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Java - How to check if two objects, from the same abstract class, are equal

I'm writing a program which contains an abstract class of 'Book', and I have two classes ('LearnBook' and 'ReadingBook') which inherit from 'Book'.
Book:
Public abstract class Book {
protected String name;
protected String author;
LearningBook:
public class LearningBook extends Book {
private String subject;
ReadingBook:
public class ReadingBook extends Book {
private int numberOfPages;
At the main class I have Book array which can include any instance of Book.
I want to add a method which checks if two Book objects are exactly the same, to prevent duplicating in the Book array. it looks like this:
public boolean sameBookCheck(Book book1, Book book2)
So my first idea was to write an isEqual() method in the Book class, which checks if the "name" and the "author" are equals.
But then I need to check if it's a learning book or reading book so I could know if I need to compare the "subject" value or the "numberOfPage" value.
I have no idea how to do it and I'd appreciate your help.

You can use the following design:
In Book abstract class have an equals() function and check whether the other object is of type Book and have same values in all fields.
In LearningBook and ReadingBook have equals() function which first checks whether the other object is of same class, then call Book's equals() function, checking the fields of abstract class, and then check whether field(s) of current class have same values or not.
Have a look at the code:
abstract class Book {
protected String name;
protected String author;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAuthor() {
return author;
}
public void setAuthor(String author) {
this.author = author;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((author == null) ? 0 : author.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (!(obj instanceof Book))
return false;
Book other = (Book) obj;
if (author == null) {
if (other.author != null)
return false;
} else if (!author.equals(other.author))
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}
class LearningBook extends Book{
private String subject;
public String getSubject() {
return subject;
}
public void setSubject(String subject) {
this.subject = subject;
}
#Override
public int hashCode() {
final int prime = 31;
int result = super.hashCode();
result = prime * result + ((subject == null) ? 0 : subject.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (!super.equals(obj))
return false;
if (!(obj instanceof LearningBook))
return false;
LearningBook other = (LearningBook) obj;
if (subject == null) {
if (other.subject != null)
return false;
} else if (!subject.equals(other.subject))
return false;
return true;
}
}
class ReadingBook extends Book{
private int numberOfPages;
public int getNumberOfPages() {
return numberOfPages;
}
public void setNumberOfPages(int numberOfPages) {
this.numberOfPages = numberOfPages;
}
#Override
public int hashCode() {
final int prime = 31;
int result = super.hashCode();
result = prime * result + numberOfPages;
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (!super.equals(obj))
return false;
if (!(obj instanceof ReadingBook))
return false;
ReadingBook other = (ReadingBook) obj;
if (numberOfPages != other.numberOfPages)
return false;
return true;
}
}
public class Runner {
public static void main(String[] args) {
Book learningBook = new LearningBook();
learningBook.setAuthor("auth");
learningBook.setName("sci");
Book learningBook2 = new LearningBook();
learningBook2.setAuthor("auth");
learningBook2.setName("sci");
Book readingBook = new ReadingBook();
readingBook.setAuthor("auth");
readingBook.setName("sci");
//returns false
System.out.println(learningBook.equals(readingBook) );
//returns true
System.out.println(learningBook.equals(learningBook2) );
}
}

Write an equals-implementation for each of the three classes. Every implementation is only responsible for its own fields.
The equals-implementations from the sub-classes ReadingBook and LearningBook should somewhere call super.equals() - the equals-implementation of Book.

