First it starts off with the class College tester, it askes the user for a command.
The first command will be to add. what adding does is it askes the user to enter a name ( with at least 2 words by identifying a space) and an address.
Then it creates a student object with it. and I add the student object to a arraylist.
Issue #1: How would i add the collegetester input and create a student object out of it
Issue #2: after how would i add it to the array College
Collegetester (where i get user input)
import java.util.Scanner;
public class CollegeTester {
Scanner input = new Scanner(System. in );
private String command;
public String name;
public static void main(String[] args) {
CollegeTester collegeTester = new CollegeTester(); //creates object
collegeTester.getCommand(); //goes to command
}
//Ask user for a command
public void getCommand() {
System.out.println("Enter a command: ");
command = input.nextLine();
if (command.equals("add"))
addCommand(); //If command is add go to addcommand
}
//Add name and address to student object
public void addCommand() {
String name = "";
do {
System.out.println("Enter a Name: ");
name = input.nextLine();
} while (!(name.contains(Character.toString(' ')))); //To check if the name has at least 2 words
System.out.println("Enter an Address: ");
String address = input.nextLine();
Student student = new Student(name, address);
getCommand(); //repeat to see if user wishes to add another
}
}
Student object (The student object)
public Student(String name, String address) {
if (name == null)
name = "";
if (address == null)
address = "";
this.name = name;
this.address = address;
lastAssignedNumber++;
studentNum = lastAssignedNumber;
}
and the arraylist (In a different file)
import java.util.ArrayList;
public class College {
private ArrayList <College> entries = new ArrayList<College>();
}
Let's do a quick analysis of your code:
Your Student object looks like a constructor for the Student class object type which is most likely in this case an inner class of the CollegeTester class.
So here's the deal, your addCommand() already connects your CollegeTester class with your Student class, by executing this command after you provide the input for name and address it creates a new instance of the Student object.
This means that at this point, you need to add this newly created Student object to your College list.
However if you take a good look at your College list you will see that:
It's marked private (meaning it can be accessed only from within itself)
It is a list of College type objects (but you need a list of Student type objects)
So your options at this point are:
Make the list public
Create a public setter method that will do the adding
You will also need to change the list object type to Student if you wish to store Student objects in the list.
Also unless you want to have multiple colleges you might consider declaring your list as static otherwise you will have to declare an instance of it to add the Student objects to it.
Hopefully this is enough information to get you started in the right direction.
Also an advice is to not look at classes as actual objects, instead look at them as blueprints that can be used to construct those real objects.
The main method should call a constructor in the Object array class that create's the Object CollegeTester ct.
ct = new CollegeTester();
Then a method to insert new students, that calls the Student constructor in the student class to create a new student Object.
ct.insert("input1", "input2");
write an insert method that calls to create a new student and adds that object to College.
Constructor to add values to the new student object:
String studentName;
String studentAddress;
public Student(String name, String address) {
studentName = name;
studentAddress = address;
}
note: with ArrayList you could simply use the method add of ArrayList.
Related
I have the class "1". In that class I defined the variables "name" "phone number" and "ID". I defined all the sets and gets methods and the constructor for those variables.
In the class "2" I want to fill from the consol those variables. That will be the fist "option" of my CRUD, so everytime the user selects opcion=1, the system has to let add separately each of those variables. I know I have to use Array List but I haven't been able to do it successfully. This is an example of the code. CAPITAL LETTERS code is where I'm stuck. Thanks.
------First Class---------
package VV;
public class 1
{
private String name;
private String phone_number;
private String id;
public 1(String name, String phone_number, String id)
{this.name=name;
this.phone_number=phone_number;
this.id=id;
}
public String getName()
{ return name;
}
public void setName(String name)
{ this.name = name;
}
public String getPhone_number()
{ return phone_number;
}
public void setPhone_number(String phone_number)
{ this.phone_number = phone_number;
}
public String getId()
{ return id;
}
public void setId(String id)
{ this.id = id;
}
----------Second class------------
package VV;
public class 2
{
public 2()
{"Insert the name of the student:"
A METHOD TO INSERT THE NAME OF THE STUDENT
"Insert the phone number of the student:"
A METHOD TO INSERT THE PHONE NUMBER OF THE STUDENT
"Insert the ID of the student:"
A METHOD TO INSERT THE ID OF THE STUDENT
..And so on, each time user selects the opcion "add new student"
(I didn't put the while-case here with all its options to simply)
}
}
I actually don't know why you need that second class's constructor to take input from user and fill the fields of first class.
