Develop Reference

How can I create a new object in a loop in java?

I have an ArrayList of String named "info" and each string holds the information about an instance of my class Student.
I try converting the Arraylist into an array and then parsing it using split method.
String[] Stringinfo = new String[info.size()];
Stringinfo = info.toArray(Stringinfo);
after I parse each line I try to make a new object in my for loop and adding it to the Students ArrayList. What happens is that every time I create this new object Student, the other objects which were added to Students ArrayList before, are all changed to this new object.
String[] temp;
for(int i = 1; i < LineCount + 1 ; i++)
{
temp = Stringinfo[i].split(" ");
Student s = new Student(Integer.parseInt(temp[0]), Integer.parseInt(temp[1]), Integer.parseInt(temp[2]), Integer.parseInt(temp[3]), Integer.parseInt(temp[4]));
Students.add(s);
}
I tried printing Students in different points and everthing is alright until the new object is created.
At any time all the objects have the same attribute values as the last object created.
this is the Student class constructor:
public Student(int certif, int class_id, int ave, int i, int a)
{
certification_num = certif;
class_id = class;
average_point = ave;
student_id = i;
age = a;
}
I searched a lot but couldn't find an answer. Sorry if the answer is obvious I am new to Java.
Any help would be appreciated.
Thanks in advance.
Edit:
public class Student{
public Student(){}
public Student(int certif, int class, int ave, int i, int a)
{
certification_num = certif;
class_id = class;
average_point = ave;
student_id = i;
age = a;
}
public static int get_certification_num(){
return certification_num;
}
public static int get_class_id(){
return class_id;
}
public static int get_average_point(){
return average_point;
}
public static int get_id(){
return student_id;
}
public static int get_age(){
return age;
}
private static int certification_num;
private static int class_id;
private static int age;
private static int node_id;
private static int student_id;
}

Take the word static out of your Student class.
static means "one of these for all Students". But every student should have a different age, id etc., so those fields (and associated methods) should not be static.

Static members means that the member belong to the class and not to the object and thus your members keeps on changing.
My guess is your members are static, so try to define your Student class like this:
public class Student {
private int certification_num;
private int class_id;
private int average_point;
private int student_id;
private int age;
public Student(int certif, int clazz, int ave, int i, int a)
{
this.certification_num = certif;
this.class_id = clazz;
this.average_point = ave;
this.student_id = i;
this.age = a;
}
}

First of all I advice you to change your Student class as follow:
public class Student
{
private int certification_num;
private int class_id;
private int average_point;
private int student_id;
private int age;
public int getCertification_num() {
return this.certification_num;
}
// Do this for all variables
public void setCertification_num(int certif) {
this.certification_num = certif;
}
// Do this for all variables
}
After that you can easy use your Student class. Also in other situations, when you don't have or need all information.
Fill your Array:
ArrayList<Student> students = new ArrayList<Student>();
for(int i = 1; i < LineCount + 1 ; i++)
{
String[] temp;
temp = Stringinfo[i].split(" ");
Student s = new Student();
s.setCertification_num(Integer.parseInt(temp[0]);
//Do for all other fields
Students.add(s);
}

Constructor + Inheritance support

I am fairly new to Inheritance, and I'm not sure if I am doing it right but I seem to be on the right track. The program runs fine except the output I am getting isn't right. I think the problem is to do with my constructors.
public class Person {
protected static String name;
protected static int birthYear;
public Person(String name, int birthYear) {
}
public String name (String n) {
n = name;
return n;
}
public int birthYear (int bY) {
bY = birthYear;
return bY;
}
#Override
public String toString() {
return String.format(name + birthYear);
}
}
public class Student extends Person {
protected String major;
public Student(String name, int birthYear, String major) {
super(name, birthYear);
major = "";
}
public String major(String maj) {
maj = major;
return maj;
}
public String toString() {
super.toString();
return super.toString() + major;
}
}
public class Instructor extends Person {
protected static int salary;
public Instructor(String name, int birthYear, int salary) {
super(name, birthYear);
salary = 0;
}
public int salary(int sal) {
sal = salary;
return sal;
}
public String toString() {
super.toString();
return super.toString() + salary;
}
}
public class PersonTester {
public static void main(String[] args) {
Person p = new Person("Perry", 1959);
Student s = new Student("Sylvia", 1979, "Computer Science");
Instructor e = new Instructor("Edgar", 1969, 65000);
System.out.println(p);
System.out.println("Expected: Person[name=Perry,birthYear=1959]");
System.out.println(s);
System.out.println("Expected:" +
"Student[super=Person[name=Sylvia,birthYear=1979],major=Computer]");
System.out.println(e);
System.out.println("Expected:" + "Instructor[super=Person[name=Edgar,birthYear=1969],salary=65000.0]");
}
}
OUTPUT I AM GETTING:
null0
Expected: Person[name=Perry,birthYear=1959]
null0null
Expected: Student[super=Person[name=Sylvia,birthYear=1979],major=Computer Science]
null00
Expected: Instructor[super=Person[name=Edgar,birthYear=1969],salary=65000.0]

