Develop Reference

Java array list returning 0 value

I have created a class like this, which contains a bunch of arraylist as you can see. I've been setting the array with the methods add.. and then retrieving it with get.., when i tried to System.out.println numberofcitizen for example it is returning 0. Note that i have instantiated the class in another class to set the values.
public int numberOfCitizen;
private final ArrayList<Integer> citizenid = new ArrayList<>();
private final ArrayList<String> citizenName = new ArrayList<>();
private final ArrayList<Integer> citizenWaste = new ArrayList<>();
private final ArrayList<Float> longitude = new ArrayList<>();
private final ArrayList<Float> latitude = new ArrayList<>();
private final ArrayList<String> address = new ArrayList<>();
public void working() {
System.out.println("executing fine");
}
public void setnoOfcit(int number) {
this.numberOfCitizen = number;
}
public int getnumber() {
return this.numberOfCitizen;
}
public void addCitizenId(int citizen) {
citizenid.add(citizen);
}
public int getCitizenid(int i) {
int citId = citizenid.get(i);
return citId;
}
public void addCitizenName(String citizenname) {
citizenName.add(citizenname);
}
public String getCitizenName(int i) {
return citizenName.get(i);
}
public void addCitizenWaste(int waste) {
citizenWaste.add(waste);
}
public int getCitizenWaste(int i) {
return citizenWaste.get(i);
}
public void addLatitude(float lat) {
latitude.add(lat);
}
public float getLat(int i) {
return latitude.get(i);
}
public void addlng(float lng) {
longitude.add(lng);
}
public float getlng(int i) {
return longitude.get(i);
}
com.graphhopper.jsprit.core.problem.VehicleRoutingProblem.Builder vrpBuilder = com.graphhopper.jsprit.core.problem.VehicleRoutingProblem.Builder.newInstance();
public void runVPRSolver() {
System.out.println(numberOfCitizen);
System.out.println(getCitizenName(0));
//create a loop to fill parameters

Probable source of problem :
numberOfCitizen is a member attribute that you seem to never change. If you want it to represent the number of elements in your lists, either use citizenName.size() or increment the value of numberOfCitizen in one of the add methods.
Design flaw :
Your design takes for granted that your other class always use that one properly. Anytime you or someone uses that class, he must make sure that he add every single element manually. This adds code that could be grouped inside your class, which would be cleaner and easier to maintain.
So instead of several add method like this :
addCitizenid();
addCitizenName();
addCitizenWaste();
addLongitude();
addLatitude();
addAddress();
Design an other Citizen class which will contain those elements, and use a single list of instances of that class. That way you can use only one method :
private List<Citizen> citizenList = new ArrayList<>();
public void addCitizen(Citizen c) {
/*Add element in your list*/
citizenList.add(c);
}
This programming methodology is called "Encapsulation" which you can read about here

You need to increment numberOfCitizen in your add methods. For example:
public void addCitizenId(int citizen){
citizenid.add(citizen);
numberOfCitizen++;
}
I would also suggest encapsulating your variables into Objects, so create a citizen class:
public class Citizen {
private Integer id;
private Integer name;
private Integer waste;
}
And change your variable to an ArrayList of objects:
ArrayList<Citizen> citizens;

Java - Using an object inside an object

I am working on a project ( I had a problem yesterday and so many people helped me!) so I decided to ask for help again.
My code has 3 classes. ProjectMain,Students,Classroom. I created an array of Classroom objects. Right now I have 3 Classroom objects. But I have to assign student objects to these Classroom objects. For example : classarray[0] is an object from Classroom class and studentobject.get(0) , studentobject.get(1) ... will be students objects inside classarray[0] object. But I have failed on this while coding. Here are my classes :
public class Classroom
{
private String classname;
private String word[] = null;
protected ArrayList<Students> studentobject = new ArrayList<Students>(10);
public String[] getWord()
{
return word;
}
public void setWord(String[] word)
{
this.word = word;
}
public ArrayList<Students> getStudentobject()
{
return studentobject;
}
public void setStudentobject(ArrayList<Students> studentobject)
{
this.studentobject = studentobject;
}
public String getClassname()
{
return classname;
}
public void setClassname(String classname)
{
this.classname = classname;
}
public void classroomreader(String filename)
{
// This method gets the name of Classroom
File text = new File("C:/Users/Lab/Desktop/classlists/" + filename
+ ".txt");
Scanner scan;
try
{
scan = new Scanner(text);
String line = scan.nextLine();
word = line.split("\t");
line = scan.nextLine();
word = line.split("\t");
} catch (FileNotFoundException e1)
{
e1.printStackTrace();
}
}
}
This is my student class :
public class Students extends Classroom
{
private String name,id;
private int age;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public String getId()
{
return id;
}
public void setId(String id)
{
this.id = id;
}
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
And my main class :
public class ProjectMain
{
public static void main(String[] args)
{
Classroom[] classarray = new Classroom[3];
//I got 3 Classroom objects here
classarray[0]=new Classroom();
classarray[1]=new Classroom();
classarray[2]=new Classroom();
classarray[0].classroomreader("class1");
classarray[0].studentobject.get(0).setName(classarray[0].getWord()[1]);
//The problem is in here. When my code comes to the line above,
// at java.util.ArrayList.rangeCheck(Unknown Source) error comes out.
// I tried to get first object in studentobject Arraylist, and tried to set it's name
// to the variable which my text reader reads.
How can I write what I have in my mind?

