Here I am trying to override both equals method as well as hash code method. But containsValue() method throwing False. Even the hashcode overridden class is getting called but i think equals method is not getting properly called. please help me with this.
import java.util.*;
class Test{
int i;
Test(int i)
{
this.i=i;
}
public boolean equals(Test t)
{
if(this.i==t.i){
return true;
}
else{
return false;
}
}
public int hashCode() { //Overriding hashCode class
int result = 17;
result = 37*result + Integer.toString(i).hashCode();
result = 37*result;
return result;
}
}
class TestCollection13{
public static void main(String args[]){
HashMap<Integer,Test> hm=new HashMap<Integer,Test>();
hm.put(1,new Test(1));
hm.put(2,new Test(2));
hm.put(3,new Test(1));
hm.put(4,new Test(4));
for(Map.Entry m:hm.entrySet()){
Test t2=(Test)m.getValue();
System.out.println(m.getKey()+" "+t2.hashCode());
}
System.out.println(hm.containsValue(new Test(2)));
}
}
Your method public boolean equals(Test t) does not override Object.equals(Object). You need to update your method signature and check the class type:
#Override
public boolean equals(Object o) {
return o instanceof Test
&& ((Test)o).i == this.i;
}
equals should be defined to take an Object, not a Test:
#Override
public boolean equals(Object other)
You could easily detect this by explicitly annotating the method with #Override in which case the compiler would detect this bug.
Method equals() takes Object as a parameter, so in your code you are not overriding equals() method but overloading it. So you need to change the incoming parameter to Object. Your method should be something like this:
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Test)) return false;
Test test = (Test) o;
return this.i == test.i;
}
Also I would add getter and setter for your i member.
Public Class ClassATest {
public static void main(String a[]) {
ClassA a = new ClassA(10);
ClassA b = new ClassA(10);
if(a.equals(b)) {
//do something.
}
}
}
Public ClassA {
private int someValue;
ClassA(int val) {
someValue = vall
}
// Overriding equals method as..
public boolean equals(object o) {
if((o instance of Object) && (((ClassA)o).getSomeMethod() == **this**.someValue) {
return true;
}
}
Question is.. What is this referring to?
Assuming implementation a.equals(b) as -- b instance of Object && b.getSomeMethod == this.someValue;
Can you please explain.. whis 'this' refer to here ? this.someValue means what??
/
It means the instance of the class that the code is in, in your example it would be an instance of the class ClassA
I have two classes Person and Teacher. In the Person class I check if the two objects passed in are equal using the compareTo method. In the Teacher class, the problem I'm working on states that I need to override the equals method in Person. In this equals method, the only way it would return true is if it's equal in both the equals method in Person and Teacher. My question is, when I check in the Teacher's equals method, do I just call super.equals(other) in order to check if the two objects are equal by the parent class or is there something else I need to do?
Person:
public class Person implements Comparable<Person> {
public boolean equals(Object other) {
try {
return this.compareTo(other) == 0;
}catch(Exception e) {
return false;
}
}
}
Teacher
public class Teacher extends Person {
private String facultyID;
#Override
public boolean equals(Object other) {
boolean personEquals = super.equals(other);
try {
Teacher teach1 = (Teacher)other;
boolean idEquals = this.facultyID.equals(teach1.facultyID);
if(idEquals && personEquals)
return true;
return false;
}catch(Exception e) {
return false;
}
}
}
Basically, the contract of Object#equals states:
It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true
And the implementation of Teacher#equals doesn't meet this requirement. For cases when implementing equals method in a class that inherits from another that is not Object e.g. Teacher, you should verify if the type of the object to be compared is the same as the class you're comparing. To achieve this, you should use getClass().equals:
public class Teacher extends Person {
private String facultyID;
#Override
public boolean equals(Object other) {
//this line is nonsense, not all Persons are Teachers
//boolean personEquals = super.equals(other);
//avoid using try-catch
//try {
//verify that the other object is not null
if (other == null) {
return false;
}
//verify that the other object is specifically a Teacher, not a super or a subclass of it
if (this.getClass().equals(other.getClass()) {
//here comes the real check
Teacher otherTeacher = (Teacher)other;
return this.facultyID.equals(otherTeacher.facultyID);
}
return false;
}
}
Do similar for Person in case it should not allow comparing against subclasses of it.