Try changing your constructor in Person to:
public Person(String name, int birthYear) {
this.name = name;
this.birthYear = birthYear;
}
Currently, the constructor has an empty body, so when you call super(name, birthYear); in the subclass constructor, nothing actually happens.
Your Student constructor also has an error. You forgot to initialize the major field.
public Student(String name, int birthYear, String major) {
super(name, birthYear);
this.major = major;
}
You have the same problem in the Instructor constructor...
public Instructor(String name, int birthYear, int salary) {
super(name, birthYear);
this.salary = salary;
}
Finally, you need to take away the static keywords before the fields in Person. This is because static ensures, that there will always be one (and only one) instance of those fields per class, as opposed to one per instance, like you want it to be:
protected String name;
protected int birthYear;
Same thing for the salary field in Instructor.

n = name; this causing your problem. It must be name = n;. All your setter function contain this problem, correct them all and tell me result.

Inheritance super variables

I'm writing a simple program in which I have a super class Person inherited by the sub-classes Customer and Employee (they inherit the variables ID, name and surname).
public class Person {
int id;
String name;
String surname;
public Person() {}
public Person(int i, String n, String s) {
id = i;
name = n;
surname = s;
}
}
public class Employee extends Person implements Serializable {
String username;
String password;
String date;
int hpw;
int recordSold;
float hourPay;
public Employee() {}
public Employee(String u, String n, String s, String p, int i, int h, String d, int rSold, float hPay) {
username = u;
super.name = n;
super.surname = s;
password = p;
super.id = i;
hpw = h;
date = d;
recordSold = rSold;
hourPay = hPay;
}
}
However the problem is here: when I try to get the variables ID, name and surname through my main class, they fail to return (0,null,null). Why is this? I have get-Methods in my sub-classes which should return the super variables, but they are not. Thanks for your time and patience.
public String getName() {
return super.name;
}
UPDATE:
ok so I sorted out the super(id,name,surname) in the Employee class constructor. I also removed all the getters and setters in the employee class since those are inherited from the Person superclass (correct me if I'm wrong?..)
Person superclass:
public class Person {
private int id;
private String name;
private String surname;
public Person () {
}
public Person(int i, String n, String s) {
this.id = i;
this.name = n;
this.surname = s;
}
public void setID(int i) {
this.id = i;
}
public void setName(String n) {
this.name = n;
}
public void setSurname(String s) {
this.surname = s;
}
public int getID() {
return id;
}
public String getName() {
return name;
}
public String getSurname() {
return surname;
}
}
Employee subclass:
import java.io.*;
public class Employee extends Person implements Serializable {
protected String username;
protected String password;
protected String date;
protected int hpw;
protected int recordSold;
protected float hourPay;
public Employee() {
super();
}
public Employee(int i, String u, String n, String s, String p, int h, String d, int r, float hP) {
super(i,n,s);
username = u;
password = p;
date = d;
hpw = h;
recordSold = r;
hourPay = hP;
}
public void setUser(String u) {
username = u;
}
public void setPassword(String p) {
password = p;
}
public void setHWeek (int h) {
hpw = h;
}
public void setDate (String d) {
date = d;
}
public void setRSold (int r) {
recordSold = r;
}
public void setHPay (float p) {
hourPay = p;
}
public String getUser() {
return username;
}
public String getPassword() {
return password;
}
public int getHWeek() {
return hpw;
}
public String getDate() {
return date;
}
public int getRSold() {
return recordSold;
}
public float getHPay() {
return hourPay;
}
however, when I run the main program the ID, name and surname variables are still null, they are not being returned by the superclass. Am I missing something please? Thanks