Your classroomreader method reads the file but don't do much of it... maybe you want to create some instance of Students within it.
scan = new Scanner(text);
String line = scan.nextLine();
word = line.split("\t"); // won't be used
line = scan.nextLine();
word = line.split("\t"); // erased here
There you only have the last line (split) of the file in word attribute.

When creating Classroom instance studentobject list is created empty and it stays that way so you can't access first (or any) object in it.
To populate your list you may add to Classroom method like this:
public void addStudent(Student s)
{
studentobject.add(s);
}

classroom contains the following field declaration
String word[] = null;
the main class, incl the classroomreader does not set a value to this field. Yet you are going to invoke
classarray[0].getWord()[1]
which then must fail.
tip: don't use expressions like this, which can be found in your main class (at least not in early stages of development, or learning)
classarray[0].studentobject.get(0).setName(classarray[0].getWord()[1]);
resolve into variables and several steps. Compilers are smart enough to produce the same code if the context is not disturbed, ie the long expression is resolved into a single block.
Never forget that the purpose of programming languages is to make programs readable for humans. :) Code with abbreviations or "tricks" simply shows some philodoxical attitude (imho)

Java Swing Object[][] getData() confusion

Hi I'm having some trouble getting started with a problem in a Java course learning Swing and starting on JTables and getting data into them. It's going to be hard to explain so I'm just going to post the code I was given, along with the question.
The question is:
The getData() method needs to return an Object[][] containing the data represented by the class.
The first class is MusicAlbum
class MusicAlbum {
private String id;
private String name;
private String genre;
private boolean isCompilation;
private int track_count;
public MusicAlbum(String id, String name, String genre, boolean isCompilation, int track_count) {
this.id = id;
this.name = name;
this.genre = genre;
this.isCompilation = isCompilation;
this.track_count = track_count;
}
public String getId() {
return id;
}
public String getName() {
return name;
}
public String getGenre() {
return genre;
}
public boolean isCompilation() {
return isCompilation;
}
public int getTrackCount() {
return track_count;
}
public boolean equals(Object obj) {
if (obj instanceof MusicAlbum)
return this.id.equalsIgnoreCase(((MusicAlbum)obj).id);
return super.equals(obj);
}
}
The class I have to implement the methods in is MusicDataObject (at the bottom)
import java.util.Random;
import java.util.Scanner;
public class MusicDataObject {
private List<MusicAlbum> albums = new ArrayList<>();
private Random random = new Random(); // for generating IDs
public void addAlbum(MusicAlbum album) throws IllegalArgumentException {
if (searchAlbum(album.getId()) != null)
throw new IllegalArgumentException("Album ID is not new!");
albums.add(album);
}
public MusicAlbum searchAlbum(String id) {
for (MusicAlbum album : albums) {
if (album.getId().equalsIgnoreCase(id)) {
return album;
}
}
return null;
}
public MusicAlbum removeAlbum(String id) {
MusicAlbum album = searchAlbum(id);
albums.remove(album);
return album;
}
public void updateAlbum(MusicAlbum album)
throws IllegalArgumentException {
if (removeAlbum(album.getId()) == null)
throw new IllegalArgumentException("Album ID does not exist!");
addAlbum(album);
}
public String generateID() {
String formatter = "A%0" + (int)Math.ceil(Math.log10(albums.size() * 2) + 1) + "d";
String ID;
do {
ID = String.format(formatter, random.nextInt(albums.size() * 2 + 1));
} while (searchAlbum(ID) != null);
return ID;
}
public void saveData(String fileName) throws IOException {
// make sure that the file exists or try to create it
File fout = new File(fileName);
if (!fout.exists() && !fout.createNewFile())
return;