What I understood from the whole discussion is, you need to set the variables of the class by taking user input, and you need a dynamic way to take that input and set the values.
First way:
Well, this is a bit dynamic. You can use reflection to access all the variables of a class, and you can set their values dynamically.
First of all, create a default constructor in the first class. Then In the main class/whereever you want to take the input from user, create an object of first class and initialize it with default constructor. Then use reflection to work with individual fields, taking input from users and setting the values in the fields. This way doesn't require to know what fields are there to take input from user explicitly. Remember, you can use this method only if there's no problem to expose the names of the variables to the end user. I used only String input as you have only strings in the first class as variables. You need that field.setAccessible(true) if the variables are private. So, let's see the code in the main class/where you'll call the function to take user input:
Field[] arrayOfFields = Student.class.getDeclaredFields();
Scanner sc = new Scanner(System.in);
Student student = new Student();
for (Field field : arrayOfFields) {
try {
field.setAccessible(true);
System.out.println("Please enter the Student " + field.getName());
field.set(student, sc.nextLine());
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
That sets the fields accordingly.
Second Way:
If you know the variables you need to initialize the objects, just use scanner to scan them one by one from the user console input and initialize the object:
Scanner sc = new Scanner(System.in);
String id,name,phone_number;
System.out.println("Please enter the Student ID:");
id = sc.nextLine();
System.out.println("Please enter the Student Name:");
name = sc.nextLine();
System.out.println("Please enter the Student Phone Number:");
phone_number = sc.nextLine();
Student student = new Student(id, name, phone_number);
Simple as that.
Problem:
I'm trying to figure out how to access the Student Array class in order to create four entries for each Student object, but I'm not sure how to do so, while also allowing the program to create more than just one Student.
public class ClassRoster<T> {
public static void main(String[]args) {
ClassRoster<Student> classroster = new ClassRoster<Student>();
Scanner keyboard = new Scanner(System.in);
System.out.println("Add/Drop/Search?");
String action = keyboard.nextLine();
boolean done = false;
Object temp, c, d, e;
int fresh, soph, jun, sen;
Student test = new Student();
while(!done) {
if(action.equalsIgnoreCase("Add"))
{
int counter = 0;
System.out.print("Enter Student ID");
temp = test.setID(keyboard.nextInt());
System.out.println("First name?");
c = test.setFirstName(keyboard.nextLine());
System.out.println("Last name?");
d = test.setLastName(keyboard.nextLine());
System.out.println("Academic Level?");
e = test.setLevel(keyboard.nextLine());
...
}
And I have another class called Student, where there are four different data entries (ID, FirstName, LastName, Academic Level).
I'm not sure how to access the object which I have created in the correct way. It just gives me an error in this Driver class, and I don't know how to correctly access the array bag.
but I'm not sure how to do so while also allowing the program to create more than just one Student
Currently you are only creating one specific instance of student with Student test = new Student(); To actually create more than one student, you will have to iterate the whole process of reading all four data entries (ID, FirstName, LastName, Academic Level). Instead of having to initialize the fields (your four data entries) with specific set methods, I would recommend you letting the Student class initialize them with the class constructor. Meaning the Student class should look something like this:
public class Student{
private final int ID;
private final String firstname;
private final String lastname;
private String level;
public Student(int ID, String firstname, String lastname, String level){
this.ID = ID;
this.firstname = firstname;
this.lastname = lastname;
this.level = level;
}
ID, firstname and lastname are set to final as you foresee them not to change. However the academic level is ought to change and therefore is not set to final. Now that you have set up a constructor for your Student class, we can get to how to allow the program to insert multiple students at once.
