I want to override "public boolean equals(Object obj)" function, for name and age, in my class named MyObject whose structure is given below
public class MyObject{
private String name;
private int age;
}
How can i ?
#balusC :
What about this ?
vo = new MyObject() {
public boolean equals(Object obj) {
return ((MyObject)obj).name().equals(this.getName());
}
vo = new MyObject() {
public boolean equals(Object obj) {
return ((MyObject)obj).age() == (this.getAge());
Your question is a bit vague, but if the sole purpose is to have different sorting algorithms depending on what property you'd like to use, then rather use a Comparator.
public class Person {
private String name;
private int age;
public static Comparator COMPARE_BY_NAME = new Comparator<Person>() {
public int compare(Person one, Person other) {
return one.name.compareTo(other.name);
}
}
public static Comparator COMPARE_BY_AGE = new Comparator<Person>() {
public int compare(Person one, Person other) {
return one.age > other.age ? 1
: one.age < other.age ? -1
: 0; // Maybe compare by name here? I.e. if same age, then order by name instead.
}
}
// Add/generate getters/setters/equals()/hashCode()/toString()
}
which you can use as follows:
List<Person> persons = createItSomehow();
Collections.sort(persons, Person.COMPARE_BY_NAME);
System.out.println(persons); // Ordered by name.
Collections.sort(persons, Person.COMPARE_BY_AGE);
System.out.println(persons); // Ordered by age.
As to the actual equals() implementation, I'd rather let it return true when the both Person objects are techically or naturally identical. You can use either a DB-generated PK for this to compare on technical identity:
public class Person {
private Long id;
public boolean equals(Object object) {
return (object instanceof Person) && (id != null)
? id.equals(((Person) object).id)
: (object == this);
}
}
or just compare every property to compare on natural identity:
public class Person {
private String name;
private int age;
public boolean equals(Object object) {
// Basic checks.
if (object == this) return true;
if (object == null || getClass() != object.getClass()) return false;
// Property checks.
Person other = (Person) object;
if (name == null ? other.name != null : !name.equals(other.name)) return false;
if (age != other.age) return false;
// All passed.
return true;
}
}
Don't forget to override hashCode() as well when you override equals().
See also:
Object ordering
Sorting an ArrayList of objects
Overriding equals() and hashCode()
I'm not exactly sure what you're aiming at with this. The general expectation of equals() is that it returns false for null and objects of other classes and performs value equality on the relevant fields of the class in question.
While you can certainly handle String and Integer in the following way:
public boolean equals(Object o) {
if (o == null) return false;
if (o instanceof String) return name.equals(o);
if (o instanceof Integer) return ((Integer)o) == age;
...
}
this breaks the contract for equals so you can't do it (except not without things going wrong in very weird ways).
equals is an equivalence relation, so it has to be reflexive, symmetric and transitive. The symmetric part here is key, since if a.equals(b) then b.equals(a). Both String and Integer won't do that for you.
If you want just helper functions that check whether the name or the age is equals to a given name/age, then you can do that without using equals():
public boolean equalsName(String name) { return name.equals(this.name); }
public boolean equalsAge(int age) { return age == this.age; }
Just keep it short and simple (aka KISS principle): write setters and getters. Something like in the following example:
public class Person {
private String name;
private int age;
public String getName() {
return name;
}
public int getAge() {
return age;
}
And then in the method you need to do the check you can write:
Person person = new Person();
if(person.getName().equals("Something")) doThis();
if(person.getAge() == 1337) doThat();
Not sure what you mean by "multiple equals()". If you want compare both your fields, you just need to override the equals method like this,
public boolean equals( Object o )
{
if ( o != null && o instanceof MyObject )
{
MyObject m = (MyObject) o;
if (this.name == null)
return false;
return this.name.eqauls(m.name) && this.age == m.age;
}
return false;
}
/// Compute a hash code for the pair.
public int hashCode()
{
int code = name == null ? 0 : name.hashCode();
return code ^ age;
}
It's a good practice to change hashCode whenever you change equals so HashMap works efficiently with your object.
if you do want to override equals, it should look something like this:
static private <T> boolean checkEquals(T t1, T t2)
{
return (t1 == null) ? (t2 == null) : t1.equals(t2);
}
#Override public boolean equals (Object o)
{
if (o instanceof MyObject)
{
MyObject obj = (MyObject)o;
return checkEquals(this.name, obj.getName())
&& this.age == o.getAge();
}
else
return false;
}
#Override public int hashCode()
{
// implement hashCode
}
You need to override both hashCode() and equals() or neither. And you also should make sure your class is final, otherwise there are potential pitfalls with equals.
public class MyObject {
private String name;
private int age;
#Override
public boolean equals(Object o){
if(o instanceof MyObject){
MyObject otherObject = (MyObject)o;
if(name == null){
return otherObject.name == null && otherObject.age == age;
} else {
return name.equals(otherObject.name) && otherObject.age == age;
}
} else {
return false;
}
}
// When we overriding equals it is a good practice to override hashCode
// for consistecy
#Override
public int hashCode(){
int nameCode = (name == null) ? 0 : name.hashCode();
// See Item 9 in book Effective Java 2nd Edition
return 31 * nameCode + age;
}
}
Related
This question already has answers here:
What issues should be considered when overriding equals and hashCode in Java?
