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How do I generate objects randomly in java?

I have an abstract class called "Student" and 2 subclasses "Graduate" and "Undergraduate", they both have the same parameters.
I want to create 10 Student objects randomly, some from the "Graduate" and some from the "Undergraduate" classes.
I want to print the displayStudent() method for all objects created, I got stuck on how to randomly generate the 10 students so they are all random of type graduates and undergraduates.
public abstract class Student {
private int ID;
private double GPA;
public Student(int ID, double GPA) {
this.ID = ID;
this.GPA = GPA;
}
public int getID() {
return ID;
}
public double getGPA() {
return GPA;
}
public abstract String getLevel();
public abstract String getStatus();
public final String displayStudent() {
return getLevel() + " ID>> " + getID() + ", GPA>> " + getGPA() + ", Status>> " + getStatus();
}
}
public class Graduate extends Student{
public Graduate(int ID, double GPA) {
super(ID, GPA);
}
#Override
public String getLevel() {
return "graduate";
}
#Override
public String getStatus() {
if( getGPA() >= 3) {
return "honor";
} else if (getGPA() >= 2 && getGPA() <= 3) {
return "good";
} else {
return "probation";
}
}
}
public class Undergraduate extends Student {
public Undergraduate(int ID, double GPA) {
super(ID, GPA);
}
#Override
public String getLevel() {
return "undergraduate";
}
#Override
public String getStatus() {
if( getGPA() >= 3) {
return "honor";
} else if (getGPA() >= 2 && getGPA() <= 3) {
return "good";
} else if( getGPA() > 0 && getGPA() < 2) {
return "probation";
} else {
//for any number that is not in the range of the GPA
return "invalid GPA!";
}
}
}

Try this.
using the ternary operator ?: is key.
exp ? a : b says if exp is true, do a, else do b
I used the loop index+1 as the id.
If the boolean is true, graduate, otherwise undergrad.
The double is multiplied by 4 to get the GPA.
Random r = new Random();
List<Student> students = new ArrayList<>();
for (int i = 0; i < 10; i++) {
students.add(r.nextBoolean() ?
new Graduate(i+1, r.nextDouble() * 4) :
new Undergraduate(i+1, r.nextDouble() * 4));
}
Note, If you change the displayStudent() method to toString() like so
#Override
public String toString() {
return getLevel() + " ID>> " + getID() + ", GPA>> "
+ getGPA() + ", Status>> " + getStatus();
}
You can just print the object directly without having to call any method.
You will probably need to format the GPA to eliminate unnecessary precision. Check out String.format.