PrintWriter out = new PrintWriter(fout);
for (MusicAlbum album: albums) {
out.println(serializeAlbum(album));
}
out.close();
}
public String serializeAlbum(MusicAlbum album) {
return String.format(
"%s;%s;%s;%b;%d",
album.getId(),
album.getName(),
album.getGenre(),
album.isCompilation(),
album.getTrackCount());
}
public void loadFile(String fileName) throws FileNotFoundException {
albums = new ArrayList<>();
Scanner in = new Scanner(new File(fileName));
while (in.hasNext()) {
// --- split the next line with the character ";"
String line = in.nextLine();
String[] tokens = line.split(";");
// --- construct a new MusicAlbum using the resulting tokens. NOTE: This isn't very robust.
// If a line doesn't contain enough data or the data is invalid, this will crash
albums.add(new MusicAlbum(
tokens[0],
tokens[1],
tokens[2],
Boolean.parseBoolean(tokens[3]),
Integer.parseInt(tokens[4])
));
}
}
// ----- these methods need to be implemented
public Object[][] getData() {
// TODO
}
public String[] getColumnNames() {
// TODO
}
}
The sample data being used is in a txt file, formatted as so:
A01;Defiance;Soundtrack;true;24
A02;Insomniac;Punk Rock;false;14
A03;A Great Day For The Race;Gypsy Jazz;false;10
A04;Viva La Internet;Ska;false;31
A05;New Surrender;Rock;false;17
So basically it's this getData() method they want me to implement that is giving me grief. I don't fully understand what they want me to do, nor do I fully understand what the Object[][] does.
I hope I have been clear enough, and I will appreciate all help given. Also please try to explain things as best you can and dumb them down as much as possible, I'm new to a lot of this :)
Thanks for your time.

Object[][] is a 2-dimensional array. Each of its element is an Object[], a one-dimensional array.
Your task is to create a 2 dimensional array, having one element (Object[]) for each of your MusicAlbum. An Object[] should hold the properties of a MusicAlbum like id, name, genre, isCompilation and track_count.
You can create an object array like this:
Object[] arr = new Object[] { "some", "values", 23, true };
You can create a 2 dimensional array like this:
Object[][] arr2d = new Object[size][];
And you can iterate over all your MusicAlbums, create an Object[] for each of them containing the properties of that music album, and set it in the arr2d.
You can set/get elements of a 2-dimensional array just like any other arrays:
// Set first element:
arr2d[0] = arr;
// Get first element:
Object[] firstElement = arr2d[0];
The getColumnNames() method should just return a String[] (a String array) containing the column names, the names of the properties.
And it might be obvious but note that the order you return the column names and the order of the property values (in the elements of the Object[]) should be the same.

Update a single variable of a class in an ArrayList of class in java

I have a class Components:
public class Components {
int numberOfNets;
String nameOfComp;
String nameOfCompPart;
int numOfPin;
public components(int i, String compName, String partName, int pin) {
this.numberOfNets = i;
this.nameOfComp = compName;
this.nameOfCompPart = partName;
this.numOfPin = pin;
}
}
Inside another class I created an arraylist of Components class:
List<Components> compList = new ArrayList<Components>();
Later in the code, I am adding the elements in List in this way:
compList.add(new Components(0,compName,partName,0));
See, here numberOfNets and numOfPin variables in Components class are initiated with 0 values. But these values are getting calculated/incremented in a later part of code and hence I need to update the new values of only these two variables in each list element. Now from ArrayList doc I get the idea of updating a list element using its index by set operation. But I am confused how to set/update a particular variable of a class in an ArrayList of a class. I need to update only these two mentioned variables, not all of the four variables in Components class. Is there any way to do that?