(11 answers)
Closed 10 months ago.
When i compare two instances in java of the same type class, with the same values of their Attributes, why i get false?
I'm new with OOP and don't Understand some OOP logic.
public static void main(String[] args) {
Object o1= new Object(24,"Omar");
Object o2= new Object(24,"Omar");
System.out.println(o1.equals(o2));
}
The equals method returns true if and only if o1 and o2 refer to the same object. And if We want to compare with members variable, we need to override the equals method in our custom class like below:
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
CustomObjClass obj = (CustomObjClass) o;
return age == obj.age && Objects.equals(name, obj.name);
}
And then it will return true for the below code:
CustomObjClass o1= new CustomObjClass(24,"Omar");
CustomObjClass o2= new CustomObjClass(24,"Omar");
System.out.println(o1.equals(o2)); // return true
By default, the equals operator verifies if the two objects are the same one. So, for example, if we perform these operations:
Object o1= new Object(24,"Omar");
Object o2= o1
System.out.println(o1.equals(o2));
you would get true, as o2 is the same object as o1. In your case, you have two different objects (since you instantiate them with the "new") that have the same content. Therefore, the equal operator returns false. If you want it to return true, you have to override the equals operator and implement your own logic.
The default implementation of equals method compares the reference of both the objects using == operator. You can find the details here. If you want to compare the value of your Class override the equals method.
Example:
public class TestClass {
private int age;
private String name;
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof TestClass)) return false;
TestClass testClass = (TestClass) o;
return getAge() == testClass.getAge() && Objects.equals(getName(), testClass.getName());
}
#Override
public int hashCode() {
return Objects.hash(getAge(), getName());
}
public TestClass(int age, String name) {
this.age = age;
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
I am trying to Implement a class named Parade using an ArrayList, which will manage instances of class Clown. Each Clown needs to be identified by all object data String for their name, int id and double size. I join a new Clown to the end of the Parade. Only the Clown at the head of the Parade (i.e., the first one) can leave the Parade. In addition, I write a method called isFront that takes a Clown as parameter and returns true if the passed clown is at the front of the parade otherwise returns false. Create a test application to demonstrate building a parade of three or four clowns and include your own name. Then, remove one or two, and add another one or two. Also, test the isFront method by passing different clowns to the method.
I have a code but it is not returning true for the isFront method, I am trying to use contains method I also tried to use Comparable interface Clown but it did not work that well. Not sure what to do.
import java.util.ArrayList;
public class Main
{
public static void main(String[] args)
{
Parade circus = new Parade();
circus.addClown(new Clown("Bobby",9,12.0));
circus.addClown(new Clown("Clair", 2, 11.0));
circus.addClown(new Clown("Tony",6,10.0));
circus.addClown(new Clown("Sarah",3,5.0));
circus.display();
System.out.println(circus.isFront(new Clown("Bobby", 9, 12.0)));
}
}
import java.util.ArrayList;
public class Parade
{
private static ArrayList<Clown> parade;
private int top;
public Parade()
{
top=0;
parade= new ArrayList<Clown>();
System.out.println(parade);
}
public void addClown(Clown c)
{
parade.add(c);
top++;
}
public void removeClown() //(Clown c)
{
parade.remove(0);
top--;
}
public void display()
{
System.out.println(parade);
}
public void getData()
{
parade.get(0);
}
public void setData()
{
parade.set(1,new Clown("Claire",2,5.0));
System.out.println(parade);
}
public int getTop()
{
return top;
}
public boolean isFront(Clown c)
{
return !parade.isEmpty() && c.equals(parade.get(0));
}
//exceptions
}
public class Clown
{
private String name;
private int id;
private double size;
public Clown(String name, int id, double size)
{
this.name=name;
this.id=id;
this.size=size;
}
public String getName()
{
return name;
}
public int getId()
{
return id;
}
public double getSize()
{
return size;
}
public String toString()
{
return name.toString() + id + size;
}
public boolean equals(Object o) {
if (o instanceof Clown c) {
return this.getName().equals(c.getName()) && this.getId() == c.getId() && this.getSize() == c.getSize();
}
return false;
}
}
their is not much info in our textbook about this stuff Java FOundations 5th e Lewis like working with objects and arraylists it skips it and assumes you already know it lol..
