () {
return id;
}
public void setforename (String forename) {
this.forename = forename;
}
public String getforename () {
return forename;
}
public void setSurname(String surname) {
this.surname = surname;
}
public String getSurname () {
return surname;
}
public void setDegreeScheme(String degreeScheme) {
this.degreeScheme = degreeScheme;
}
public String getDegreeScheme () {
return degreeScheme;
}
public void setNext (Student next) {
this.next = next;
}
public Student getNext () {
return next;
}
} //End Student Class
This should be a good base to get you started. Remember, when creating data structures you'll avoid a lot of headaches by making sure that you have a clear separation of concerns. Make the student class purely responsible for storing and maintaining the data, and let the Registry class be responsible for maintaining the structure itself!
At this point, you can utilize the Student class within your Registry class, just insert the nodes as you want, search for them, delete them as needed.

Reading in file to array and using it to initialize objects

I have 5 classes (they're small). PersonDemo (test class), Person (superclass), and Student, Instructor and Graduate Student (sub classes). All the classes except for PersonDemo are finished.
I need to read in a file (data.txt) and store it to array Person. Then need I need to determine which object to initialize depending on the first value of the array. ( 1 - person, 2 - student, 3 - instructor or 4 - graduate student ) - I'm having trouble with this part.
Can someone point me in the right direction? My classes are below along with what the input file (data.txt) looks like and what the output file should look like.
PersonDemo.Java
public class PersonDemo
{
public static void main ()
{
JFileChooser chooser = new JFileChooser();
Scanner fileScanner = null;
Person [] ins = new Person [10];
try {
if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION)
{
File selectedFile = chooser.getSelectedFile();
fileScanner = new Scanner(selectedFile);
while(fileScanner.hasNextLine())
{
// Need to load "data.txt" into array
// Then need I need to determine which object to initialize depending on the
// first value of the array in "data.txt"
//( 1 - person, 2 - student, 3 - instructor or 4 - graduate student )
}
fileScanner.close();
}
}
catch (FileNotFoundException e)
{
System.out.println("Could not find file");
}
}
public static void showAll(Person [] ins)
{
// Future code here
}
}
Person.java (superclass)
public class Person
{
private String name;
private int age;
public Person()
{
name="";
age=0;
}
public Person(String name, int age)
{
this.name = name;
this.age = age;
}
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
public String toString()
{
return "Name: " + name + "\t" + "Age: " + age;
}
}
Student.java (subclass)
public class Student extends Person
{
private int studentID;
private String major;
public Student()
{
studentID = 0;
major = "";
}
public Student(String name, int age, int studentID, String major)
{
super(name, age);
this.major = major;
this.studentID = studentID;
}
public int getID()
{
return studentID;
}
public void setID(int studentID)
{
this.studentID = studentID;
}
public String getMajor()
{
return major;
}
public void setMajor(String major)
{
this.major = major;
}
public String toString()
{
return super.toString() + "Student ID: " + studentID + "Major: " + major;
}
}
GraduateStudent.java (subclass)
public class GraduateStudent extends Student
{
private String researchArea;
public GraduateStudent()
{
researchArea = "";
}
public GraduateStudent(String name, int age, int studentID, String major, String researchArea)
{
super(name, age, studentID, major);
this.researchArea = researchArea;
}
public String getArea()
{
return researchArea;
}
public void setArea(String researchArea)
{
this.researchArea = researchArea;
}
public String toString()
{
return super.toString() + "Research Area: " + researchArea;
}
}
Instructor.java (subclass)
public class Instructor extends Person
{
private int salary;
public Instructor()
{
salary = 0;
}
public Instructor(String name, int age, int salary)
{
super(name, age);
this.salary = salary;
}
public int getSalary()
{
return salary;
}
public void setSalary(int salary)
{
this.salary = salary;
}
public String toString()
{
return super.toString() + "Salary: " + salary;
}
}