How do I create an boolean equals method compares objects in an array

My programming assignment tasked me with writing an increase/decreasePay abstract method that must be put in my abstract employee class. I can't seem to get the the method correct in HourlyWorker so that it will take increase or decrease the pay by a "percentage". My math is sound (monthly pay - or + (monthly pay * the percentage), but my output in my test class is coming out the same after increasing/decreasing pay. Any help?
Employee class:
abstract public class Employee
{
private String lastName;
private String firstName;
private String ID;
public abstract void increasePay(double percentage);
public abstract void decreasePay(double percentage);
public abstract double getMonthlyPay();
public Employee(String last, String first, String ID)
{
lastName = last;
firstName = first;
this.ID = ID;
}
public void setLast(String last)
{
lastName = last;
}
public void setFirst(String first)
{
firstName = first;
}
public void setIdNumber(String ID)
{
this.ID = ID;
}
public String getLastName()
{
return lastName;
}
public String getFirstName()
{
return firstName;
}
public String getName()
{
return firstName + lastName;
}
public String getIdNumber()
{
return ID;
}
}
HourlyWorkerClass
public class HourlyWorker extends Employee
{
private int hours;
private double hourlyRate;
private double monthlyPay;
public HourlyWorker(String last, String first, String ID, double rate)
{
super(last, first, ID);
hourlyRate = rate;
}
public void setHours(int hours)
{
this.hours = hours;
}
public int getHours()
{
return hours;
}
public void setHourlyRate(double rate)
{
if ( hours > 160 )
this.hourlyRate = hourlyRate * 1.5;
else
this.hourlyRate = rate;
}
public double getHourlyRate()
{
return hourlyRate;
}
public void setMonthlyPay(double monthlyPay)
{
monthlyPay = hourlyRate * hours;
}
public double getMonthlyPay()
{
return hourlyRate * hours;
}
public void increasePay(double percentage)
{
monthlyPay = monthlyPay* percentage;
}
public void decreasePay(double percentage)
{
monthlyPay = monthlyPay* percentage;
}
public String toString()
{
String result = "Name: " + getFirstName() + " " + getLastName() + "\nID: "
+ getIdNumber() + " \nHourly Rate: " + hourlyRate;
return result;
}
}
Testing class (currently testing increase
public class TestEmployee2
{
public static void main(String[] args)
{
Employee [] staff = new Employee[3];
Supervisor sup = new Supervisor("Boss", "Jim", "JB7865", 54000);
HourlyWorker hw1 = new HourlyWorker("Bee", "Busy", "BB1265", 11.95);
hw1.setHours(200);
staff[0] = sup;
staff[1] = hw1;
System.out.println(staff[0].getMonthlyPay());
staff[0].increasePay(5);
System.out.println(staff[0].getMonthlyPay());
System.out.println(staff[1].getMonthlyPay());
staff[1].increasePay(10);
System.out.println(staff[1].getMonthlyPay());
}
}
Supervisor class:
public class Supervisor extends Employee
{
private double annualSalary;
private double monthlyPay;
public Supervisor(String last, String first, String ID, double salary)
{
super(last, first, ID);
annualSalary = salary;
}
public void setAnnualSalary(double salary)
{
annualSalary = salary;
}
public double getAnnualSalary()
{
return annualSalary;
}
public double getMonthlyPay()
{
return ((annualSalary + (annualSalary * .02)) / 12);
}
public void increasePay(double percentage)
{
monthlyPay = monthlyPay* percentage;
}
public void decreasePay(double percentage)
{
monthlyPay = monthlyPay* percentage;
}
public String toString()
{
String result = "Name: " + getFirstName() + " " + getLastName() + "\nID: "
+ getIdNumber() + "\nAnnual Salary: " + annualSalary;
return result;
}
}
Output is:
4590.0 4590.0 2390.0 2390.0
Doesn't appear to be modifying getMonthlyPay()
Should be:
4590.00 4819.50 2390.00 2629.00

Generally, when implementing equals(), you compare “key” fields whose values don’t change for the entity, and don’t compare “state” fields whose values change from time to time.
You are comparing sharePrice, when I believe you should be comparing symbol.

When you do list.indexOf(temp), what that does, right now, is look for a Stock that is equals to the argument passed to it -- so it looks for a Stock with price zero, not caring about the symbol at all. That's what the code does right now.
Honestly, using indexOf and equals is not really appropriate for this problem. indexOf is really only useful when you have something that's totally equal to the target you're looking for.
The best way to do something like this is
Optional<Stock> foundStock = list.stream().filter(stock -> stock.getName().equals(symbol)).findAny();
if (foundStock.isPresent()) {
// do something with foundStock.get()
} else {
// no found stock
}

indexOf() is a method return the index of the first occurrence of the specified element in the returned list. If the list does not contain this element, value -1 is returned.
More formally, return the lowest index i that meets the following conditions:
if(o==null? get(i)==null :o.equals(get(i))){
return i;
}
return -1;
If there is no such index, return -1.
And you have override the equals method, I guess you just want to focus on the same price Stock?:
#Override
public boolean equals(Object obj){
if (obj instanceof Stock){
Stock other = (Stock) obj;
return getPrice() == other.getPrice();
}
return false;
}
As my opinion, you have use List<Stock> list so the Object in the list is all Stock. Maybe it could be simplifed:
#Override
public boolean equals(Object obj){
Stock other = (Stock) obj;
return getPrice() == other.getPrice();
}