You should add getter/setter to your component class so that outer class can update component's members
public class Components {
private int numberOfNets;
private String nameOfComp;
private String nameOfCompPart;
private int numOfPin;
public components(int i, String compName, String partName, int pin) {
setNumberOfNets(i);
setNameOfComp(compName);
setNameOfCompPart(partName);
setNumOfPin(pin);
}
public void setNumberOfNets(int numberOfNets) {
this.numberOfNets = numberOfNets;
}
// Similarly other getter and setters
}
You can now modify any data by using following code because get() will return reference to original object so modifying this object will update in ArrayList
compList.get(0).setNumberOfNets(newNumberOfNets);

Example code.
public class Main {
public static void main(String[] args) {
List<Components> compList = new ArrayList<Components>();
compList.add(new Components(0, "compName", "partName", 0));
System.out.println(compList.get(0).toString());
compList.get(0).numberOfNets = 3;
compList.get(0).numOfPin = 3;
System.out.println(compList.get(0).toString());
}
}
Your class.
public class Components {
int numberOfNets;
String nameOfComp;
String nameOfCompPart;
int numOfPin;
public Components(int i, String compName, String partName, int pin) {
this.numberOfNets = i;
this.nameOfComp = compName;
this.nameOfCompPart = partName;
this.numOfPin = pin;
}
public String toString() {
return this.numberOfNets + " " + nameOfComp + " " + nameOfCompPart
+ " " + numOfPin;
}
}
The output:
0 compName partName 0
3 compName partName 3

Problems when creating methods in my task

The goal is to produce this:
Picture of the task summary here
These are the errors I get when I try to compile:
screen shot
I have changed and fixed most of the more obvious errors I think which was mainly just stupid of me. Sorry.
I have this code
public class Ex5Program {
public void start() {
Tutor[] tutors = createTutorsArray();
printTutors(tutors);
printOnLeaveList(tutors);
updateTutorDetails(tutors[1]);
printNewTutorDetails(tutors[1]);
Tutor tutorWithMostPapers = getTutorWithMostPapers(tutors);
printTutorWithMostPapers(tutorWithMostPapers);
}
private Tutor[] createTutorsArray() {
String[] noPapers = {};
String[] introductoryPapers = {"CompSci101", "CompSci111"};
String[] coreStage1Papers = {"CompSci101", "CompSci105"};
String[] allStageOnePapers = {"CompSci111", "CompSci101", "CompSci105"};
String[] stageTwoPapers = {"CompSci210", "CompSci220", "CompSci225", "CompSci230"};
Tutor[] tutors = new Tutor[7];
tutors[5] = new Tutor("Sad Sack", 86302, introductoryPapers, false);
tutors[4] = new Tutor("Crystal Ball", 49123, introductoryPapers, false);
tutors[2] = new Tutor("Earl Lee Riser", 40879, allStageOnePapers, true);
tutors[3] = new Tutor("Tom Katt", 50876, stageTwoPapers, false);
tutors[1] = new Tutor("Candy Kane", 30869, noPapers, false);
tutors[0] = new Tutor("Carrie Oakey", 30987, coreStage1Papers, true);
tutors[6] = new Tutor("Sonny Day", 49586, stageTwoPapers, true);
return tutors;
}
private void printTutors(Tutor[] tutors) {
System.out.println("Current Tutors");
System.out.println("==============");
for (int i = 0; i < tutors.length; i++) {
System.out.print(i + 1 + ". ");
System.out.println(tutors[i].toString());
}
}
private void printOnLeaveList(Tutor[] tutors) {
System.out.println();
System.out.println("Tutors Currently on Leave");
System.out.println("=========================");
for (int i = 0; i < tutors.length; i++) {
if (tutors[i].isOnLeave()) {
System.out.println(tutors[i].getName());
}
}
}
private void updateTutorDetails(Tutor tutor) {
tutor.setName("Ali Katt");
tutor.setStaffId(23456);
String[] stage1Papers = {"CompSci101", "CompSci105", "CompSci111"};
tutor.setPapers(stage1Papers);
tutor.setOnLeave(true);
}
private void printNewTutorDetails(Tutor tutor) {
System.out.println();
System.out.println("Updated details");
System.out.println("===============");
System.out.println("Name: " + tutor.getName());
System.out.println("Id: " + tutor.getStaffId());
String[] papers = tutor.getPapers();
System.out.print("Papers: ");
if (papers.length > 0) {
for (int i = 0; i < papers.length; i++) {
System.out.print(papers[i] + " ");
}
} else {
System.out.print("None");
}
System.out.println();
if (tutor.isOnLeave()) {
System.out.println("Currently on leave");
}
}
private Tutor getTutorWithMostPapers(Tutor[] tutors) {
Tutor tutorWithMostPapersSoFar = tutors[0];
for (int i = 0; i < tutors.length; i++) {
if (tutors[i].teachesMorePapersThan(tutorWithMostPapersSoFar)) {
tutorWithMostPapersSoFar = tutors[i];
}
}
return tutorWithMostPapersSoFar;
}
private void printTutorWithMostPapers(Tutor tutorWithMostPapers) {
System.out.println();
System.out.println("Most papers");
System.out.println("===========");
System.out.println(tutorWithMostPapers.getName() + " teaches more papers than any other tutor.");
}
}
and I created this code here(It has been changed):
public class Tutor {
// instance variables
private String name;
private int staffId;
private String[] papers;
private boolean onLeave;
public Tutor(String name, int staffId, String[] papers, boolean onLeave) {
// Complete this constructor method
this.name = name;
this.staffId = staffId;
this.papers = papers;
this.onLeave = onLeave;
}
// Insert getName() method here
public String getName(){
return name;
}
// Insert setName() method here
public void setName(String name){
this.name = name;
}
// Insert getStaffId() method here
public int getStaff(){
return staffId;
}
// Insert setStaffId() method here
public void setStaffId(int StaffId){
this.staffId = staffId;
}
// Insert getPapers() method here;
public String[] getPapers(){
return papers;
}
// Insert setPapers() method here
public void setPapers(String[] papers){
this.papers = papers;
}
// Insert isOnLeave() method here
public boolean isOnLeave(){
return onLeave;
}
// Insert setOnLeave() method here
public void setOnLeave(boolean OnLeave){
this.onLeave = onLeave;
}
// Insert toString() method here
public String toString(){
return name + "(Staff id:"+staffId+")";
}
// Insert teachesMorePapersThan() method here
public Tutor teachesMorePapersThan(Tutor other){
return(papers.length>other.papers.length);
}
}

