I've following the following Vertex class and it implements equals, hashCode and compareTo method. Even then my HashMap returns null. I don't know why?
public class Vertex implements Comparable<Vertex> {
int id;
public Vertex(int number) {
id = number;
}
public boolean equals(Object other) {
if (other == null)
return false;
else if (other.getClass() != this.getClass())
return false;
else {
Vertex copy = (Vertex) other;
if (copy.id == this.id)
return true;
else
return false;
}
}
public int hasCode() {
int prime = 31;
int smallPrime = 3;
int hashCode = this.id ^ smallPrime - prime * this.hasCode();
return hashCode;
}
public int compareTo(Vertex other) {
if (this.id < other.id)
return -1;
else if (this.id > other.id)
return 1;
else
return 0;
}
}
Your method is called hasCode(). Make it hashCode() instead.
I'd suggest using your IDE to automatically generate hashCode() and equals(..). That will generate the proper methods (right now you have a recursive call in hashCode())
Also, in your equals() method
else if(other.getClass()!=this.getClass())
return false;
can be changed to
else if(!(other instanceof Vertex))
return false;
Try this based on what Integer does. Note: the use of #Override would have shown you were overriding the wrong method.
public class Vertex {
final int id;
public Vertex(int number){
id = number;
}
#Override
public boolean equals(Object other){
if(!(other instanceof Vertex)) return false;
return ((Vertex)other).id == id;
}
#Override
public int hashCode() { return id; }
}
Related
I have 3 entities, student, grade and class. Code shows below. It is just a sample.
Student class
public class Student implements Serializable{
private static final long serialVersionUID = 1L;
private String fullName;
private long studentId;
//omit getter/setter column mapped to db
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((getFullName() == null) ? 0 : getFullName().hashCode());
result = prime * result + (int) (getStudentId() ^ (getStudentId() >>> 32));
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (obj instanceof Student)
return false;
test other = (test) obj;
if (getFullName() == null) {
if (other.getFullName() != null)
return false;
} else if (!getFullName().equals(other.getFullName()))
return false;
if (getStudentId() != other.getStudentId())
return false;
return true;
}
}
SchoolClass class:
public class SchoolClass implements Serializable{
private static final long serialVersionUID = 1L;
private String className;
private long classId;
//omit getter/setter column mapped to db
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + (int) (getClassId() ^ (getClassId() >>> 32));
result = prime * result + ((getClassName() == null) ? 0 : getClassName().hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (obj instanceof SchoolClass)
return false;
SchoolClass other = (SchoolClass) obj;
if (getClassId() != other.getClassId())
return false;
if (getClassName() == null) {
if (other.getClassName() != null)
return false;
} else if (!getClassName().equals(other.getClassName()))
return false;
return true;
}
}
Grade Class:
public class Grade implements Serializable{
private static final long serialVersionUID = 1L;
private SchoolClass schoolClass;
private Student student;
//omit getter/setter column mapped to db
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((getSchoolClass() == null) ? 0 : getSchoolClass().hashCode());
result = prime * result + ((getStudent() == null) ? 0 : getStudent().hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (obj instanceof Grade)
return false;
Grade other = (Grade) obj;
if (getSchoolClass() == null) {
if (other.getSchoolClass() != null)
return false;
} else if (!getSchoolClass().equals(other.getSchoolClass()))
return false;
if (getStudent() == null) {
if (other.getStudent() != null)
return false;
} else if (!getStudent().equals(other.getStudent()))
return false;
return true;
}
}
So I checked hibernate doc for hashcode and equals, it works perfectly fine for entity that exists in DB. The problem I have is for new transient entity objects before save to db. I did separate tests specifically on Student and SchoolClass using HashSet, the size of set won't increase if it tries to add same object.
Student s1 = studentRepo.findById(studentId).get();
SchoolClass sc = scRepo.findById(classId).get();
Grade grade = new Grade();
grade.setStudent(s1);
grade.setSchoolClass(sc);
grades.add(grade);
logger.info(grades.size());
Here I have a new Set of grades and preparing this set and save to db. Here comes the problem, this set will contains duplicate grade object. Meaning there will be 2 entries with same student and same class. In grade class, I override its hashcode and equals to Student and SchoolClass, it should NOT have duplicate entries. I figure it probably because the new Grade object is in transient state? Not really sure what is the cause.
Of course I can do unique check for grade in an manual way, but hashcode and equals should be the right way to go, isn't it?
So how to solve this? Need some help.
Thanks to #samabcde. He is right, I missed ! on the condition check.
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.
I have a class having more than 30 attributes.
I want to override the equals method in order to compare two instance of my class.
