I have 2 differents programs that use class and class child.
In both of the program I use private field. But in the first program that has for parent class "Car" I have no problem with private field, the children can has access to. But in the second program that has "Hamburger" for parent class, the children can not has acces to the fields :
First program (the program is working)
public class Car {
private boolean engine;
private int cylinders;
private String name;
private int wheels;
public Car(int cylinders, String name) {
this.cylinders = cylinders;
this.name = name;
this.wheels = 4;
this.engine = false;
}
}
public class Mitsubishi extends Car {
public Mitsubishi() {
super(5, "Mitsubishi");
}
}
Second program (not working)
public class Hamburger {
private String name;
private String meat;
private double price;
private String breadRollType;
private String addition1;
private String addition2;
private String addition3;
private String addition4;
public Hamburger(String name, String meat, double price, String breadRollType) {
if (price < 0) {
price = 0;
}
this.name = name;
this.meat = meat;
this.price = price;
this.breadRollType = breadRollType;
this.addition1 = "none";
this.addition2 = "none";
this.addition3 = "none";
this.addition4 = "none";
}
}
public class DeluxeBurger extends Hamburger{
public DeluxeBurger() {
super("Deluxe Burger", "Steak", 19.10, "Chic bread");
}
}
I have seen that I can use protected and it's fix the probelem, but I don't know why I have to use protected on the second program (with the Hamburger) but not on the first (with the Car).
I copy your code and past to my project and see your problem.
i dont know Why, maybe its a bug. but if you create new package and drop this two class into new package Your problem will be solved and you will not see this problem again. It was interesting for me.
thanx
public class Playlist
{
String title;
String genre;
Boolean privatePlaylist = true;
Song[] listOfSongs;
public Playlist(String tl, String gn, Boolean priv)
{
tl = title;
gn = genre;
priv = privatePlaylist;
}
Song[] mostPlayedSongs()
{
return listOfSongs;
}
}
Above is my code. I am trying to create a java class that returns a playlist. I want it to return a title, genre, whether the playlist is private or public. All three of these things are already constructed above. However, to that list, as a fourth property I would like to add an array that lists all the song in the playlist. The array is already a property I created above(Song[]listOfSongs). However, I do not know how to join this array to the other three properties and how to put the songs in this array. Any suggestions?
if you don't want to change the songs, you could just place it in the constructor:
public PlayList(String title, String genre, Boolean privatePlaylist, Song[] songs) {
this.title = title;
this.genre = genre;
this.privatePlaylist = privatePlaylist; //original constructor wasn't assigning this property
this.listOfSongs = listOfSongs;
}
if the list of songs shouldn't be added in the constructor (i.e, it needs to be updated) I would use an ArrayList so you can change the size:
// Field:
private ArrayList<Song> listOfSongs;
// Constructor
public PlayList(String title, String genre, Boolean privatePlaylist) {
this.title = title;
this.genre = genre;
this.privatePlaylist = privatePlaylist; //original constructor wasn't assigning this property
this.listOfSongs = new ArrayList<>();
}
// To edit the songs:
public void setListOfSongs (ArrayList<Song> listOfSongs) {
this.listOfSongs = listOfSongs;
}
import java.util.ArrayList;
public class Playlist {
public final String title;
public final String genre;
public final Boolean privatePlaylist;
public final ArrayList<Song> listOfSongs;
public Playlist(String tl, String gn, Boolean priv) {
title = tl;
genre = gn;
privatePlaylist = priv;
listOfSongs = new ArrayList<>();
}
}
Example usage:
public class Main {
public static void main(String[] args) {
Playlist pl = new Playlist("title", "genre", false);
pl.listOfSongs.add(...);
System.out.println(pl.title);
}
}
I need to create a method call loadBooks. When called, this method create 5 books and adds them to the collection.
What i Have
public class Library
{
private ArrayList<Book> library;
public Library()
{
library = new ArrayList<>();
}
public void loadBooks()
{
library.add(new Book("T"));
library.add(new Book("A"));
library.add(new Book("C"))
library.add(new Book("O"));
library.add(new Book("S"));
}
Book Class:
class Book
{
private String author;
private String title;
private int numberOfPages;
private String refNumber;
private int borrowed;
private int total;
private boolean courseText;
public Book(String bookAuthor, String bookTitle, int numPages,boolean isCourseText)
{
author = bookAuthor;
title = bookTitle;
numberOfPages = numPages;
refNumber = "";
courseText = isCourseText;
}
I get a constructor Book in class Book cannot be applied to given types error then if i change it to something else it says cannot find class error.
Any ideas what to do or fix my code?
You're book class constructor accsepts 4 perameters, yet you're only passing it one in your loadBooks() method.
Just change your lines in loadBooks() to something like:
library.add(new Book("Author", "T", 1205 , true));
In my CD class and TrackList class I have an object called tracklist that comes up as an error. I don't know what I need to add or remove to fix it.