Typo: toString() not tostring(), which results in Object.toString() is being invoked and the intended formatted string is not being returned. Change to:
#Override public String toString()
Using the #Override annotation would have produced a compiler error in the case of tostring() being the method name and alerted you to the error, because no method of that name exists in a superclass.
Several of the setter methods have missing parameters:
// Insert setPapers() method here
public void setPapers(){
this.papers = papers;
}
// Insert setOnLeave() method here
public void setOnLeave(){
this.OnLeave = OnLeave;
}

First error: setStaffID()
You are calling it using an int, but on your method you say it doesn't have any parameter.
Take a look that you have some others errors caused by the same mistake. Correct them first...

You need to look at the error text to find the problems. While a newbie may instinctively just dismiss the error messages as uselesss (as a result of years of clicking the x or cancel or whatever on windows dialogues), The error text is actually the most useful resource for figuring out what the error is, 90% of the time.
For instance, the first error reads
File: F:\course related stuff\101\Lab06\Ex5\Ex5Program.java [line: 54]
Error: method setStaffId in class Tutor cannot be applied to given types;
required: no arguments
found: int
reason: actual and formal argument lists differ in length
If you read it carefully, you can see it tells you the name of the file, the line number, the method call name, the class name containing the method, and some additional information about the exact type of error. It is even telling you what you did wrong in calling the method, by putting an "int" where "no arguments" were required, that the "actual and formal argument lists differ in length".
Read the other error messages, and you will see that they actually tell you what the problem is.
This code also needs newlines inserted to group blocks of stuff, comments added to explain exactly how it works, and a few java style violations fixed - some teachers grade for style and clarity as well as just functionality.
Also, if the reason you are failing your class is because you don't understand how to program, it may be because of excessive use of stack overflow to solve the problems. In the real world, if you can just use somebody else's code, that's great, but the point of a programming class is is to teach you how to come up with your own code, not how to use somebody else's.

Well it's not easy to help, because i think you don't know what you are doing. But first thing when you create a set method like this:
public void setPapers(){
this.papers = papers;
}
you should declare the arguments like this:
public void setPapers(String[] papers){
this.papers = papers;
}
and you should know that variables names is caseSensitive so :
private boolean onLeave;
public boolean isOnLeave(){
//return OnLeave; this variable is not declared
return onLeave;
}
I think you need to study a little more, because you can't read the compilation errors.