Develop Reference

Using Eclipse, trying to get a program to work

I am currently learning Java with eclipse for my computer science course, and I need some assistance trying to figure out how to fix the error that is currently showing.
package sec4Les2;
public class RoseDS4L2Person {
//creating the variables
public String name = "Uma Thurman";
public int Age = 0;
//constructor
public RoseDS4L2Person()
{
}
public String getname()
{
//will return first name
return name;
}
public int getAge()
{
//will return age
return Age;
}
public void setAge(int Age)
{
//will set age to int
this.Age = Age;
}
}
And here is the running code:
package sec4Les2;
public class RoseDS4L2ManagingPeople {
public static void main(String[] args) {
//runs information in first class file
RoseDS4L2Person p1 = new RoseDS4L2Person("Arial", 37);
RoseDS4L2Person p2 = new RoseDS4L2Person ("Joseph", 15);
if(p1.getAge()==p2.getAge())
{
System.out.println(p1.getname()+" is the same age as "+p2.getname());
}
else
{
System.out.println(p1.getname()+" is NOT the same age as "+p2.getname());
}
}
}
It says there are no errors on the first one, but the second one has errors on the p1/p2 lines. How can I fix this error? thank you!

You should have a constructor for RoseDS4L2Person accepting a string and a number, like so:
public RoseDS4L2Person(String name, int age)
{
this.name = name;
this.Age = age;
}
This will allow you to create an instance of the class passing a name and an age as parameters.

You need to add this code to your class. Basically, a constructor was missing in your class. Also note that by convention, member variables start in lowercase. So your Age should really be age.
Another thing is, you have kept name as constant. Probably you want to remove "Uma Thurman". If you want to keep that as default name for all objects where name is not specified at initialization time, you would want to add that in the constructor. *
public class RoseDS4L2Person
{
// other lines ....
RoseDS4L2Person(String name, int age) {
this.Age = age;
this.name = name;
}
}
*
Something like this:
public class RoseDS4L2Person
{
private static final String UMA_THURMAN = "Uma Thurman";
// other lines ....
RoseDS4L2Person( int age) {
this.Age = age;
this.name = UMA_THURMAN;
}
}

The error is popping up because you have created objects with a non-existing constructor.Making objects with the default constructor or creating a constructor
which can accept the arguments you pass should do the trick.
Happy Coding

I am trying to print the name of the list but somehow the reference from the arraylist is not working?

Customer is a class. The Class list is arraylist of Customer.
I have added the Customers to list but when I want to print all the customer names from the list I get null only.
import java.util.*;
public class Assignment1 {
public static void main(String args[])
{
List list = new List();
list.addCustomer("man");
list.addCustomer("man");
//System.out.println(list);
list.printx();
}
}
class Customer{
public String name;
public Customer(String name)
{
name = this.name;
}
}
class List
{
ArrayList<Customer> list = new ArrayList<Customer>();
public void addCustomer(String name1)
{
Customer x = new Customer(name1);
list.add(x);
System.out.println(list.get(0));
}
public void printx()
{
for(int i =0;i < list.size();i++)
{
System.out.println(list.get(i).name);
}
}
}

Inside your Customer constructor, you need to set ::
this.name = name;
and not the other way round! :P
What you have done right now is that you change the function parameter name to the class parameter name which is currently null(default initialization). So, you never initialize name variable of the Customer class, and hence you always get null when you print it.

I suggest you override the toString method in class Customer, it helps you debug your Customer objects. For example, you can change the local variable to assignedName as below:
class Customer{
public String name;
public Customer(String name)
{
this.name = name;
}
#Override
public String toString(){
return "customer name:" + this.name;
}
}
this.name and name are different things in the Customer constructor:
this.name is an instance variable and name is a local variable defined in your constructor.
// you should narrow the modifier to private, and implement getter and setter for it
public String name;
public Customer(String assignedName){
this.name = assignedName;
}

New to OOP, manage variables in classes [closed]

Closed. This question needs details or clarity. It is not currently accepting answers.
Want to improve this question? Add details and clarify the problem by editing this post.
Closed 7 years ago.
Improve this question
So. I got a mission from my teacher to make a program that manages different students. The program will hold the name, education, information and points.
You will have the option to:
Add new students
Change education and information
Manage points
Making new students is not a problem, but managing education, info and points for specific students is the hard part, that's where I need your help. My main.java does not contain anything for now.
Student.java
package student;
public class Student {
String namn;
String program;
String info;
int points;
public void managePoints(){
}
public void changeProgram(){
}
public void changeInfo(){
}
}
Main.java
package student;
public class main {
public static void main(String[] args) {
}
}