Reference of a non static variable

I have class Employe that has variables like id , name ... and 2 other classes that inherit from Employe : Seller and Cashier.
To calculate their salaries, I created a method in each one of Seller and Cashier but I need to access the name via the name getter method in Employe so I'd have :
System.out.println("The salary is "+Seller.getName() +" is : " +salary);
Once I type that, I get an error sayingI need to make the name variable to static, but I need it as non static since I'm creating multiple objects using the name variable.
Any solution to this problem?
EDIT :
This is the Employe class :
public class Employe {
protected int id;
protected String name;
protected String adresse;
protected int nbrHours;
public Employe () {
}
public Employe (int id, String name, String adresse, int nbHours)
{
this.id=id;
this.name=name;
this.adresse=adresse;
this.nbrHours=nbHours;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setNom(String name) {
this.name = name;
}
public String getAdresse() {
return adresse;
}
public void setAdresse(String adresse) {
this.adresse = adresse;
}
}
This is the Seller class :
public class Seller extends Employe {
private int prime;
public Seller (int id, String name, String adresse, int nbHours,int prime)
{
super(id,name,adresse,nbHours);
this.prime=prime;
}
public int getPrime() {
return prime;
}
public void setPrime(int prime) {
this.prime = prime;
}
#Override
public String toString() {
return super.toString() +" [prime=" + prime + "]";
}
public void salaireSeller ()
{
int salaire = 0;
if(nbrHours<160)
{
salaire = nbrHours * 10 + prime;
System.out.println("La salaire de "+Responsable.getName() +" est : " +salaire);
}
else if(nbrHours >160)
{
int Extra= (160 - nbrHours) * 12;
int salaire1 = 160 * 10000 + prime;
salaire= Extra + salaire1;
System.out.println("La salaire de "+Seller.getName() +" est : " +salaire);
}
}
In the Main class I created a Seller object :
Seller Sel1 = new Seller(2, "Alex", "Miami", 18, 200);
now I want to calculat its salary using the SalaireSeller() method in the Main class of course :
Sel1.salaireSeller();
but in the Seller class :
System.out.println("La salaire de "+Responsable.getName() +" est : " +salaire);
it says I need to set Name to static, this will give every object the same name

You need a Seller instance, to call getName() and getSalary() on.
Seller s = new Seller();
// ...
System.out.println("The salary is " + s.getName() +
" is : " + s.getSalary());

You're certainly trying to access an instance variable from a static method.
What you want to do here is to create an instance of your class, then call the getName() method on the object created.
Seller sell = new Seller();
sell.setName("Jean-Paul"); // This is just an example
System.out.println("His name is " + sell.getName()); // Prints : His name is Jean-Paul

I figuered out a solution, I just need to add the name to the toString() method in class Employee, then add the salary variable to the toString() method in Seller class, without System.out.println(..) in SalaireSeller().
or instead of Seller.getName(), I use this.getName() and it works.