Here is my CD class:
public class CD {
public String artist;
public String title;
public trackist myTracklist;
public CD(String artist, String title){
artist = artist;
title = title;
myTracklist = new tracklist(100);
}
public static void main(String[] args) {
String mainArtist;
String mainTitle;
CD myCD = new CD("RisingNation", "Natioins");
mainArtist = myCD.getArtist();
System.out.println(mainArtist);
mainTitle = myCD.getTitle();
System.out.println(mainTitle);
myCD.display();
}
public String getArtist(){
String person;
person = artist;
return person;
}
}
Also here is my tracklist class,
public class TrackList {
int length;
int numUsed;
String[] storage;
public tracklist(int size){// this shows up underlined in red
length = size;
numUsed = 0;
storage = new String[length];
}
public int count(){
return numUsed;
}
}
The error occurs because the compiler doesn't know whether tracklist should be a method, since there was no return type, Or a constructor since it doesn't match the class name. But it seems to me that it should be a constructor.
So change the code to :
public TrackList(int size)
You should also change the CD class constructor like :
public CD(String artist, String title){
this.artist = artist;
this.title = title;
myTracklist = new tracklist(100);
}
Since the constructor parameters have the same names as the class global variables, you need to refer to the global variables with this..
That line should be Constructor right?
Constructor name should be the same as Class name. Case is sensitive. This mean, if your class are
public class TrackList {}
So the constructor must be
public TrackList () {}
I'm just working through a few things as practice for an exam I have coming up, but one thing I cannot get my head round, is using a variable that belongs to one class, in a different class.
I have a Course class and a Student class. Class course stores all the different courses and what I simply want to be able to do is use the name of the course, in class Student.
Here is my Course class:
public class Course extends Student
{
// instance variables - replace the example below with your own
private Award courseAward;
private String courseCode;
public String courseTitle;
private String courseLeader;
private int courseDuration;
private boolean courseSandwich;
/**
* Constructor for objects of class Course
*/
public Course(String code, String title, Award award, String leader, int duration, boolean sandwich)
{
courseCode = code;
courseTitle = title;
courseAward = award;
courseLeader = leader;
courseDuration = duration;
courseSandwich = sandwich;
}
}
And here is Student:
public class Student
{
// instance variables - replace the example below with your own
private int studentNumber;
private String studentName;
private int studentPhone;
private String studentCourse;
/**
* Constructor for objects of class Student
*/
public Student(int number, String name, int phone)
{
studentNumber = number;
studentName = name;
studentPhone = phone;
studentCourse = courseTitle;
}
}
Am I correct in using 'extends' within Course? Or is this unnecessary?
In my constructor for Student, I am trying to assign 'courseTitle' from class Course, to the variable 'studentCourse'. But I simply cannot figure how to do this!
Thank you in advance for your help, I look forward to hearing from you!
Thanks!
Am I correct in using 'extends' within Course? Or is this unnecessary?
Unfortunately not, if you want to know whether your inheritance is correct or not, replace extends with is-a. A course is a student? The answer is no. Which means your Course should not extend Student
A student can attend a Course, hence the Student class can have a member variable of type Course. You can define a list of courses if your model specifies that (a student can attend several courses).
Here is a sample code:
public class Student{
//....
private Course course;
//...
public void attendCourse(Course course){
this.course = course;
}
public Course getCourse(){
return course;
}
}
Now, you can have the following:
Student bob = new Student(...);
Course course = new Course(...);
bob.attendCourse(course);
I assume a Course is not a Student, so inheritance between those classes is probably a bad idea.
You have to declare them public.
A better way is the keep them private, and code a public getter for that variable. for example:
public Award getCourseAward(){
return this.courseAward;
}
Course should not extend Student. If you want to access the courseTitle field of Course, you need to pass a reference to a Course object to the Student and then do course.CourseTitle.
You cannot access private attributes of a class from another, this is one of the main principles of OOP: encapsulation. You have to provide access method to those attribute, you want to publish outside the class. The common approach is setter/getters - getters only, if you want to have your class immutable. Look here: http://en.wikipedia.org/wiki/Mutator_method#Java_example
It does not make sense to arbitrarily extend classes. Student is not a Course or vice versa, so you cannot extend them like that.
What you need to do is:
create a Course first:
Course aCourse = new Course(..);
create a Student:
Student aStudent = new Student(..);
assign the Course to the Student:
aStudent.setCourse(aCourse.title);
Extending Student with Couse because they are not of the same kind. Extending one class with another happens when specializing a more general (in a sense) one.
The solution would be to pass courseTitle as an argument of the Student constructor
There should be 3 separate objects here, a Course, a Student, and an Enrollment. An enrollment connects a Student to a Course, a Course has many Students, and a Student can enroll in many courses. None of them should extend each other.
First,
You are extending Student class in Course class, which means, student class gets all the coruse class properties. So, the student class does not have the courseTitle property.
