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Java FX observableList. Observation scheme [closed]

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FXCollections.observableList
Documentation said:
Note that mutation operations made directly to the underlying list are
not reported to observers of any ObservableList that wraps it.
Code for example:
public class Test {
public static void tst() {
ArrayList<Person> arrayList = new ArrayList<>();
ObservableList<Person> people = FXCollections.observableList(arrayList);
Person person = new Person("Tom");
arrayList.add(person);
person.setName("Mary");
// How update people after that
}
public static class Person {
private String name;
public Person(String name) {
this.name = name;
}
public void setName(String name) {
this.name = name;
}
}
}
What are some ways to observe the change or addition of SIMPLE objects?

How to create a private array and access it by the main method in java? [closed]

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I want to create a class outside the public class but in the same file, and in that class I wanna create a private array which can only be accessed by creating object through the previous public class. I also wanna store data to that array through the public class.

I suggest you to start learning java, this will save you from asking this kind of question next time. For the moment you can find how to achieve what you are asking for in the example bellow :
public class Learning {
public static void main(String[] args) {
Course course = new Course();
List<String> materials = new ArrayList<>();
materials.add("java basic courses");
materials.add("OOP courses");
// here we use setters to set course materials and notes
course.setMaterials(materials);
course.setNotes(new int[] {19,20});
System.out.println("Display course materials");
for (String material : course.getMaterials()) {
System.out.println("Material : " + material);
}
System.out.println("Display Notes");
for (int note : course.getNotes()) {
System.out.println("Note : " + note);
}
}
}
class Course {
private List<String> materials;
private int[] notes;
public List<String> getMaterials() {
return materials;
}
public void setMaterials(List<String> materials) {
this.materials = materials;
}
public int[] getNotes() {
return notes;
}
public void setNotes(int[] notes) {
this.notes = notes;
}
}

Creating a class that has integers, getter/setters, with input validation [closed]

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I am working on this Java program where I am supposed to write a class called kumquat that has an integer age with getter and setters. Additionally, I need to do input validation. I can't figure out what I'm doing wrong. I'm sorry if this is really simple I'm just still really new to this.
public class Person
{
private int age;
public int getAge()
{
return age;
}
public void setAge(int newAge)
{
this.age = newAge;
}
}
and then my main
public class Kumquat
{
public static void main(int[] args)
{
Person myObj = new Person();
myObj.setAge("5");
System.out.println(myObj.getAge());
}
}

Everything is ok in the Person class. Although in your main the method Person::setAge receives an int as parameter and you're trying to pass a String in line myObj.setAge("5");. Try passing an int like myObj.setAge(5); instead.

you should write String instead of int and setAge should not be string
public static void main(String[] args)
{
Person myObj = new Person();
myObj.setAge(5);
System.out.println(myObj.getAge());
}

Comparator in Java [closed]

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I'm trying to solve my homework. This is the code I use. Comparator for sorting a student list in 2 ways.
public class Student {
public static final Comparator<Student> BY_NAME = new ByName();
public static final Comparator<Student> BY_Gpa = new ByGpa();
private static class ByName implements Comparator<Student> {
public int compare(Student v, Student w) {
return v.getName().compareTo(w.getName());
}
}
private static class ByGpa implements Comparator<Student> {
public int compare(Student v, Student w) {
if (v.getGpa() == w.getGpa()) return 0;
else if (v.getGpa() < w.getGpa()) return -1;
else return 1;
}
}
}
I don't understand Collection.sort. Why does it have to use a complicated form like that? Why not just a static function in the class like C++ for sorting, instead of returning a class that implements Comparator which has a method compare? It's too complex.

Java 8's enhancements to the Comparator interface make it a lot more elegant:
public static final Comparator<Student> BY_NAME = Comparator.comparing(Student::getName);
public static final Comparator<Student> BY_Gpa = Comparator.comparingInt(Student::getGpa);

How to make this method use ArrayList? [closed]

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class Course {
private String courseName;
ArrayList<String> students = new ArrayList<>();
private int numberOfStudents;
public Course(String courseName) {
this.courseName = courseName;
}
public void addStudent(String student) {
students[numberOfStudents] = student;//<-- Line 15
numberOfStudents++;
}
public ArrayList getStudents() {
return students;
}
public int getNumberOfStudents() {
return numberOfStudents;
}
public String getCourseName() {
return courseName;
}
}
Line 15 I am getting error "Array required, but ArrayList found.
I am unsure what to do here as I am new to strings and such.

students is declared as an ArrayList. This notation
students[numberOfStudents] = student;
only works for array types. You should use
students.add(student);
Please read the javadoc for ArrayList.
You also don't need to keep a field to hold the number of students, as
students.size();
will give you that.

If you want to use ArrrayList in your program:
public void addStudent(String student) {
students.add(student);
}
public int getNumberOfStudents() {
return students.length();
}
Also then you do not require numberOfStudents variable