public static void main(String[]args) {
ClassRoster<Student> classroster = new ClassRoster<Student>();
Scanner keyboard = new Scanner(System.in);
System.out.println("Add/Drop/Search?");
String action = keyboard.nextLine();
boolean done = false;
while(!done) {
if(action.equalsIgnoreCase("Add")) {
System.out.print("Enter Student ID");
int ID = keyboard.nextInt();
System.out.println("First name?");
String firstname = keyboard.nextLine();
System.out.println("Last name?");
String lastname = keyboard.nextLine();
System.out.println("Academic Level?");
String level = keyboard.nextLine();
Student student = new Student(ID, firstname, lastname, level);
//we have now created a new instance of Student, now we have to save it in your classroster
classroster.add(student);
}
System.out.println("Next action?");
action = keyboard.nextLine();
if(action.equals("done") done = true; //if you write 'done', your loop will finish executing
}
I don't know about your implementation of classroster, but I assume you have implemented it with some kind of list or map, which is why I call the add(Student s) method after creating an instance of Student. To actually then access all students, you will have to implement a method in classroster that returns the saved list of classroster and then iterate through the returned list in the main loop. To actually see what the students look like, you will also have to implement methods for the student instances to for example print out their full names.
I see that you are having a little trouble with arrays, maps and lists as you don't know how to access your students yet. I recommend you reading up on the difference between these three data structure types and simply try to implement them in a small example to see how they work.
I am working on a Java app for a school project where we have to enter user information: Name, Student ID and their points. How can I store all the data for each user on an ArrayList (or an Array or really whatever type) so I can keep track of all the data.
Example:
John Doe - 401712 - 20 points
Jack Young - 664611 - 30 points
The I want to be able to call methods like setPoints to change the point values for whatever the student selected is.
Here's the problem: How can I link the ArrayList together. If I have three ArrayLists, how does Java know what name, student id and points are associated together?
All the ArrayLists are stored in a class called Data.
Data data = new Data();
Also, all the ArrayLists in the Data class should be outputted to a file which will be loaded next time the app is opened.
I will try to answer any questions.
You need to define a class which contain 3 data fields as follows
Name
Student ID
their points
But not to forget, the class has to have other necessary elements of a class like:
Constructor
Overloaded Constructors if they are necessary
Accessors
Mutators
Note: For accessing each part of an object in your arrayList, you can use accessors. For manipulating each part of an object in your arrayList, you can use mustators.
After having such a class, you can define a arrayList that contain elements with type of class you have already define
Like:
List<Your Type of class > students = new ArrayList<Your Type of class>;
After Java 7, you can do
List<Your Type of class > students = new ArrayList<>;
which is diamond inference.
If you are looking for a specific id number in your arrayList, you can do something like:
public boolean findIdNumber(int idNumber){
for(int i=0; i< students.size; i++)
if(students.get(i).getID() == idNumber)
return true;
else
return false;
}
Warning:
what I have done are suggestions for you to be able to look for what you want smoother. You need to do necessary changes in order to comply what you were
asked to do
You need to create a class named Student, and then declare an array/ArrayList of the Student type. Your Student class must have a constructor that sets the fields of an instance of the Student class (the created instance is now called an object).
So first create a Student class in the same package in which your other class is (the class in which your main method is):
public class Student {
private String firstName;
private String lastName;
private String studentId;
private int points;
public Student(String firstName, String lastName, String studentId, int points) {
this.firstName = firstName;
this.lastName = lastName;
this.studentId = studentId;
this.points = points;
}
public String getFirstName() {
return firstName;
}
public String getLastName() {
return lastName;
}
public int getPoints() {
return points;
}
public void setPoints(int points) {
this.points = points;
}
}
Then in your main method or wherever you like, create a Hashmap to hold your Student objects. A map/hashmap is a collection just like an ArrayList to hold a set of objects. In your use case, it is better to use a hashmap because finding/retrieving a specific student object is much faster and easier when you use a hashmap.
import java.util.HashMap;
import java.util.Map;
public class Main {
public static void main(String[] args) {
// a map is a "key-value" store which helps you search items quickly
// (by only one lookup)
// here you consider a unique value of each object as its 'key' in the map,
// and you store the whole object as the value for that key.