You can ask the book instance for its class and check class equality.
book1.getClass().equals(book2.getClass())

You can use instanceof method to compare the type of the Object. To check if it is a type of LearningBook or ReadingBook example
Answer for your comment,
Lets say when you check the two instance it says they are different, then there is no issue you can return false. But if the instances are also same then you can check it with something like this after that
if (both instances are same) {
if (yourObjectIs instanceof LearningBook) {
you can check the two values of LearningBook here and return true if the are equals
} else {
you can check the two values of ReadingBook here and return true if the are equals
}
}

As it was mentioned you should overwrite equals(Object object) method. In your example you can do it like this:
public abstract class Book{
#NonNull protected String name;
#NonNull protected String author;
public Book(String name, String author) {
this.name = name;
this.author = author;
}
#Override
public boolean equals(Object object) {
if (object instanceof Book) {
var book = (Book) object;
return this.name.equals(book.name) && this.author.equals(book.author);
} else
return false;
}
}
public class LearningBook extends Book{
#NonNull private String subject;
public LearningBook(String name, String author,String subject) {
super(name, author);
this.subject = subject;
}
#Override
public boolean equals(Object object) {
if (object instanceof LearningBook) {
var book = (LearningBook) object;
return this.subject.equals(book.subject) && super.equals(book);
} else
return false;
}
}
public class ReadingBook extends Book{
#NonNull private int numberOfPages;
public ReadingBook(String name, String author,int numberOfPages) {
super(name, author);
this.numberOfPages = numberOfPages;
}
#Override
public boolean equals(Object object) {
if (object instanceof ReadingBook) {
var book = (ReadingBook) object;
return super.equals(book) && this.numberOfPages == book.numberOfPages;
} else
return false;
}
}
I've used #NonNull annotation to avoid NPE in equals method.

How to group values in list based on id in java?

Hello folks this may be dumb question but as a beginner am struggling with this how to group values based on id in list, Now let me clarify you briefly am having set of objects like this :
ID:1,UserID:330
ID:2,UserID:303
ID:3,UserID:090
ID:1,UserID:302
ID:2,UserID:306
How my list should look like is(Json Format):
[{"ID":1,"UserID":[330,302]},{"ID":2,"UserID":[303,306]},{"ID":3,"UserID":[090]}]
Now let me post what i have tried so far:
final List<Integer>list=new ArrayList<>();
final List<SpareReturnModel>lisobj=new ArrayList<>();
int duplicate=0;
for(int i=0;i<tView.getSelected().size();i++){
Object o= tView.getSelected().get(i).getValue();
SpareReturnModel asset=(SpareReturnModel) o;
int flag=asset.getFlag();
if(flag==2) {
int warehouseid = asset.getWareHouseID();
asset.setWareHouseID(warehouseid);
int partid = asset.getSerialNoID();
list.add(partid);
}
else {
Log.d("s","no value for header");
}
if(duplicate!=asset.getWareHouseID()){
asset.setParlist(list);
asset.setWareHouseID(asset.getWareHouseID());
lisobj.add(asset);
list.clear();
}
duplicate=asset.getWareHouseID();
}
Gson gson=new Gson();
//this will convert list to json
String value=gson.toJson(listobj);
SpareReturn Model Class:
public class SpareReturnModel {
private Integer SerialNoID;
private String SerialNumber;
private List<Integer>parlist;
public List<Integer> getParlist() {
return parlist;
}
public void setParlist(List<Integer> parlist) {
this.parlist = parlist;
}
public Integer getFlag() {
return flag;
}
public void setFlag(Integer flag) {
this.flag = flag;
}
private Integer flag;
public Integer getWareHouseID() {
return WareHouseID;
}
public void setWareHouseID(Integer wareHouseID) {
WareHouseID = wareHouseID;
}
private Integer WareHouseID;
public Integer getSerialNoID() {
return SerialNoID;
}
public void setSerialNoID(Integer serialNoID) {
SerialNoID = serialNoID;
}
public String getSerialNumber() {
return SerialNumber;
}
public void setSerialNumber(String serialNumber) {
SerialNumber = serialNumber;
}
}
Can someone let me know how to achieve this am struggling with this.

