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How to extract field value from another field that is of type Object

I'm new to java reflexion and I'm trying to integrate SQLite with java.
I have 2 objects Person and Department. There is relation OneToMany between them.
As I'm working on save functionality (SQLite) I want to extract field names and its values so I can build full query. I have no problem with extracting names and values of fields that are of primitive type (String, int etc.). I have problem with type of Object (in this case it is Department field in Person object).
I'm able to print object but unable to access its fields (namely pk).
Could you help me please?
METHOD FOR EXTRACTING FIELDS
// method for extracting fields
private StringBuilder getFieldsWithValues(Object entity) throws IllegalAccessException, NoSuchFieldException {
StringBuilder query = new StringBuilder();
for (Field field : entity.getClass().getDeclaredFields()) {
System.out.print((field.getName() + " - "));
field.setAccessible(true);
// TODO: eliminate if statement from for cycle
if (field.isAnnotationPresent(ManyToOne.class)) {
// HERE I want to extract the pk value from Department object
System.out.println(field.get(entity));
} else {
System.out.println(field.get(entity));
}
}
return query;
}
DEPARTMENT OBJECT
import javax.persistence.Entity;
import javax.persistence.Id;
#Entity
public class Department {
#Id
private long pk;
private String name;
private String code;
public Department() {
}
public Department(String name, String code) {
this.name = name;
this.code = code;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getCode() {
return code;
}
public void setCode(String code) {
this.code = code;
}
public String toString() {
return String.format("Department %d: %s (%s)", pk, name, code);
}
}
PERSON OBJECT
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
#Entity
public class Person {
#Id
private long id;
private String surname;
private String name;
private int age;
#ManyToOne
private Department department;
public Person(String surname, String name, int age) {
this.surname = surname;
this.name = name;
this.age = age;
}
public Person() {
}
public String getSurname() {
return surname;
}
public void setSurname(String surname) {
this.surname = surname;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public long getId() {
return id;
}
public Department getDepartment() {
return department;
}
public void setDepartment(Department department) {
this.department = department;
}
#Override
public String toString() {
return String.format("Person %d: %s %s (%d)", id, surname, name, age);
}
}

How to Serialize circular referenced object using JSON-B (Java API for JSON Binding)

I am using JSON-B for output object to json and there is a circular reference in the object (please do not ask me to remove the circular reference), sample code as follows
The Person class contains a list of Property
and the Property class reference back the person which form a circular reference.
In the first print the json can be output, however in the second print statement, stack overflow error due to touch the circular reference of the object, I do not want to use #JsonbTransient to ignore any of them, how can I solve this?
I am expecting the json output as
{"id":1,"name":"Jhon","propertyList":[{"person":1, "propertyName":"Palace"},{"person":1, "propertyName":"Apartment"}]}
Sample Code:
import java.util.ArrayList;
import java.util.List;
import javax.json.bind.Jsonb;
import javax.json.bind.JsonbBuilder;
public class JsonTest {
public static void main(String[] args) throws InterruptedException {
Person person = new Person(1, "Jhon");
Jsonb jsonb = JsonbBuilder.create();
//no error as no property is added
System.out.println("jsonPerson without property: " + jsonb.toJson(person));
Property p1 = new Property();
p1.setPropertyName("Palace");
p1.setPerson(person);
Property p2 = new Property();
p2.setPropertyName("Apartment");
p2.setPerson(person);
person.getPropertyList().add(p1);
person.getPropertyList().add(p2);
/**
* stackoverflow here
*/
System.out.println("jsonPerson with property: " + jsonb.toJson(person));
}
public static class Property {
private Person person;
private String propertyName;
public Person getPerson() {
return person;
}
public void setPerson(Person person) {
this.person = person;
}
public String getPropertyName() {
return propertyName;
}
public void setPropertyName(String propertyName) {
this.propertyName = propertyName;
}
}
public static class Person {
private int id;
public Person() {
super();
}
public Person(int id, String name) {
super();
this.id = id;
this.name = name;
}
private String name;
private List<Property> propertyList = new ArrayList<>();
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Property> getPropertyList() {
return propertyList;
}
public void setPropertyList(List<Property> propertyList) {
this.propertyList = propertyList;
}
}
}