Calling methods from different classes | Java

I'm writing code for an application that keeps track of a student’s food purchases at a campus cafeteria. There's two classes - Student, which holds overloaded constructors & appropriate getter & setter methods; and MealCard, which holds a class variable to track the number of meal cards issued, appropriate getter & setter methods, a purchaseItem() method, a purchasePoints() method & an overriddden toString() method. There's a Tester class also.
In my MealCard class, the methods are written but in the Tester when I call them, they don't work correctly. I want to get the itemValue from the user but how do I do this in the MealCard class?
Same goes for the purchasePoints method, how do I get the topUpValue from user in MealCard class?
Code so far is:
public class Student {
// Instance Variables
private String name;
private int age;
private String address;
// Default Constructor
public Student() {
this("Not Given", 0, "Not Given");
}
// Parameterized constructor that takes in values
public Student(String name, int age, String address) {
this.name = name;
this.age = age;
this.address = address;
}
// Getters and Setters
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 getAddress(){
return address;
}
public void setAddress(String address) {
this.address = address;
}
// toString() to be overriden
#Override
public String toString() {
return "Name: " + this.name + "\n" + "Age: " + this.age + "\n" + "Address: " + this.address;
}
}
`
public class MealCard extends Student {
static int numberOfMealCards;
private final static int DEFAULT_BALANCE = 1000;
private int itemValue;
private int topUpValue;
public int newBalance;
// Getters and Setters
public int getItemValue() {
return itemValue;
}
public void setItemValue(int itemValue) {
this.itemValue = itemValue;
}
public int getTopUpValue() {
return topUpValue;
}
public void setTopUpValue(int topUpValue) {
this.topUpValue = topUpValue;
}
// purchaseItem method for when students buy food
public int purchaseItem() {
newBalance = DEFAULT_BALANCE - itemValue;
return newBalance;
}
// purchasePoints method for students topping up their meal card balance
public int purchasePoints() {
newBalance = DEFAULT_BALANCE + topUpValue;
return newBalance;
}
// Overriden toString method
#Override
public String toString() {
return super.toString() + "Meal Card Balance: " + this.newBalance + "\n" + "Number of Meal Cards: " + numberOfMealCards;
}
}
`
import java.util.Scanner;
public class TestMealCard {
public static void main(String[] args) {
// Create instances of MealCard class
MealCard student1 = new MealCard();
MealCard student2 = new MealCard();
Scanner keyboard = new Scanner(System.in);
System.out.println("Name: ");
student1.setName(keyboard.nextLine());
System.out.println("Age: ");
student1.setAge(keyboard.nextInt());
System.out.println("Address: ");
student1.setAddress(keyboard.nextLine());
System.out.println("Meal Card Balace: ");
student1.newBalance = keyboard.nextInt();
System.out.println("Number of Meal Cards Issued: ");
student1.numberOfMealCards = keyboard.nextInt();
System.out.println("Name: ");
student2.setName(keyboard.nextLine());
System.out.println("Age: ");
student2.setAge(keyboard.nextInt());
System.out.println("Address: ");
student2.setAddress(keyboard.nextLine());
System.out.println("Meal Card Balace: ");
student2.newBalance = keyboard.nextInt();
System.out.println("Number of Meal Cards Issued: ");
student2.numberOfMealCards = keyboard.nextInt();
// Call purchaseItem
student1.purchaseItem();
// Call purchasePoints
student2.purchasePoints();
// Call tString to output information to user
}
}

In order to set the itemValue from the user you need to get get that input from the user using this setItemValue() method.
Ex.
System.out.print("Please enter the item value: ");
student1.setItemValue(keyboard.nextInt());
or
int itemVal = 0;
System.out.print("Please enter the item value: ");
itemVal = keyboard.nextInt();
student1.setItemValue(itemVal);
as for the other method call just call the setter for toUpValue.
Ex.
System.out.print("Please enter the item value: ");
student1.setTopUpValue(keyboard.nextInt());
or
int toUpVal = 0;
System.out.print("Please enter the item value: ");
itemVal = keyboard.nextInt();
student1.setTopUpValue(topUpVal);
Hope that helps =).