// that is why we defined Student as the second type in the following
// HashMap, it is the type of the "value" we are going to store
// in each entry of this map.
Map<String, Student> students = new HashMap<String, Student>();
Student john = new Student("John", "Doe", "401712", 20);
Student jack = new Student("Jack", "Young", "664611", 30);
students.put("401712", john);
students.put("664611", jack);
Student johnRetrieved = students.get("401712");
// each hashmap has a get() method that retrieves the object with this
// specific "key".
// The following line retrieves the student object with the key "664611".
Student jackRetrieved = students.get("664611");
// set/overwrite the points "field" of this specific student "object" to 40
johnRetrieved.setPoints(40);
int johnsPoints = johnRetrieved.getPoints();
// the value of johnsPoints "local variable" should now be 40
}
}
The classical object-oriented approach would be to create a Student class including name, ID and points and storing list of Student objects in a single ArrayList.
class Student{
private String id;
private String name;
private int points;
public Student(String id, String, name, int points){
this.id = id;
this.name = name;
this.points = points;
}
}
..
ArrayList<Student> students = new ArrayList<Student>();
students.add(new Student(1, 'John Doe', 1000));
String id = students.get(0).id;
I am attempting to create a list of bank records. Each record consists of a first name, last name, phone number, and balance. In the first class I ask the user for this information, then create a new instance of the records class to add to the list. However, as I add more records it replaces all records with the most recent one, which you can see with my showAllRecords() method. How do I fix this?
The add and showAllRecords method in the main class. These methods are called from a switch statement in the main method:
private static void showAllRecords()
{
if(records.bankRecords.size() == 0)
System.out.println("There are no records.");
else
for (int i = 0; i < records.bankRecords.size(); i++)
{
System.out.println(records.bankRecords.get(i));
}
}
private static void add()
{
Scanner scan = new Scanner(System.in);
System.out.print("Please enter the first name: ");
String firstName = scan.next();
System.out.print("Please enter the last name: ");
String lastName = scan.next();
System.out.print("Please enter the phone number: ");
String phoneNumber = scan.next();
System.out.print("Please enter the balance: ");
int balance = scan.nextInt();
bankRecords.add(new records(firstName, lastName, phoneNumber, balance));
}
The records class
public class records
{
public static String firstName;
public static String lastName;
public static String phoneNumber;
public static int balance;
LinkedList<records> bankRecords = new LinkedList<records>();
public records(String tFirstName, String tLastName, String tPhoneNumber, int tBalance)
{
firstName = tFirstName;
lastName = tLastName;
phoneNumber = tPhoneNumber;
balance = tBalance;
}
}
The problem occurs because all the fields in records class are static. Remove the static keyword from the declarations of fields. As they are static whenever you create a new object of records class you overwrite those static fields.
Static fields belong to the class not to the object.
Remove the LinkedList instance that you have declared in records class. Why are u doing that. Declare it in your main class and try to use ArrayList which I think is better in your case. The reason is that records has static fields
Why your class name starts with small letter. Its a very very bad practice.
You have an inherent planning problem.
There is a difference between the entity "Bank Record", which includes, as you said, a name, balance etc., and the entity "List of Bank Records", which includes, well, a variable number of bank records.
Your "records" class (please use a capital letter in the beginning of a class name) tries to mix both. So you have both a record and a list inside it. You should separate the two entities. You then create a new "Record", and add it to the "ListOfBankRecords" objects.
Also, it seems that you have both a variable and a variable called "records". This is also why a capital letter would have been good. You shouldn't have a variable that has the same name as a class.