I simplify your class to make solution clearer:
public class SpareReturnModel implements Comparable<SpareReturnModel> {
private Integer id;
private String userId;
public SpareReturnModel(Integer id, String userId) {
this.id = id;
this.userId = userId;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getUserId() {
return userId;
}
public void setUserId(String userId) {
this.userId = userId;
}
#Override
public int compareTo(SpareReturnModel other) {
return this.getId().compareTo(other.getId());
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
SpareReturnModel model = (SpareReturnModel) o;
if (id != null ? !id.equals(model.id) : model.id != null) return false;
return userId != null ? userId.equals(model.userId) : model.userId == null;
}
#Override
public int hashCode() {
int result = id != null ? id.hashCode() : 0;
result = 31 * result + (userId != null ? userId.hashCode() : 0);
return result;
}
}
and add JsonSpareReturnModel
public class JsonSpareRuturnModel implements Comparable<JsonSpareRuturnModel> {
private final List<SpareReturnModel> modelList;
private final Integer id;
public JsonSpareRuturnModel(List<SpareReturnModel> modelList) {
this.modelList = modelList;
this.id = modelList.get(0).getId();
}
private final String toJson() {
return String.format("{\"ID\":%s,\"UserID\":%s}", id, formatUserIdList());
}
private String formatUserIdList() {
StringBuilder builder = new StringBuilder("[");
Iterator<SpareReturnModel> modelIterator = modelList.iterator();
while (modelIterator.hasNext()) {
builder.append(modelIterator.next().getUserId());
if (modelIterator.hasNext()) {
builder.append(",");
}
}
builder.append("]");
return builder.toString();
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
JsonSpareRuturnModel that = (JsonSpareRuturnModel) o;
return id != null ? id.equals(that.id) : that.id == null;
}
#Override
public int hashCode() {
return id != null ? id.hashCode() : 0;
}
#Override
public int compareTo(JsonSpareRuturnModel other) {
return this.id.compareTo(other.id);
}
#Override
public String toString() {
return toJson();
}
if you need to group by user id you need to sort your models according to id's
and place them to json format model:
public class Main {
public static void main(String[] args) {
List<SpareReturnModel> models = new ArrayList<>(Arrays.asList(
new SpareReturnModel(1, "330"),
new SpareReturnModel(2, "303"),
new SpareReturnModel(3, "090"),
new SpareReturnModel(1, "302"),
new SpareReturnModel(2, "306")
));
Map<Integer, List<SpareReturnModel>> groupById = new HashMap<>();
for (SpareReturnModel model : models) {
List<SpareReturnModel> listById = groupById.get(model.getId());
if (listById == null) {
groupById.put(model.getId(), new ArrayList<>(Arrays.asList(model)));
} else {
listById.add(model);
}
}
List<JsonSpareRuturnModel> jsonList = new ArrayList<>();
for (Map.Entry<Integer, List<SpareReturnModel>> pair : groupById.entrySet()) {
jsonList.add(new JsonSpareRuturnModel(pair.getValue()));
}
System.out.println(jsonList);
final String expected = "[{\"ID\":1,\"UserID\":[330,302]}, {\"ID\":2,\"UserID\":[303,306]}, {\"ID\":3,\"UserID\":[090]}]";
System.out.println(jsonList.toString().equals(expected));
}
}

Find if an object is already present in a HashMap

I am trying to create a HashMap, that adds objects to a line, if they are not already present in this line. This is how I check it:
if (!waiting.containsKey(p)) {
waiting.put(current, p);
current++;
}
Where p is our object, which is stored with an Integer. However, when I run this code. It will store the same object several times under different integers, how can this be prevented?

thats because you call containsKey with the object and not the key:
parameter must be an Integer key
Integer lKey = 0;
if(!waiting.containsKey(lKey)){
waiting.put(current, p);
current++;
}
if your object has an identifier use this identifier for the map.
if(!waiting.containsKey(p.getId())){
waiting.put(p.getId(), p);
current++;
}
otherwise use containsValue():
if(!waiting.containsValue(p)){
waiting.put(current, p);
current++;
}
but then you have to overwrite the equals method.