Finally I give up using JSON-B and instead use Jackson, use the annotation #JsonIdentityInfo here is my solution for information:
import java.util.ArrayList;
import java.util.List;
import javax.json.bind.Jsonb;
import javax.json.bind.JsonbBuilder;
import com.fasterxml.jackson.annotation.JsonIdentityInfo;
import com.fasterxml.jackson.annotation.ObjectIdGenerators;
import com.fasterxml.jackson.databind.ObjectMapper;
public class JsonTest {
private static Person person = null;
private static List<Property> propertyList = new ArrayList<>();
public static void main(String[] args) throws Exception {
person = new Person(1, "Jhon");
propertyList.add(new Property(1, person, "Palace"));
propertyList.add(new Property(2, person, "Apartment"));
person.setPropertyList(propertyList);
jacksonTest();
//jsonbTest();
}
private static void jacksonTest()
throws Exception
{
String result = new ObjectMapper().writeValueAsString(person);
System.out.println("result: " + result);
}
private static void jsonbTest()
throws Exception
{
Jsonb jsonb = JsonbBuilder.create();
/**
* stackoverflow here
*/
System.out.println("jsonPerson with property: " + jsonb.toJson(person));
}
public static class Property extends BaseEntity {
private Person person;
private String propertyName;
public Property(int id, Person person, String propertyName) {
super();
setId(id);
this.person = person;
this.propertyName = propertyName;
}
public Person getPerson() {
return person;
}
public void setPerson(Person person) {
this.person = person;
}
public String getPropertyName() {
return propertyName;
}
public void setPropertyName(String propertyName) {
this.propertyName = propertyName;
}
}
public static class Person extends BaseEntity {
public Person() {
super();
}
public Person(int id, String name) {
super();
setId(id);
this.name = name;
}
private String name;
private List<Property> propertyList = new ArrayList<>();
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Property> getPropertyList() {
return propertyList;
}
public void setPropertyList(List<Property> propertyList) {
this.propertyList = propertyList;
}
}
#JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
public static abstract class BaseEntity {
private int id;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
}
}
Jackson output:
result: {"id":1,"name":"Jhon","propertyList":[{"id":1,"person":1,"propertyName":"Palace"},{"id":2,"person":1,"propertyName":"Apartment"}]}

Do child/sub classes have to be in their own separate file in order to test? [duplicate]

This question already has answers here:
Why is each public class in a separate file?
(11 answers)
Closed 3 years ago.
I am using Eclipse to run this code program to test a Person class and its subclasses. In Eclipse it shows there are errors--that each child class must be defined in its own file.
I am learning Java, and would like to know if this is a must? Or can I make it work with parent and child classes all in one file? If I'm missing something, please point me in the right direction. Thank you!
Here is my code: [I put this is all in one file on Eclipse]
import java.util.*;
//Test program to test Person class and its subclasses
public class Test {
public static void main(String[] args) {
Person person = new Person("person");
Student student = new Student ("student");
Employee employee = new Employee("employee");
Faculty faculty = new Faculty("faculty");
Staff staff = new Staff("staff");
//invoke toString() methods
System.out.println(person.toString());
System.out.println(student.toString());
System.out.println(employee.toString());
System.out.println(faculty.toString());
System.out.println(staff.toString());
}
}
//Defining class Person
public class Person {
protected String name;
protected String address;
protected String phoneNum;
protected String email;
public Person(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress () {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public String getPhoneNum() {
return phoneNum;
}
public void setPhoneNum(String phoneNum) {
this.phoneNum = phoneNum;
}
public String getEmail() {
return email;
}
public void setEmail (String email) {
this.email = email;
}
#Override
public String toString() {
return "Name:"+getName()+"Class:"+this.getClass().getName();
}
}
//Defines class Student extends Person
public class Student extends Person {
public static final String FRESHMAN = "freshman";
public static final String SOPHMORE = "sophmore";
public static final String JUNIOR = "junior";
public static final String SENIOR = "senior";
protected String classStatus;
public Student(String name) {
super(name);
}
public Student(String name, String classStatus) {
super(name);
this.classStatus = classStatus;
}
#Override
public String toString() {
return "Name:"+getName()+"Class:"+this.getClass().getName();
}
}
//Defines class Employee extends Person
public class Employee extends Person {
protected double salary;
protected String office;
protected MyDate dateHired;
public Employee(String name) {
this(name, 0, "none", new MyDate());
}
public Employee(String name, double salary, String office, MyDate dateHired) {
super(name);
this.salary = salary;
this.office = office;
this.dateHired - dateHired;
}
public double getSalary() {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
public String getOffice() {
return office;
}
public void setOffice (String office) {
this.office = office;
}
public MyDate getDateHired() {
return dateHired;
}
public void setDateHired(MyDate dateHired) {
this.dateHired = dateHired;
}
#Override
public String toString() {
return "Name:"+getName()+"Class:" + this.getClass().getName();
}
}
//Defines class Faculty extends Employee
public class Faculty extends Employee {
public static String LECTURER = "lecturer";
public static String ASSISTANT_PROFESSOR = "assistant professor";
public static String ASSOCIATE_PROFESSOR + "associate professor";
public static PROFESSOR = "professor";
protected String officeHours;
protected String rank;
public Faculty(String name) {
this(name, "9-5 PM", "Employee");
}
public Faculty(String name, String officeHours, String rank) {
super(name);
this.officeHours = officeHours;
this.rank = rank;
}
public String getOfficeHours() {
return officeHours;
}
public void setOfficeHours(String officeHours) {
this.officeHours = officeHours;
}
public String getRank() {
return rank;
}
public void setRank(String rank) {
this.rank=rank;
}
#Override
public String toString() {
return "Name:"+getName()+"Class:"+this.getClass().getName();
}
}
//Defines class Staff extends Employee
public class Staff extends Employee {
protected String title;
public Staff(String name) {
this(name, "none");
}
public Staff(String name, String title) {
super(name);
this.title=title;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
#Override
public String toString() {
return "Name:"+getName()+"Class:"+this.getClass().getName();
}
}
//Define class MyDate
public class MyDate {
private int month, day, year;
public MyDate (int month, int day, int year) {
this.day=day;
this.month=month;
this.year=year;
}
}