I am a new learner of java. I have overridden equals and hashcode for my Animal class. I would expect that, using a HashSet<Animal> as below, only one Animal will be inserted into the HashSet. However the second Animal object is getting added to the set despite having the same properties. Can anyone look into it?
import java.util.*;
class SetTest
{
public static void main(String [] args)
{
Set<Animal>s=new HashSet<Animal>();
Animal a=new Animal(2);
boolean b1=s.add(a);
System.out.println("hi "+b1);
Animal b=new Animal(2);
boolean b2=s.add(b);
System.out.println("hi "+b2);
}
}
class Animal
{
int t;
Animal(int a)
{
t=a;
}
public boolean equals(Object O)
{
if(O instanceof Animal && ((Animal)O).t==t)
return true;
return false;
}
public int hashcode()
{
return t;
}
}
Output:
hi true
hi true
Thanks a Lot.
You didn't override hashCode(). You introduced another method called hashcode(). Case matters.
When your intention is to override a method, always use the #Override annotation:
#Override
public int hashcode()
If you had done that, the compiler would have noticed the typo and have refused to compile the class.
Your code has a mistype hashcode: write hashCode not hashcode .
import java.util.*;
class SetTest {
public static void main(String [] args) {
Set<Animal>s=new HashSet<Animal>();
Animal a=new Animal(2);
boolean b1=s.add(a);
System.out.println("hi "+b1);
Animal b=new Animal(2);
boolean b2=s.add(b);
System.out.println("hi "+b2);
}
}
class Animal {
int t;
public Animal(int a) {
t=a;
}
#Override
public boolean equals(Object O) {
return O instanceof Animal
&& ((Animal)O).t==t;
}
#Override
public int hashCode() {
return t;
}
}
If you are using eclipse, you can make use of the source code generator and there by avoiding the common human error(the like you did it over here. Right click on the class -> source -> generate hashcode and equals methods. which will pop up window. There you can choose the fields which you need to consider for equals(equality of object) and hashcode generation.
Java uses hashCode and equals method to check if two objects are equal. HashSet internally uses HashMap. You can refer HashMap.put(K key, V value) method for more details.
I have an abstract class named Xpto and two subclasses that extend it named Person and Car. I have also a class named Test with main() and a method foo() that verifies if two persons or cars (or any object of a class that extends Xpto) are equals. Thus, I redefined equals() in both Person and Car classes. Two persons are equal when they have the same name and two cars are equal when they have the same registration.
However, when I call foo() in the Test class I always get "false". I understand why: the equals() is not redefined in Xpto abstract class. So... how can I compare two persons or cars (or any object of a class that extends Xpto) in that foo() method?
In summary, this is the code I have:
public abstract class Xpto {
}
public class Person extends Xpto{
protected String name;
public Person(String name){
this.name = name;
}
public boolean equals(Person p){
System.out.println("Person equals()?");
return this.name.compareTo(p.name) == 0 ? true : false;
}
}
public class Car extends Xpto{
protected String registration;
public Car(String registration){
this.registration = registration;
}
public boolean equals(Car car){
System.out.println("Car equals()?");
return this.registration.compareTo(car.registration) == 0 ? true : false;
}
}
public class Teste {
public static void foo(Xpto xpto1, Xpto xpto2){
if(xpto1.equals(xpto2))
System.out.println("xpto1.equals(xpto2) -> true");
else
System.out.println("xpto1.equals(xpto2) -> false");
}
public static void main(String argv[]){
Car c1 = new Car("ABC");
Car c2 = new Car("DEF");
Person p1 = new Person("Manel");
Person p2 = new Person("Manel");
foo(p1,p2);
}
}
As the others say, the signature of the method you override must be exactly the same. When overriding methods, to make sure you are overriding, use the #Override annotation above the function, so IDEs like Eclipse will warn you if you changed the method.
This is what it would look like:
#Override
public boolean equals(Object obj){
...Your code here...
}
I would suggest to override hashCode() as well because when inserting items into lists, sets, hastables, etc... for equality (and performande) hashCode() is used (and sometimes equals() is not!)
So your final code would be:
#Override
public boolean equals(Object obj){
...Your code here...
}
#Override
public int hashCode(){
...Your code here...
}
More info at the javadoc
I understand why: the equals() is not
redefined in Xpto abstract class.