If you want to use an object as a key, you can override the equals() and hashCode() methods to return and compare the id of the object.
Driver.java
import java.util.HashMap;
import java.util.Map;
public class Driver {
public static void main(String[] args) {
Map<MyObject, Integer> map = new HashMap<MyObject, Integer>();
map.put(new MyObject(1000L, "One"), 1);
map.put(new MyObject(1001L, "Two"), 2);
map.put(new MyObject(1002L, "Three"), 3);
Long id = 1001L;
System.out.println(contains(map, id)); // true
System.out.println(get(map, id)); // 2
}
public static <T, U> boolean contains(Map<T, U> map, T obj) {
return map.containsKey(obj);
}
public static boolean contains(Map<MyObject, Integer> map, Long id) {
return contains(map, new MyObject(id, ""));
}
public static <T, U> U get(Map<T, U> map, T obj) {
return map.get(obj);
}
public static Integer get(Map<MyObject, Integer> map, Long id) {
return get(map, new MyObject(id, ""));
}
}
MyObject.java
public class MyObject {
private Long id;
private String name;
protected Long getId() {
return id;
}
protected void setId(Long id) {
this.id = id;
}
protected String getName() {
return name;
}
protected void setName(String name) {
this.name = name;
}
public MyObject(Long id, String name) {
this.id = id;
this.name = name;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((id == null) ? 0 : id.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj) return true;
if (obj == null) return false;
if (getClass() != obj.getClass()) return false;
MyObject other = (MyObject) obj;
if (id == null) {
if (other.id != null) return false;
} else if (!id.equals(other.id)) return false;
return true;
}
#Override
public String toString() {
return "MyObject { id : " + id + ", name : " + name + "}";
}
}