Yes, there should be one class per file. Moreover, you are using the MyDate class in the Employee class, which you need to extend and you cannot extends more than one class, so it's better use the predefined Date class which is present java.util.Date. Import this in the Employee class.
import java.util.Date;
instead of this:
public Employee(String name, double salary, String office, MyDate dateHired)
use:
public Employee(String name, double salary, String office, Date dateHired)
There are some careless mistakes:
in Employee class
public static String ASSOCIATE_PROFESSOR + "associate professor";
change to:
public static String ASSOCIATE_PROFESSOR = "associate professor";
Similarly in faculty class
public static String ASSOCIATE_PROFESSOR + "associate professor";
put = instead of +.
Now this code will work.

Yes it is a must. One class per file. Class can have inner classes. You can define subclasses as inner classes. But I recommend putting them in separate files and don't use inner classes.

Deserialize yaml to list of objects

Say I have a Yaml file like this,
people:
- name : Joe
surname : Barber
age : 16
- name : Andy
surname : Lots
age : 17
And I have a class like this,
public class people {
private String name;
private String surname;
private String age;
<!-- With getters and setters -->
}
How would i go about getting a list of people objects from the Yaml file?
Just getting the value from a key in the file is fairly simple but mapping it to a collection of objects is not.
I am using the snakeYaml lib.

i hope this can help you.
public class StackOverflow {
public static void main(String[] args) {
final URL resource = StackOverflow.class.getResource("people.yaml");
final Constructor peopleContructor = new Constructor(Group.class);
final TypeDescription peopleDescription = new TypeDescription(People.class);
peopleDescription.putMapPropertyType("people", People.class, Object.class);
peopleContructor.addTypeDescription(peopleDescription);
final Yaml yaml = new Yaml(peopleContructor);
try {
final Group group = (Group) yaml.load(resource.openStream());
for (final People people : group.getPeople()) {
System.out.println(people);
}
} catch (IOException e) {
e.printStackTrace();
}
}
public static class People {
private String name;
private String surname;
private int age;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSurname() {
return surname;
}
public void setSurname(String surname) {
this.surname = surname;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
#Override
public String toString() {
return "People: {name: " + this.name + ", surname: " + this.surname + ", age: " + this.age + "}";
}
}
public static class Group {
private List<People> people;
public List<People> getPeople() {
return people;
}
public void setPeople(List<People> people) {
this.people = people;
}
}}