According to the comments, I guess the three methods in your class are supposed to change the points, program and info of the student to a desired value. In Java, we call these setters.
You should rename your methods to setPoints, setProgram and setInfo. It's a pattern, you know.
Next, how are you going to know the "desired" value of those fields? You might say, I get them from the text boxes in the methods. But a better approach would be to get the values from another method and pass the value to the setters as a parameter.
To add a parameter to your methods, add a variable-like thingy in the brackets of the method declaration:
public void setPoints (int p)
And for the setInfo
public void setInfo (String i)
And so on.
In the method bodies, you set the fields to the parameters. E.g. In the setInfo method you write
info = i;
I think you can figure out the others.
Now how do you use these methods? For instance, suppose you have a student variable called student. And you got the info of him/her and stored it in a string variable called studentInfo. You can set the student variable's info to studentInfo by
student.setInfo (studentInfo);
Or if you don't want to use a variable, you can just use a string literal.
student.setInfo("this is my info. Blah blah blah");

I don't exactly know what do you want to actually do but your Student class (if I think correctly what you will need) should look more like this:
public class Student {
private String name; // private because you don't want anyone to interact with the variable too much.
private String program;
private String info;
private int points;
public Student( String name, String program, String info, int points ) { // contructor with variables to initialize. You can remove some of the variables if you do not consider they should be here.
this.name = name;
this.program = program;
this.info = info;
this.points = points;
// without `this` you would change parameter's value to itself which isn't what you want.
}
public String getName( ) { // getter because I guess you would like to know students name
return name;
}
public int getPoints( ) {
return points;
}
public void addPoints( int points ) { // setter so you can modify points
this.points += points;
}
public String getProgram( ) { // same as getName
return program;
}
public void setProgram( String program ) {
this.program = program;
}
public String getInfo( ) {
return info;
}
public void setInfo( String info ) {
this.info = info;
}
}
But how to use these methods? You use them as the example below shows
Student s1 = new Student("Abc Xyz", "IT", "Some informations", 12);
Student s2 = new Student("Cba Zyx", "Some other program", "Some more informations, 0);
s2.setInfo( s1.getInfo( ) );
s1.setPoints(1234);
s2.setProgram("Axbzcy");
Getter is a method which returns (most likely) private variable's value.
Setter is a method which sets private variable's value to another value which is passed as a parameter to the method.

Final code:
package student;
// The student class definition
public class Student {
private String name;
private String address;
private String info;
private String kurs;
private int points;
// Constructor
public Student(String name, String address, String info, String kurs, int points) {
this.name = name;
this.address = address;
this.points = points;
this.kurs = kurs;
this.info = info;
}
// Public getter for private variable name
public String getName() {
return name;
}
// Public getter for private variable address
public String getAddress() {
return address;
}
public String getInfo() {
return info;
}
public int getPoints() {
return points;
}
// Public setter for private variable address
public void setAddress(String address) {
this.address = address;
}
public void setPoints(int points){
this.points = points;
}
public void setInfo(String info){
this.info = info;
}
public void setKurs(String kurs){
this.kurs = kurs;
}
// Describe itself
public String toString() {
return name + ", Adress: " + address + ", Info: " + info + ", Kurs: " + kurs + ", Poäng: " + points +" ";
}
}
Main
package student;
// A test driver program for the Student class
public class main {
public static void main(String[] args) {
Student ArHa = new Student("A H", "Jysgaan 61", "ADHD", "Teknik", 5);
ArHa.setPoints(10);
ArHa.setKurs("TEINF");
System.out.println(ArHa);
Student DaSk = new Student("Dael Sklbr", "Fegea 65", "Svart", "Teknik", 5);
DaSk.setInfo("Riktigt svart");
System.out.println(DaSk);
Student FaMe = new Student("Falafel Medusa", "Fågel 123", "Maten", "Kock", 123);
System.out.println(FaMe);
}
}
Thank you everyone for the help.

Method override returns null

I'm newbie in Java. So question might sound simple, but I'm stuck and can not figure out why this code returns null and 0.0 ?
file: Transport.java
public class Transport {
private String name;
private double price;
public Transport(String name, double price) {
this.name = name;
this.price = price;
}
public String carName() {
return name;
}
public double carPrice(){
return price;
}
}
file: Car.java
public class Car extends Transport{
protected String name;
protected double price;
public Car(String name, double price) {
super(name, price);
}
#Override
public String carName(){
return name;
}
#Override
public double carPrice(){
return price * 1.5;
}
}
file: Main.java
public class Main {
public static void main(String[] args) {
Car c = new Car("CarBrand", 1000);
System.out.println("Name: " + c.carName());
System.out.println("Price: " + c.carPrice());
}
}
Output
Name: null
Price: 0.0

You've declared separate name and price variables in Car, and never assigned a value to them - they're not the same as the name and price variables declared (and initialized) in Transport. So you're seeing the default values for String and double, basically. Get rid of those extra variables in Car, and use super.carPrice() to get the original price from Transport:
public class Car extends Transport {
public Car(String name, double price) {
super(name, price);
}
#Override
public double carPrice(){
return super.carPrice() * 1.5;
}
}
Note that there's no need to override carName() at all unless you really want it to change behaviour.
I'd also suggest changing carName() and carPrice() to getName() and getPrice() to be more idiomatic.