I'm working on a homework project that requires me to create an object from data entered by a user. I have a class called Person which takes the basic information, a class called Customer which extends the Person class and includes a customer number and a class called Employee which extends the Person class and returns a social security number.
I have pasted the code from my main program below. I'm a little confused on a couple of things. First when I'm collecting the information (first name, last name etc) amd I supposed to be accessing my Person class in there somehow?
Second I guess more plainly, how do I create the object? so far in all of the examples I have read online I find they seem to enter the information already like if I were to have it say
Person firstName = new Person(Jack);
Although I am collecting the information from the user so I don't see how to tell it like
Person firstName = new Person (enter info from user here);
Finally and again this is a really dumb question but I have to create a static method that accepts a Person object.
To create the static method I'm assuming it is
Public Static print()
but how do I tell it to print something from the person class? how does it know?
Most of my examples in the book include a class that contains all of the information instead of making the user enter it which is confusing because now I'm being told the user has the freedom to type what they want and I need to collect that information.
import java.util.Scanner;
public class PersonApp
{
public static void main(String[] args)
{
//welcome user to person tester
System.out.println("Welcome to the Person Tester Application");
System.out.println();
Scanner in = new Scanner(System.in);
//set choice to y
String choice = "y";
while (choice.equalsIgnoreCase("y"))
{
//prompt user to enter customer or employee
System.out.println("Create customer or employee (c/e): ");
String input = in.nextLine();
if (input.equalsIgnoreCase("c"))
{
String firstName = Validator.getString(in, "Enter first name: ");
String lastName = Validator.getString(in, "Enter last name: ");
String email = Validator.getEmail(in, "Enter email address: ");
String custNumber = Validator.getString(in, "Customer number: ");
}
else if(input.equalsIgnoreCase("e"))
{
String firstName = Validator.getString(in, "Enter first name: ");
String lastName = Validator.getString(in, "Enter last name: ");
String email = Validator.getEmail(in, "Enter email address: ");
int empSoc = Validator.getInt(in, "Social security number: ");
}
}
System.out.println("Continue? y/n: ");
choice = in.next();
}
}
First, I observe that there isn't a Person object. I assume you'll get around to creating that, so I'm not going to concern myself too much with it.
Insofar as actually getting the data, you're halfway there. Depending on how you want to frame the Person object, you can create a new Customer or Employee object by passing the values which you received from the user.
Customer customer = new Customer(firstName, lastName, email, custNumber);
or
Employee employee = new Employee(firstName, lastName, email, empSoc);
Here's the snippet of both:
public class Person {
public Person (String first, String last, String email) {
// You'd fill in code here for handling the variables
}
// ...
}
public class Customer extends Person {
public Customer (String first, String last, String email, String custNo) {
super(first, last, email);
// You'd fill in code here for handling the variables
}
// ...
}
public class Employee extends Person {
public Employee (int social) {
super(first, last, email);
// You'd fill in code here for handling the variables
}
// ...
}
To print something from the Person class, using that static method (why? You could override toString() instead), you frame it such that your Person object has accessors to each of the fields relevant to a Person. This would mean you have a getFirstName(), getLastName(), and so forth, relevant to the object if it's an employee or a customer. (I leave that as an exercise to you.)
In that sense, one would then only require calls to those accessors to print the value.
public static void print(Person p) {
System.out.println(p.getFirstName()) + " " + p.getLastName()); // You can get the trend from here.
}
To print the Person object you can just use the System.out.println() if you just want to print it to the command line, but you'll get some unreadable nonsense.
What the println() method does is, if the object is not a String call it's toString() method, because all objects have one, it is defined in java.lang.Object. But that method gives us unreadable things mentioned above, so you have to override it to do something like
public class Person
{
String firstName;
String Lastname;
public Person(String firstName, String lastName)
{
this.firstName = firstName;
this.lastName = lastName;
}
public String toString()
{
// Create a String that represents this object
return "This person is called " + firstName + " " + lastName;
}
}
To create an object you can read the Strings from the commandline and then pass them into the constructor as Makoto suggests.