Actually equals() isn't redefined anywhere in your code. To override it, your method has to have Object as parameter type and you have to cast it (after testing with instanceof to return false when instances of two different subclasses are compared).
declaring public boolean equals(Person p) or public boolean equals(Car p) does not override Object's public boolean equals(Object o), it's just a new method that is never called.
Here's how I would go about it:
public abstract class Xpto {
}
public class Person extends Xpto{
protected String name;
public Person(String name){
this.name = name;
}
public boolean equals(Object o){
if(o == null || !getClass().equals(o.getClass())
return false;
Person p = (Person) o;
System.out.println("Person equals()?");
return this.name.compareTo(p.name) == 0 ? true : false;
}
}
public class Car extends Xpto {
protected String registration;
public Car(String registration){
this.registration = registration;
}
public boolean equals(Object o){
if(o == null || !getClass().equals(o.getClass())
return false;
Car car = (Car) o;
System.out.println("Car equals()?");
return this.registration.compareTo(car.registration) == 0 ? true : false;
}
}
public class Teste {
public static void foo(Xpto xpto1, Xpto xpto2){
if(xpto1.equals(xpto2))
System.out.println("xpto1.equals(xpto2) -> true");
else
System.out.println("xpto1.equals(xpto2) -> false");
}
public static void main(String argv[]){
Car c1 = new Car("ABC");
Car c2 = new Car("DEF");
Person p1 = new Person("Manel");
Person p2 = new Person("Manel");
foo(p1,p2);
}
}
Every class inherit an equals(Object) method from the Object class. Thus, Xpto does not need to define such a method.
When one overrides this method in subclasses (namely: Person, Car) one must define it with the exact same signature. In other words, the parameter of the equals method must be of type Object, and the method implementation must downcast it.
The Javadoc states that you need to override the equals method with object as a parameter.
Indicates whether some other object is
"equal to" this one.
Therefore your subclasses equals methods should look something like this:
public class Car extends Xpto
{
protected String registration;
public Car(String registration)
{
this.registration = registration;
}
public boolean equals(Object obj)
{
if (obj == null)
{
return false;
}
if (obj == this)
{
return true;
}
if (!obj.getClass().isAssignableFrom(getClass()))
{
return false;
}
Car car = (Car) obj;
return this.registration.compareTo(car.registration) == 0 ? true : false;
}
}
It is generally very difficult/impossible to fully fulfill the equals contract and still have two different classes in the hierarchy equal to each other, and it is generally not done. Generally an equals method tests for the class being the same (so two instances of the same subclass will equal each other, but two instances of two different subclasses will not).
However, in your case it is possible to implement an equals in Xpto since there is only one property. The obvious way to do this is to define an abstract method in Xpto, and then override equals in Xpto as well:
public class Xpto {
protected abstract String getIdentity();
#Override
public boolean equals(Object o) {
if (o == null) return false;
//Typical implementation
//if (getClass() != o.getClass()) return false;
if (!(o instanceof Xpto)) return false; //risky implementation, but will allow a car to compare to a person
return getIdentity().equals((Xpto) o.getIdentity());
}
#Override
public int hashCode() {
return getIdentity().hashCode();
}
}
Others have pointed out that you did not actually override equals in your implementation. In the future you can get the compiler to help you out with that by using the #Override annotation. In your case you would have gotten a compile error early which would have saved you some time.
Your equals method should look like:
#Override public boolean equals(Object o) {
if (!(o instanceof YourType)) {
return false;
}
YourType yt = (YourType)o;
... // rest here
}
Also, don't forget to override hashCode as well, to be able to properly use your types in collections.
You are not overriding the equals() method, instead you overload it. Change the signature to
public boolean equals(Object o)
And then cast o to Person/Car and do the comparison.
And BTW, you could compare the strings with equals() as well:
return registration.equals(car.registration);
Your subclasses are defining equals(Person) or equals(Car), neither of which is going to like being passed an Xpto. If you declare them both as equals(Xpto), or better yet equals(Object) so that they'll work in collections, then your problem should go away.
Note, if you redeclare the equals() methods this way, (1) you'll need to check the classes of the objects you get passed, since you can't guarantee they're Cars or Persons anymore, and (2) you'll probably want to override getHashCode() as well, especially if you decide to make them both equals(Object), cause getHashCode() should return equal hash codes for two objects that are equal.