You are passing both the values to parent class Transport through super(). So
Car c = new Car("CarBrand", 1000);
will eventually set
Transport class attributes name & price.
You dont need to declare both the attributes in Car class. Car will have both attributes implicitly through inheritance. Here you are creating separate attributes for Car.

The problem is that you have two different variables for name, one in Car and one in Transport. c.carName() returns Car.name which has not been initialized.
If your car class is the one below, it will work
public class Car extends Transport {
public Car(String name, double price) {
super(name, price);
}
#Override
public double carPrice(){
return price * 1.5;
}
}
the same goes for the variable price

The derived class Car is hiding the instance variables of class Transport .So although you are inheriting the correctly initialized data members from Transport class ,but the Car class instance variables initilized to their default values are getting returned from Car class methods

When you create the 'c' object of type Car, you assign values only for 'name' and 'price' variables of class Transport (because in your constructor you call super(name, price) that will call the constructor from your Parent class).
Here: c.carName() you call the method from your Car class (because is marked as #Override) and this one returns the value of the 'name' variable from class Car. And this variable in your case, is null because you didn't assign any value for it yet.
You assigned the value "CarBrand" for 'name' variable of type Transport.
The same for 'price' variable.

The use of super will return the values which you already stored in the parent class by calling the constructor super(name, price), the use of super followed by dot notation will access the parent class method. So super.carPrice() will return the value stored in the parent class.
Also, #Override annotation should only used to change an existing method from the parent class with a new functionality in the child class with out changing the name. So in case of the #Overide for carname() you need to call the super.carname() because you are returning the value from the parent class.
In short, The reason why you are getting null and 0.0 because you are accessing the child class values when you should be accessing the parent class values.
public class Car extends Transport{
protected String name;
protected double price;
public Car(String name, double price) {
super(name, price);
}
#Override
public String carName(){
return name;
}
#Override
public double carPrice(){
return price * 1.5;
}
}
Your class should be
public class Car extends Transport{
public Car(String name, double price) {
super(name, price);
}
public String getName(){
return super.carName();
}
#Override
public double carPrice(){
return super.carPrice()* 1.5;
}
}
your main class should now be
public class Main {
public static void main(String[] args) {
Car c = new Car("CarBrand", 1000);
System.out.println("Name: " + c.getName());
System.out.println("Price: " + c.carPrice());
}
}

How can I print from this object? Java

I need to print the first name, last name, and salary from two employee objects but I keep getting a cannot find symbol error. What would I do to fix this?
Here is the constructor class:
public class Employee
{
private String firstName;
private String lastName;
private double monthlySalary;
public Employee( String firstName1, String lastName1, double monthlySalary1) {
setfirstName(firstName1);
setlastName(lastName1);
setmonthlySalary(monthlySalary1);
}
String getfirstName() {
return firstName;
}
String getlastName() {
return lastName;
}
double getmonthlySalary() {
return monthlySalary;
}
public void setfirstName (String firstName1) {
firstName = firstName1;
}
public void setlastName (String lastName1) {
lastName = lastName1;
}
public void setmonthlySalary (double monthlySalary1) {
monthlySalary = ( monthlySalary1 >= 0 ? monthlySalary1 : 0);
}
}
And here is what I have so far to print the objects:
public class EmployeeTest {
public static void main(String[] args) {
Employee a = new Employee("John", "Smith", 10000);
Employee b = new Employee("Jane", "Smith", 11000);
System.out.print(a.firstName1);
}
}
I need to be able to have it print out something along the lines of "Name: Salary:" But I am clueless as to how to make this work. Any help would be greatly appreciated.

In your employee class, you need to override the toString() method.
You can try something like:
#Override
public String toString()
{
System.out.println("Name: "+name+"Salary: "+salary);
}
Then for each of your employees, when you want to print them, just call
System.out.println(employee);

You cant print out firstName (or firstName1, because that doesnt exist in your class), because its marked as private. You should do something like this:
System.out.print(a.getfirstName())

firstName is private, which means that it cannot be seen outside of the object/class it resides in. I suggest you try overriding the toString() method on your Employee class. That method would have access to all the private members of Employee.
Alternately, you could use getfirstName() to return the first name.
Also, this may be a typo, but there is no firstName1 in Employee - it is firstName.