Inheritance only works for methods NOT for variables. It is also bad practice to implement methods in subclasses that access super class variables directly. You'd better implement access methods in your superclass. Due to inheritance, those methods will be available in the sub-classes ass well.
Another thing is the visibility of you instance varibles. You are using the default visibility which is "package-wide". So if your sub-classes are not in the same package, they can't access those variables. If you use "private" or "protected" visibility you are much safer accessing the variables.
Another point is that you are initializing the objects not correctly. Calling the sub-class constructor has to call the super-class constructor as well because your Employee object relies on the functionality that your Person object provides. A more scientific description of this principle exists:
Barbara Liskov - Liskov substitution principle
public class Person {
private int id;
private String name;
private String surname;
public Person() {}
public Person(int i, String n, String s) {
id = i;
name = n;
surname = s;
}
public int getId() {
return this.id;
}
public String getName() {
return this.name;
}
public int getSurname() {
return this.surname;
}
}
Add access methods for super class instance variables and set visibility to private.
public class Employee extends Person implements Serializable {
private String username;
private String password;
private String date;
private int hpw;
private int recordSold;
private float hourPay;
public Employee() {}
public Employee(String u, String n, String s, String p, int i, int h, String d, int rSold, float hPay) {
super(id, name, surname);
this.username = u;
this.password = p;
this.hpw = h;
this.date = d;
this.recordSold = rSold;
this.hourPay = hPay;
}
}
Call the super class constructor for initialization of the super class.

Your code should look something like this:
public class Person {
private int id;
private String name;
private String surname;
public Person (int id, String name, String surname) {
this.id = id;
this.name = name;
this.surname = surname;
}
public int getId() {
return id;
}
... //similarly for getName() and getSurname()
}
public class Employee extends Person {
private String username;
private String password;
private String date;
private int hpw;
private int recordSold;
private float hourPay;
public Employee (int id, String name, String surname, String username, String password, String date, int hpw, int recordSold, float hourPay) {
super(id, name, surname);
this.username = username;
... //similarly for other parameters.
}
}
The important bit is super(id, name, surname).
EDIT
lionc claims that I did not answer the question, which is true. I did this because the original poster seems to be new to Java and, hence, might be asking the "wrong" question. I should have highlighted this in my original response. Given that my answer is currently marked as the best, I believe that I made the right decision.

You haven't initialized those variables, that's why it is returning default value for those variables. In java following are default values for variables.
int -> 0
String -> null (because String is Object in Java)

You define those attributes in both of your classes so you override them in the subclass. Moreover, your Employee constructor is not the way it should. You should call the adapted super-constructor as your first statement.
public class Person {
protected int id;
protected String name;
protected String surname;
public Person(int id, String name, String surname) {
this.id = id;
this.name = name;
this.surname = surname;
}
}
public class Employee extends Person implements Serializable {
private String username;
private String password;
private String date;
private int hpw;
private int recordSold;
private float hourPay;
public Employee(String username, String name, String surname, String pswd, int id,
int hpw, String date, int rSold, float hPay) {
super(id,name,surname);
this.username = username;
this.password = pswd;
this.hpw = hpw;
this.date = date;
this.recordSold = rSold;
this.hourPay = hPay;
}
}
In your constructors, I consider a best practice to give the same name to your parameters as the name of your attributes to initialize and differenciate them thanks to this. Some people also use the same names except that they add a _ at the beginning of all the members of the class. In any case, don't use such meaningless names as "s", "n" etc when the variables they represent have a special meaning (surname, name). Keep those names for example for local variables without any particular semantic (n would be an integer, s would be a String...).

In your example, you don't need tu use super to access the attributes defined in the super class since you are using package visibility for them (and both seems to be in the same package).
However, this is NOT the proper way to write Java code.
You should define a visibility for your attributes. In most case, it is recommended to use private visibility and to define getter and setter methods to access them:
public class Person {
private int id;
private String name;
private String surname;
public Person() {}
public Person(int id, String name, String surname) {
this.id = id;
this.name = name;
this.surname = surname;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
// And so on...
}
In sub-classes, you just have to call getId() or setId(...) to access the Id attribute. No need to call super.getId(). Since Employee extends Person, it has access to all of its public, protected (and package if they are in the same package) attributes and method.
This means that in your current code, you can simply write name = n instead of super.name = n.
public class Employee extends Person implements Serializable {
private String username;
private String password;
private String date;
private int hpw;
private int recordSold;
private float hourPay;
public Employee() {}
public Employee(String username, String name, String surname, String password, int id, int hpw, String date, int rSold, float hPay) {
super(id, name, surname);
this.username = username;
this.password = password;
this.hpw = hpw;
this.date = date;
this.recordSold = rSold;
this.hourPay = hPay;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
// And so on...
}
Now to use these classes, you can write code like:
Employee e = new Employee("user3149152", "Ulrich", "Ser", "passwd", 1234, 0, "2014/08/13", 0, 0);
System.out.println("Employee " + e.getName() + ' ' + e.getSurname() + " has for id " + e.getId() + '.');
For reference, this code works even with your current code.
It prints:
Employee Ulrich Ser has for id 1234.