Can't Marshal java.lang.String

Here is my dilemma:
I have a dto class for marshaling back and forth from/to XML.
Here is the trick: Because of the number of dto classes our project deals with that are collections with a plural outter tag, I decided to create a delegate collection that allows me to take one of these classes and effortlessly turn them into a Collection and get the convenience that comes with it (iteration, add, etc.).
In our project we have marshaling tests to flush out annotation errors and such.
Below is my trouble code.
Problem:
Depending on the marshaler, if I extend this QuickCollection I get the below error.
When the object is unmarshaled to xml using CXF as a response to a webservice request, it fails. Exact error:
com.sun.istack.SAXException2: unable to marshal type "java.lang.String" as an element because it is missing an #XmlRootElement annotation
When it's marshaled/unmarshaled with JAXB in test it's fine.
When This same QuickCollection is used to marshal in results from 3rd parties using spring RestOperations and works fine
the mind screw:
When I remove the inheritance and manage the collection as a private member it all just works!
This makes not a stitch of sense to me as I am literally returning the exact data type in both situations.
Below is all relevant code.
This is the Inherited delegate class.
public class QuickCollection<T> implements Collection<T> {
// to be set if needed after instantiation. To behave like a normal collection, we set it to something safe
protected Collection<T> delegate = Collections.emptySet();
public QuickCollection() {
}
public QuickCollection(Collection<T> delegate) {
this.delegate = delegate;
}
#Override
public int size() {
return delegate.size();
}
#Override
public boolean isEmpty() {
return delegate.isEmpty();
}
#Override
public boolean contains(Object o) {
return delegate.contains(o);
}
#Override
public Iterator<T> iterator() {
return delegate.iterator();
}
#Override
public Object[] toArray() {
return delegate.toArray();
}
#Override
public <T> T[] toArray(T[] a) {
return delegate.toArray(a);
}
#Override
public boolean add(T t) {
return delegate.add(t);
}
#Override
public boolean remove(Object o) {
return delegate.remove(o);
}
#Override
public boolean containsAll(Collection<?> c) {
return delegate.containsAll(c);
}
#Override
public boolean addAll(Collection<? extends T> c) {
return delegate.addAll(c);
}
#Override
public boolean removeAll(Collection<?> c) {
return delegate.removeAll(c);
}
#Override
public boolean retainAll(Collection<?> c) {
return delegate.retainAll(c);
}
#Override
public void clear() {
delegate.clear();
}
#Override
public String toString() {
return "" + delegate.toString();
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
QuickCollection that = (QuickCollection) o;
if (delegate != null ? !delegate.equals(that.delegate) : that.delegate != null) return false;
return true;
}
#Override
public int hashCode() {
return delegate != null ? delegate.hashCode() : 0;
}
}
Here is the child DTO class
#XmlAccessorType(XmlAccessType.PROPERTY)
#XmlType(name = "BuddyCodes")
#XmlRootElement(name = "BuddyCodes")
public class BuddyCodes extends QuickCollection<String> implements Xml {
private Long accountId;
private Date expirationDate;
public BuddyCodes() {
super.delegate = new HashSet<String>();
}
public BuddyCodes(Long accountId, Set<String> codes, Date expirationDate) {
super(codes);
this.accountId = accountId;
this.expirationDate = expirationDate;
super.delegate = new HashSet<String>();
}
public BuddyCodes(Long accountId, Date expirationDate) {
this.accountId = accountId;
this.expirationDate = expirationDate;
super.delegate = new HashSet<String>();
}
#Override
public String toXml() {
String retVal;
try {
retVal = StringUtils.toXml(this);
}
catch (JAXBException e) {
retVal = e.toString();
}
return retVal;
}
public Long getAccountId() {
return accountId;
}
public void setAccountId(Long accountId) {
this.accountId = accountId;
}
public Set<String> getCodes() {
return (Set<String>) super.delegate;
}
#XmlElement(name = "code")
public void setCodes(Set<String> codes) {
super.delegate = codes;
}
public Date getExpirationDate() {
return expirationDate;
}
public void setExpirationDate(Date expirationDate) {
this.expirationDate = expirationDate;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
BuddyCodes that = (BuddyCodes) o;
if (accountId != null ? !accountId.equals(that.accountId) : that.accountId != null) return false;
if (delegate != null ? !super.delegate.equals(that.delegate) : that.delegate != null) return false;
if (expirationDate != null ? !expirationDate.equals(that.expirationDate) : that.expirationDate != null)
return false;
return true;
}
#Override
public int hashCode() {
int result = accountId != null ? accountId.hashCode() : 0;
result = 31 * result + (expirationDate != null ? expirationDate.hashCode() : 0);
result = 31 * result + (super.delegate != null ? super.delegate.hashCode() : 0);
return result;
}
#Override
public String toString() {
return "BuddyCodes{" +
"accountId=" + accountId +
"codes=" + super.delegate +
", expirationDate=" + expirationDate +
'}';
}
}
And it doesn't work. I get the error.
Now, here is the child class after removing the inheritance and it works!!!
import javax.xml.bind.JAXBException;
import javax.xml.bind.annotation.*;
import java.util.Collection;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;
/**
* #author christian.bongiorno
* Date: 10/3/11
* Time: 6:11 PM
*/
#XmlAccessorType(XmlAccessType.PROPERTY)
#XmlType(name = "BuddyCodes")
#XmlRootElement(name = "BuddyCodes")
public class BuddyCodes implements Xml {
private Long accountId;
private Date expirationDate;
private Set<String> delegate;
public BuddyCodes() {
delegate = new HashSet<String>();
}
public BuddyCodes(Long accountId, Set<String> codes, Date expirationDate) {
this.accountId = accountId;
this.expirationDate = expirationDate;
delegate = new HashSet<String>();
}
public BuddyCodes(Long accountId, Date expirationDate) {
this.accountId = accountId;
this.expirationDate = expirationDate;
delegate = new HashSet<String>();
}
#Override
public String toXml() {
String retVal;
try {
retVal = StringUtils.toXml(this);
}
catch (JAXBException e) {
retVal = e.toString();
}
return retVal;
}
public Long getAccountId() {
return accountId;
}
public void setAccountId(Long accountId) {
this.accountId = accountId;
}
public Set<String> getCodes() {
return delegate;
}
#XmlElement(name = "code")
public void setCodes(Set<String> codes) {
delegate = codes;
}
public Date getExpirationDate() {
return expirationDate;
}
public void setExpirationDate(Date expirationDate) {
this.expirationDate = expirationDate;
}
public boolean add(String s) {
return delegate.add(s);
}
public int size() {
return delegate.size();
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
BuddyCodes that = (BuddyCodes) o;
if (accountId != null ? !accountId.equals(that.accountId) : that.accountId != null) return false;
if (delegate != null ? !delegate.equals(that.delegate) : that.delegate != null) return false;
if (expirationDate != null ? !expirationDate.equals(that.expirationDate) : that.expirationDate != null)
return false;
return true;
}
#Override
public int hashCode() {
int result = accountId != null ? accountId.hashCode() : 0;
result = 31 * result + (expirationDate != null ? expirationDate.hashCode() : 0);
result = 31 * result + (delegate != null ? delegate.hashCode() : 0);
return result;
}
}
Why does the inheritance matter at all???
I haven't figured this out but, I have another DTO in a similar layout (BuddyTypes BuddyType). BuddyType has 2 members: Long and String. Both are annoted as XmlElement. This one works just fine.
It seems the problem that the members of the set making up the delegate are not annotated in my problem case and I don't know how to annotate a parent member. As an inherited class, it wouldn't make sense to have some sort of default name/annotation. But, I tried this madness and the annotation is ignored -- I have seen parent member annotations ignored before so this isn't new.
I don't know if it's possible, but I need to annotate a parent member.