However I want to avoid re-write all the 30 attributes in my method as this
#Override
public boolean equals(java.lang.Object o) {
if (this == o) {
return true;
}
if (o == null || getClass() != o.getClass()) {
return false;
}
Address address = (Address) o;
return Objects.equals(this.attr1, address.attr1) &&
Objects.equals(this.attr2, address.attr2) &&
Objects.equals(this.attr3, address.attr3) &&
......
Objects.equals(this.attr30, address.attr30);
}
Have you a more simple and proper way ?
Well, this is basically the type of boilerplate code that is necessary. Luckily, there are lots of developers just as annoyed of writing such code as you are. For reasons like that, Project Lombok was founded.
Please see this link.
As an example, see the following two code snippets extracted from the page that I linked above:
Lombok
import lombok.EqualsAndHashCode;
#EqualsAndHashCode(exclude={"id", "shape"})
public class EqualsAndHashCodeExample {
private transient int transientVar = 10;
private String name;
private double score;
private Shape shape = new Square(5, 10);
private String[] tags;
private int id;
public String getName() {
return this.name;
}
#EqualsAndHashCode(callSuper=true)
public static class Square extends Shape {
private final int width, height;
public Square(int width, int height) {
this.width = width;
this.height = height;
}
}
}
Vanilla Java
import java.util.Arrays;
public class EqualsAndHashCodeExample {
private transient int transientVar = 10;
private String name;
private double score;
private Shape shape = new Square(5, 10);
private String[] tags;
private int id;
public String getName() {
return this.name;
}
#Override public boolean equals(Object o) {
if (o == this) return true;
if (!(o instanceof EqualsAndHashCodeExample)) return false;
EqualsAndHashCodeExample other = (EqualsAndHashCodeExample) o;
if (!other.canEqual((Object)this)) return false;
if (this.getName() == null ? other.getName() != null : !this.getName().equals(other.getName())) return false;
if (Double.compare(this.score, other.score) != 0) return false;
if (!Arrays.deepEquals(this.tags, other.tags)) return false;
return true;
}
#Override public int hashCode() {
final int PRIME = 59;
int result = 1;
final long temp1 = Double.doubleToLongBits(this.score);
result = (result*PRIME) + (this.name == null ? 43 : this.name.hashCode());
result = (result*PRIME) + (int)(temp1 ^ (temp1 >>> 32));
result = (result*PRIME) + Arrays.deepHashCode(this.tags);
return result;
}
protected boolean canEqual(Object other) {
return other instanceof EqualsAndHashCodeExample;
}
public static class Square extends Shape {
private final int width, height;
public Square(int width, int height) {
this.width = width;
this.height = height;
}
#Override public boolean equals(Object o) {
if (o == this) return true;
if (!(o instanceof Square)) return false;
Square other = (Square) o;
if (!other.canEqual((Object)this)) return false;
if (!super.equals(o)) return false;
if (this.width != other.width) return false;
if (this.height != other.height) return false;
return true;
}
#Override public int hashCode() {
final int PRIME = 59;
int result = 1;
result = (result*PRIME) + super.hashCode();
result = (result*PRIME) + this.width;
result = (result*PRIME) + this.height;
return result;
}
protected boolean canEqual(Object other) {
return other instanceof Square;
}
}
}
If you like this approach, I'd recommend checking out Project Lombok in its entirety. It really helps cleaning up your code!
!!! Beware !!!
In order to be able to actually use methods generated by Lombok, you need to install the Lombok plugin into your IDE! Otherwise, your IDE won't know of the automatically generated methods.
You can use the Field class in the java.lang.reflect package like so:
#Override
public boolean equals(Object o) {
//instanceof check, null check, etc
Field[] fields = Address.class.getDeclaredFields();
for (Field field : fields) {
try {
field.setAccessible(true);
if (!field.get(this).equals(field.get((Address) o))) {
return false;
} //end if
} catch (IllegalAccessException e) {
//handle exception
} //end try catch
} //end for
return true;
} //equals
You have this method in apache commons library that uses reflection for compare them
org.apache.commons.lang.builder.EqualsBuilder.reflectionEquals(Object, Object)
Example
import org.apache.commons.lang.builder.EqualsBuilder;
public class MyObject {
...
#Override
public boolean equals(Object obj) {
return EqualsBuilder.reflectionEquals(this, obj);
}
}
You can use lombok project to auto-generate hashCode and equals methods at build time .