java constructor handling lack of arguments

i have this code here for a person details and the person should be intialized always whatever the number of possible arguments given ,like for example only height and age or salary and age
and i find it difficult to declare a constructor for every combination of arguments , is there a more optimal solution than that ??
class person {
public static final int defalut_salary=1000;
public static final int default_age=20;
public static final double default_height=6;
public static final String default_name="jack";
protected int salary;
protected int age;
protected double height;
protected String name;
person(){
}
}

I would suggest using the Builder pattern:
public final class Person {
private final int salary;
private final int age;
private final double height;
private final String name;
public Person(int salary, int age, double height, String name) {
this.salary = salary;
this.age = age;
this.height = height;
this.name = name;
}
// Getters or whatever you want
public static class Builder {
// Make each field default appropriately
private int salary = 1000;
private int age = 20;
private double height = 6;
private String name = "jack";
public Builder setSalary(int salary) {
this.salary = salary;
return this;
}
// Ditto for other properties
public Person build() {
return new Person(salary, age, height, name);
}
}
}
Usage:
Person person = new Person.Builder().setAge(25).setHeight(15).build();
You can perform validation in the Person constructor, and if you want to make any of the fields mandatory, you could take those in the Builder constructor.

Initialize all fields in a default constructor and use methods to set values in the desired fields, for example like this:
class A{
Field a;
Field b;
Field c;
public A(){
a = SOME_VALUE;
b = SOME_VALUE;
c = SOME_VALUE;
}
public A withA(Field a){ this.a = a; return this; }
public A withB(Field b){ this.b = b; return this; }
public A withC(Field c){ this.c = c; return this; }
/* other code */
}
Then you can use it like this:
A a = new A().withB(SOME_OTHER_VALUE).withC(YET_DIFFERENT_VALUE);
If don't want the fields' values to be changed with subsequent calls to withX() methods, you can use a full-fledged builder pattern.

You could define the constructor to take primitive wrapper classes (like java.lang.Integer or java.lang.Double) instead of primitives. The difference is that this allows you to pass null instead of a value. You can then check them for null. When they are null, you assign the default value. When they aren't, you assign their value.
public Person(Integer salary, Integer age, Double height, String name) {
if (salary == null) this.salary = salary else this.salary = default_salary;
if (age == null) this.age = age else this.age = default_age;
//...
}
The invocation new Person(1200, null, 5.3, null); would mean "create a person with start salary 1200, default age, height 5.3 and default name".
Another approach is to use the builder pattern as shown in the answer by Jon Skeet. It is less concise, but more readable.

Create a default constructor in Java

I need to create a method with a default constructor, which sets name to an empty string and sets both credits and contactHours to zero. How to do it? Thanks, Pieter.

Methods don't have constructors... classes do. For example:
public class Dummy
{
private int credits;
private int contactHours;
private String name;
public Dummy()
{
name = "";
credits = 0;
contactHours = 0;
}
// More stuff here, e.g. property accessors
}
You don't really have to set credits or contactHours, as the int type defaults to 0 for fields anyway.
You're likely to want at least one constructor which takes initial values - in which case your parameterless one can delegate to that:
public class Dummy
{
private String name;
private int credits;
private int contactHours;
public Dummy()
{
this("", 0, 0);
}
public Dummy(String name, int credits, int contactHours)
{
this.name = name;
this.credits = credits;
this.contactHours = contactHours;
}
// More stuff here, e.g. property accessors
}

public class Test {
private String name;
private int credits;
private int contactHours;
public Test {
this( "", 0, 0);
}
public Test (String name, int credits, int contactHours) {
this.name = name;
this.credits = credits;
this.contactHours = contactHours;
}
// more code here
}

public class Bibabu{
private String name;
private int credits;
private int contactHours;
public Bibabu(){
name = ""; // you could also write this.name and so on...
credits = 0;
contactHours= 0;
}
// more code here
}

You don't need a constructor:
public class Dummy
{
private int credits = 0;
private int contactHours=0;
private String name="";
/*
public Dummy()
{
name = "";
credits = 0;
contactHours = 0;
}
*/
// More stuff here, e.g. property accessors
}

//constructor
public Account(int id, double balance, Person owner){
this.id = id;
this.balance = balance;
this.owner = owner;