A bit out of the box: try Simple XML library instead of JAXB. My experience with it is the best.

Save object with Hibernate annotations?

I just wanna ask know how its possible to create object after creating your database with Hibernate annotations?
When i run the code below, it creates the database with the objects, but when i run the second time it just creates exactly the same, and none new objects are added? How come? How do i create objects using annotations with the method .save, after creating the database with annotations? Or is it not possible to do so with annotations?
Thanks in advance.
public static void main(String[] args) {
Session session = HibernateUtil.getSessionFactory().openSession();
Transaction transaction = null;
try {
transaction = session.beginTransaction();
Adress adress = new Adress("Streetname", "postcode");
Person person1 = new Person("Peter Hanks", adress);
Person person2 = new Person("Sophie Hanks", adress);
session.save(person1);
session.save(person2);
transaction.commit();
} catch (HibernateException e) {
transaction.rollback();
e.printStackTrace();
} finally {
session.close();
}
}
Heres the code person.class
#Entity
#Table(name="person")
public class Person implements Serializable {
private long id;
private String navn;
private Adresse adresse;
public Person() {
}
public Person(String navn, Adresse adresse) {
this.navn = navn;
this.adresse = adresse;
}
#ManyToOne(cascade = CascadeType.ALL)
#JoinColumn(name= "adresse_id", nullable = false)
public Adresse getAdresse() {
return adresse;
}
public void setAdresse(Adresse adresse) {
this.adresse = adresse;
}
#Id
#GeneratedValue
#Column(name= "id")
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
#Column(name = "navn", nullable= false, length= 100)
public String getNavn() {
return navn;
}
public void setNavn(String navn) {
this.navn = navn;
}
#Override
public int hashCode() {
int hash = 3;
hash = 29 * hash + (this.navn != null ? this.navn.hashCode() : 0);
hash = 29 * hash + (this.adresse != null ? this.adresse.hashCode() : 0);
return hash;
}
#Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
final Person other = (Person) obj;
if ((this.navn == null) ? (other.navn != null) : !this.navn.equals(other.navn)) {
return false;
}
if (this.adresse != other.adresse && (this.adresse == null || !this.adresse.equals(other.adresse))) {
return false;
}
return true;
}

You might need to show us how you've written and annotated your Person and Adress (sic) objects.
If you've written "correct" equals() and hashcode() implementations (i.e. that don't look at the #Id of the object) then your save() calls will do nothing the second time around because the objects you've asked to save already exist in the database.

just changed the settings for hibernate.hbm2ddl.auto from create to create-update, and now theres no problem...