You can use Unitils http://www.unitils.org/cookbook.html
import static org.unitils.reflectionassert.ReflectionAssert.*;
// Exact field-by-field comparison
assertReflectionEquals(new Person("John", "Doe", new Address("New street", 5, "Brussels")),
new Person("John", "Doe", new Address("New street", 5, "Brussels"));
Right now I have this Java code
import java.util.HashSet;
import java.util.Objects;
import java.util.Set;
public class Dummy {
private String value;
public Dummy(final String value) {
this.value = value;
}
public boolean equals(final Object that) {
return that instanceof Dummy && Objects.equals(value, ((Dummy) that).value);
}
public int hashcode() {
return Objects.hash(value);
}
public static void main(final String... args) {
final Set<Dummy> dummies = new HashSet<>();
dummies.add(new Dummy("toto"));
System.out.println(dummies.contains(new Dummy("toto")));
}
}
The output is "false", and I'm supposed to change it to "true" by changing only one character, but I have absolutely no idea how to do that... Any ideas? Thanks. :)
hashcode() is not a Object's method but hashCode() is.
public int hashcode() {
return Objects.hash(value);
}
should be
public int hashCode() {
return Objects.hash(value);
}
In my case I had lots of objects who all need a equals and hash method. I have used lombok to reduce the work and code.
#Data
#AllArgsConstructor
#EqualsAndHashCode
public class JsonPictureStuff {
private String type;
private String url;
private String width;
private String height;
}
The annotations instruct lombok to add the code for you.
your set is not working because you are implementing equals and hashcode in a not proper way...
specially because you are not even considering the string field
you are avoiding the annotation Override that will hint you about wrongly named methods like hashcode and hashCode
you can do something like
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((value == null) ? 0 : value.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;
Dummy other = (Dummy ) obj;
if (value == null) {
if (other.value != null)
return false;
} else if (!value.equals(other.value))
return false;
return true;
}
in ide like eclipse you can do right click and auto generate those methods so you can save the implementation
I have two methods for looking up the index of a particular element in a linked list. I'm trying to use contais() and indexOf() to find the index. I am unsure how to override equals() to suit my need. One method finds the index based on surname and initials the other just the telephone number. Here are my methods:
#Override
public int lookupNumber(String surname, String initials) {
Entry entry1 = new Entry(surname, initials);
if (listDirectory.contains(entry1)) {
int index = listDirectory.indexOf(entry1);
return index;
}
else {
return -1;
}
}
#Override
public int lookupName(int extension) {
Entry entry1 = new Entry(Integer.toString(extension));
if (listDirectory.contains(entry1)) {
int index = listDirectory.indexOf(entry1);
return index;
}
else {
return -1;
}
}
I've tried to use the equals() and hashCode() that eclipse provides:
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Entry other = (Entry) obj;
if (extension == null) {
if (other.extension != null)
return false;
} else if (!extension.equals(other.extension))
return false;
if (initals == null) {
if (other.initals != null)
return false;
} else if (!initals.equals(other.initals))
return false;
if (surname == null) {
if (other.surname != null)
return false;
} else if (!surname.equals(other.surname))
return false;
return true;
}
But it won't work for both my methods, as it compares all the variables in the objects not just the ones I need it to compare. What is the correct logic for this?
Help much appreciated, thanks.
Update - here is my full Entry class -
public class Entry {
private String surname;
private String initals;
private String extension;
public Entry(String surname, String initals,String extension) {
this.surname = surname;
this.initals = initals;
this.extension = extension;
}
public Entry(String surname,String initals){
this.surname = surname;
this.initals = initals;
}
public Entry(String extension) {
this.extension = extension;
}
public String getInitals(){
return initals;
}
public String getSurname(){
return surname;
}
public String getExtension(){
return extension;
}
public void setExtension(String extension) {
this.extension = extension;
}
public String toString(){
return surname + "\t " + initals + "\t" + extension;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result
+ ((extension == null) ? 0 : extension.hashCode());
result = prime * result + ((initals == null) ? 0 : initals.hashCode());
result = prime * result + ((surname == null) ? 0 : surname.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;
Entry other = (Entry) obj;
if (extension == null) {
if (other.extension != null)
return false;
} else if (!extension.equals(other.extension))
return false;
if (initals == null) {
if (other.initals != null)
return false;
} else if (!initals.equals(other.initals))
return false;
if (surname == null) {
if (other.surname != null)
return false;
} else if (!surname.equals(other.surname))
return false;
return true;
}
}
Update - My problem is in my lookupNumber() method I want to find elements by (initials,surname) whereas in my lookupNumber() I want to find elements by (extension). How would I do this?
The Java Object class contains an equals() method of its own, and the Entry class should be the blueprint for objects of type Entry. Java inheritance is such that all classes inherit from the Object class, which is of little use to many of us. Really all you would need is
#Override
public boolean equals(Entry otherEntry)
{
if(!(this.surname.equals(otherEntry.getSurname()))
return false;
else if(!(this.initials.equals(otherEntry.getInitials()))
return false;
else if(!(this.extension.equals(otherEntry.getExtension()))
return false;
else
return true;
}
The above code just checks for inequality amongst the instance variables, returning false upon any one not equal.