SUPER CSV write bean to CSV

Here is my class,
public class FreebasePeopleResults {
public String intendedSearch;
public String weight;
public Double heightMeters;
public Integer age;
public String type;
public String parents;
public String profession;
public String alias;
public String children;
public String siblings;
public String spouse;
public String degree;
public String institution;
public String wikipediaId;
public String guid;
public String id;
public String gender;
public String name;
public String ethnicity;
public String articleText;
public String dob;
public String getWeight() {
return weight;
}
public void setWeight(String weight) {
this.weight = weight;
}
public Double getHeightMeters() {
return heightMeters;
}
public void setHeightMeters(Double heightMeters) {
this.heightMeters = heightMeters;
}
public String getParents() {
return parents;
}
public void setParents(String parents) {
this.parents = parents;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
public String getProfession() {
return profession;
}
public void setProfession(String profession) {
this.profession = profession;
}
public String getAlias() {
return alias;
}
public void setAlias(String alias) {
this.alias = alias;
}
public String getChildren() {
return children;
}
public void setChildren(String children) {
this.children = children;
}
public String getSpouse() {
return spouse;
}
public void setSpouse(String spouse) {
this.spouse = spouse;
}
public String getDegree() {
return degree;
}
public void setDegree(String degree) {
this.degree = degree;
}
public String getInstitution() {
return institution;
}
public void setInstitution(String institution) {
this.institution = institution;
}
public String getWikipediaId() {
return wikipediaId;
}
public void setWikipediaId(String wikipediaId) {
this.wikipediaId = wikipediaId;
}
public String getGuid() {
return guid;
}
public void setGuid(String guid) {
this.guid = guid;
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getGender() {
return gender;
}
public void setGender(String gender) {
this.gender = gender;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getEthnicity() {
return ethnicity;
}
public void setEthnicity(String ethnicity) {
this.ethnicity = ethnicity;
}
public String getArticleText() {
return articleText;
}
public void setArticleText(String articleText) {
this.articleText = articleText;
}
public String getDob() {
return dob;
}
public void setDob(String dob) {
this.dob = dob;
}
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
public String getSiblings() {
return siblings;
}
public void setSiblings(String siblings) {
this.siblings = siblings;
}
public String getIntendedSearch() {
return intendedSearch;
}
public void setIntendedSearch(String intendedSearch) {
this.intendedSearch = intendedSearch;
}
}
Here is my CSV writer method
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import org.supercsv.io.CsvBeanWriter;
import org.supercsv.prefs.CsvPreference;
public class CSVUtils {
public static void writeCSVFromList(ArrayList<FreebasePeopleResults> people, boolean writeHeader) throws IOException{
//String[] header = new String []{"title","acronym","globalId","interfaceId","developer","description","publisher","genre","subGenre","platform","esrb","reviewScore","releaseDate","price","cheatArticleId"};
FileWriter file = new FileWriter("/brian/brian/Documents/people-freebase.csv", true);
// write the partial data
CsvBeanWriter writer = new CsvBeanWriter(file, CsvPreference.EXCEL_PREFERENCE);
for(FreebasePeopleResults person:people){
writer.write(person);
}
writer.close();
// show output
}
}
I keep getting output errors. Here is the error:
There is no content to write for line 2 context: Line: 2 Column: 0 Raw line:
null
Now, I know it is now totally null, so I am confused.

So it's been a while, and you've probably moved on from this, but...
The issue was actually that you weren't supplying the header to the write() method, i.e. it should be
writer.write(person, header);
Unfortunately the API is a little misleading in it's use of the var-args notation in the signature of the write() method, as it allows null to be passed in. The javadoc clearly states that you shouldn't do this, but there was no null-check in the implementation: hence the exception you were getting.
/**
* Write an object
*
* #param source
* at object (bean instance) whose values to extract
* #param nameMapping
* defines the fields of the class that must be written.
* null values are not allowed
* #since 1.0
*/
public void write(Object source, String... nameMapping) throws IOException,
SuperCSVReflectionException;
Super CSV 2.0.0-beta-1 is out now. It retains the var-args in the write() method, but fails fast if you provide a null, so you know exactly what's wrong when you get a NullPointerException with the following:
the nameMapping array can't be null as it's used to map from fields to
columns
It also includes many bug fixes and new features (including Maven support and a new Dozer extension for mapping nested properties and arrays/Collections).

I don't see where you create ArrayList<FreebasePeopleResults> people, but you might verify that it has more than one element. As an example of coding to the interface, consider using List<FreebasePeopleResults> people as the formal parameter.
Addendum: Have you been able to make this Code example: Write a file with a header work?
Example: Here's a simplified example. I think you just need to specify the nameMapping when you invoke write(). Those names determine what get methods to call via introspection.
Console output:
name,age
Alpha,1
Beta,2
Gamma,3
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;
import org.supercsv.io.CsvBeanWriter;
import org.supercsv.io.ICsvBeanWriter;
import org.supercsv.prefs.CsvPreference;
public class Main {
private static final List<Person> people = new ArrayList<Person>();
public static void main(String[] args) throws IOException {
people.add(new Person("Alpha", 1));
people.add(new Person("Beta", 2));
people.add(new Person("Gamma", 3));
ICsvBeanWriter writer = new CsvBeanWriter(
new PrintWriter(System.out), CsvPreference.STANDARD_PREFERENCE);
try {
final String[] nameMapping = new String[]{"name", "age"};
writer.writeHeader(nameMapping);
for (Person p : people) {
writer.write(p, nameMapping);
}
} finally {
writer.close();
}
}
}
public class Person {
String name;
Integer age;
public Person(String name, Integer age) {
this.name = name;
this.age = age;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}

CellProcessor[] processors = new CellProcessor[] { new Optional(), new NotNull(),
new Optional(), new Optional(), new NotNull(), new Optional()};
CsvBeanWriter writer = new CsvBeanWriter(file, CsvPreference.EXCEL_PREFERENCE)
writer.write(data,properties,processors);