Second, yes, it is unnesessary - you need to do the following:
public class Course
{
private Award courseAward;
private String courseCode;
public String courseTitle;
private String courseLeader;
private int courseDuration;
private boolean courseSandwich;
public Course(String code, String title, Award award, String leader, int duration, boolean sandwich)
{
courseCode = code;
courseTitle = title;
courseAward = award;
courseLeader = leader;
courseDuration = duration;
courseSandwich = sandwich;
}
}
public class Student
{
private int studentNumber;
private String studentName;
private int studentPhone;
// This is where you keep the course object associated to student
public Course studentCourse;
public Student(int number, String name, int phone, Course course)
{
studentNumber = number;
studentName = name;
studentPhone = phone;
studentCourse = course;
}
}
Example usage would be something like this:
Course course = new Course("ASD", "TITLE", null, "ME", 50, true);
Student student = new Student(1, "JOHN", "5551234", course);
And then, get the course information you need from student via, i.e.:
student.studentCourse.courseTitle;
Since now student.studentCourse will be a course object with all of its properties.
Cheers,
Maybe you do not need to add the course name to student. What I would do is add Students to some datastructure in Course. This is cleaner and reduces the coupling between Course and Student. This would also allow you to have Students being in more than one course. For example:
public class Course extends Student{
private Award courseAward;
private String courseCode;
public String courseTitle;
private Student courseLeader;//change to a student Object
private int courseDuration;
private boolean courseSandwich;
private Set<Student> students;//have course hold a collection of students
/**
* Constructor for objects of class Course
*/
public Course(String code, String title, Award award, Student leader, int duration, boolean sandwich){
courseCode = code;
courseTitle = title;
courseAward = award;
courseLeader = leader;
courseDuration = duration;
courseSandwich = sandwich;
this.students=new HashSet<Student>();
}
public boolean addStudent(Student student){
return students.add(student);
}
public Set<Student> getStudents(){
return students;
}
}
As mentioned, stay away from the "extends" for this. In general, you shouldn't use it unless the "is-a" relationship makes sense.
You should probably provide getters for the methods on the Course class:
public class Course {
...
public String getTitle() {
return title;
}
}
And then if the Student class needs that, it would somehow get a hold of the course (which is up to you in your design), and call the getter:
public class Student {
private Set<Course> courses = new HashSet<Course>();
public void attendCourse(Course course) {
courses.add(course);
}
public void printCourses(PrintStream stream) {
for (Course course : courses) {
stream.println(course.getTitle());
}
}
}
Here below find out the solution of your problem and if you want to check below code on your machine then create a file named Test.java and paste the below codes:
package com;
class Course
{
private Award courseAward;
private String courseCode;
public String courseTitle;
private String courseLeader;
private int courseDuration;
private boolean courseSandwich;
public Course(String code, String title, Award award, String leader, int duration, boolean sandwich)
{
courseAward = award;
courseCode = code;
courseTitle = title;
courseLeader = leader;
courseDuration = duration;
courseSandwich = sandwich;
}
public Award getCourseAward() {
return courseAward;
}
public void setCourseAward(Award courseAward) {
this.courseAward = courseAward;
}
public String getCourseCode() {
return courseCode;
}
public void setCourseCode(String courseCode) {
this.courseCode = courseCode;
}
public String getCourseTitle() {
return courseTitle;
}
public void setCourseTitle(String courseTitle) {
this.courseTitle = courseTitle;
}
public String getCourseLeader() {
return courseLeader;
}
public void setCourseLeader(String courseLeader) {
this.courseLeader = courseLeader;
}
public int getCourseDuration() {
return courseDuration;
}
public void setCourseDuration(int courseDuration) {
this.courseDuration = courseDuration;
}
public boolean isCourseSandwich() {
return courseSandwich;
}
public void setCourseSandwich(boolean courseSandwich) {
this.courseSandwich = courseSandwich;
}
}
class Student
{
private int studentNumber;
private String studentName;
private int studentPhone;
private Course studentCourse;
/**
* Constructor for objects of class Student
*/
public Student(int number, String name, int phone, Course course)
{
studentNumber = number;
studentName = name;
studentPhone = phone;
studentCourse = course;
}
public int getStudentNumber() {
return studentNumber;
}
public void setStudentNumber(int studentNumber) {
this.studentNumber = studentNumber;
}
public String getStudentName() {
return studentName;
}
public void setStudentName(String studentName) {
this.studentName = studentName;
}
public int getStudentPhone() {
return studentPhone;
}
public void setStudentPhone(int studentPhone) {
this.studentPhone = studentPhone;
}
public Course getStudentCourse() {
return studentCourse;
}
public void setStudentCourse(Course studentCourse) {
this.studentCourse = studentCourse;
}
}
class Award{
private long awardId;
private String awardName;
Award(long awardId, String awardName){
this.awardId = awardId;
this.awardName = awardName;
}
public long getAwardId() {
return awardId;
}
public void setAwardId(long awardId) {
this.awardId = awardId;
}
public String getAwardName() {
return awardName;
}
public void setAwardName(String awardName) {
this.awardName = awardName;
}
}
public class Test{
public static void main(String ar[]){
// use your all classes here
}
}