Java Inheritance of Classes

I am trying to figure out Inheritance and Arrays in Java and I am trying to get these classes to work together. I believe I have the Inheritance down, but I am still struggling with the array part.
There are three files: 1. Person.java -base class 2. Student.java -a derived class of Person.java 3. Family.java -not quite sure, I think it's its own base class
Person.java has two instance variables, String name and int age, and an assortment of constructors, toString, equals, and set/get methods
Student.java, again, a derived class of Person, by definition will have all the stuff contained within Person, as well as two more instance vars, String major, and double gpa. This class also have get/set methods for major and gpa, an equals method that compares one class student with another class student, and I believe it's called an overidden method of toString that returns name, age, major, and gpa all in one string.
Lastly, Family.java is where the main method resides. It creates an array of type Person, adds "Persons" to this array, then outputs them.
I am getting an error that says: "main" java.lang.ArrayIndexOutOfBoundsException: 8
I can not figure out why this program is not working properly and would appreciate any help to figure this out. Thank you.
Person.java Class
public class Person
{
private String name;
private int age;
public Person()
{
name = "John Smith";
age = 1;
}
public Person(String n, int a)
{
name = n;
age = a;
}
public String toString()
{
return ("Name: " + getName() + ", Age: " + age + " ");
}
public boolean equals(Person otherPerson)
{
return (getName().equals(otherPerson.getName()) && (age == otherPerson.age));
}
public String getName()
{
return name;
}
public void setName(String newName)
{
name = newName;
}
public int getAge()
{
return age;
}
public void setAge(int newAge)
{
age = newAge;
}
}
Student.java Class
public class Student extends Person
{
private String major;
private double gpa;
public Student()
{
super();
major = "Undecided";
gpa = 0.0;
}
public Student(String theName, int theAge, String theMajor, double theGpa)
{
super(theName, theAge);
setMajor(theMajor);
setGpa(theGpa);
}
public String toString()
{
return ("Name: " + getName() + ", Age: " + getAge() + ", Major: " + major + ", GPA: " + gpa);
}
public boolean equals(Student otherStudent)
{
return (major.equals(otherStudent.major) && (gpa == otherStudent.gpa));
}
public String getMajor()
{
return major;
}
public void setMajor(String newMajor)
{
major = newMajor;
}
public double getGpa()
{
return gpa;
}
public void setGpa(double newGpa)
{
gpa = newGpa;
}
}
Family.java Class
public class Family
{
private int famArray = 0;
private Person[] family;
public Family(int size_of_family)
{
famArray = size_of_family;
family = new Person[famArray];
}
public void addPerson(Person p)
{
boolean isPresent = false;
int i;
for(i = 0; i < family.length; i++)
{
if(family[i] != null && family[i].equals(p))
{
isPresent = true;
System.out.println(p.getName() +
" is already present in the family");
}
}
if(isPresent == false)
family[i] = p;
}
public void printOutFamily()
{
for(int i = 0; i < family.length; i++)
{
System.out.println(family[i].toString());
}
}
public static void main(String[] args)
{
Family f = new Family(8);
Person fred= new Person("Fred Flintstone", 50);
System.out.println("created " + fred);
f.addPerson(fred);
f.addPerson(fred);
Student fredStudent = new Student("Fred Flintstone", 50, "Math", 3.1);
System.out.println("created "+ fredStudent);
f.addPerson(fredStudent);
Person wilma = new Person("Wilma Flintstone", 48);
f.addPerson(wilma);
Student george= new Student("George", 21, "Politics", 3.1);
System.out.println("created " + george);
f.addPerson(george);
george.setName("Georgie");
f.addPerson(new Student("George", 21, "Politics", 3.1));
f.addPerson(new Student("John", 18, "Geology", 2.9));
f.addPerson(new Student("Jane", 21, "Music", 3.2));
f.addPerson(new Student("Tarzan", 22, "Gymnastics", 4.0));
f.addPerson(new Student("Jim", 21, "Physics", 2.5));
System.out.println("****** family listing: ");
f.printOutFamily();
}
}

Here's the problem, in Family#addPerson method:
if(isPresent == false)
family[i] = p;
You're adding the element in position i. If the element is not found, then i value will be family.length, thus giving you the exception.
Use int famArray field instead:
if(isPresent == false) {
family[famArray++] = p;
}
Or in an easier way for starters:
if(isPresent == false) {
family[famArray] = p;
famArray = famArray + 1;
}
As an addition to your current problem, you should first check if the famArray element is equals to family.length. If they're the same, then increase the array or do not allow more elements.
You state you have the same problem. This is because you're initializing famArray with the length of the array, noted in Family class constructor:
public Family(int size_of_family) {
famArray = size_of_family; //here
family = new Person[famArray];
}
Change the code to:
public Family(int size_of_family) {
famArray = 0;
family = new Person[size_of_family];
}
And you're done.

The problem is the line
family[i] = p;
This occurs after a for loop which increments i to be equal to the length of the array. I don't have any suggestions to fix it because I'm not sure what you are trying to do here.