Firstly, objects in Java are, by default, compared by reference. So, even if you create two Clown objects with the exact same properties, Java sees them as different objects because both those object references are not the same, they are both referring to different memory locations. You can override this behavior and ask Java to compare it as you want by overriding the equals() method of the Object class:
public class Clown {
private String name;
private int id;
private double size;
public Clown(String name, int id, double size) {
this.name=name;
this.id=id;
this.size=size;
}
public String getName() {
return name;
}
public int getId() {
return id;
}
public double getSize() {
return size;
}
#Override
public boolean equals(Object o) {
if (o instanceof Clown) {
Clown c = (Clown) o;
return this.getName().equals(c.getName());
}
return false;
}
#Override
public int hashCode() {
return this.getId();
}
#Override
public String toString() {
return name.toString() + id + size;
}
}
This will help with contains()(it internally uses equals()).
Secondly, you can just compare your clown with the first clown to see if it is the one at the front:
public boolean isFront(Clown c) {
return !parade.isEmpty() && c.equals(parade.get(0));
}
The isFront() method will return true if the parade is not empty and the clown c is equal to to the first clown in the parade. get(0) retrieves the first clown in the parade.
As per your comment, if you want that two clowns be equal only if all their properties are equal, change your equals method to:
#Override
public boolean equals(Object o) {
if (o instanceof Clown) {
Clown c = (Clown) o;
return this.getName().equals(c.getName()) &&
this.getId() == c.getId() &&
this.getSize() == c.getSize();
}
return false;
}
The equals() method is of the Object class which is the parent class of all Java classes. It defines how to compare two objects.
Its signature is as follows:
public boolean equals(Object obj)
As we're overriding, its signature must be the same in the derived class, in our case in class Clown. Its parameter is of type Object not Clown. Any type can be converted to Object, if I compare an object of Clown to another type, like:
Clown c = new Clown("X", 1, 10);
if ( c.equals(objectOfAnotherType) ) {..}
it will still work.
So we use the instanceof operator to check if that another object is also a Clown. If it is not an instance of Clown, we return false but if it is, we convert/cast that object to Clown, only then we can call getName() and other getter methods:
if (o instanceof Clown) {
Clown c = (Clown) o; //Casting happens here
return this.getName().equals(c.getName()) &&
this.getId() == c.getId() &&
this.getSize() == c.getSize();
}
return false;
Java 14 introduced a shortcut for this, instead of these steps:
if (o instanceof Clown) {
Clown c = (Clown) o;
we can simply write:
if (o instance of Clown c)
which does the casting for us and stores it in c.
Lastly, I have also overriden Object.hashCode() because you have to when you override equals(), here's why.
I tried to use an interface as a key in a hashMap in order to have 1 map for multiple types of keys. The following seems to work.
interface Foo {
void function();
}
static class Bar implements Foo {
private int id;
public Bar(int id) {
this.id = id;
}
#Override
public void function() {
System.out.println("this is bar");
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Bar bar = (Bar) o;
return id == bar.id;
}
#Override
public int hashCode() {
return Objects.hash(id);
}
}
public static Map<Foo, Integer> map = new HashMap<>();
static class Baz implements Foo {
String name;
public Baz(String name) {
this.name = name;
}
#Override
public void function() {
System.out.println("this is Baz");
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Baz baz = (Baz) o;
return name.equals(baz.name);
}
#Override
public int hashCode() {
return Objects.hash(name);
}
}
public static void main(String[] args) {
Bar bar = new Bar(123);
Baz baz = new Baz("some name");
map.put(bar, 10);
map.put(baz, 20);
System.out.println(map.get(bar));
}
What I am not sure about is if there is some corner case that would break this map?
Is there a case that having an interface as a key would break down? Could I have done it simpler using generics?
The only thing that's slightly out of the ordinary is that the equals methods have to compare Bar and Baz objects. When a Map only has one type of objects, the check this.getClass() == that.getClass in equals method never returns false. You have implemented this correctly though, so you don't have anything to worry about.
You may get more hash collisions than you expect. Imagine you have two classes that both have an int id field and implement hashCode with Objects.hash(id) - now objects of different classes with the same ID have the same hash code. If this use case is expected, you can perturb the hash in a way unique to each class, for example by mixing a class-specific constant to the hash:
#Override
public int hashCode() {
return Objects.hash(1, id);
}
...
#Override
public int hashCode() {
return Objects.hash(2, name);
}
In theory there could be problems with potentially more hash collisions leading to bad performance due to different implementations of hashCode, so you need to be careful, and test it with the real data. Other than that it is a valid use case.
I've been testing a code and stumbled across a problem: Should you call super.equals() method in subclass that can override some of the methods used in equals() method of the super class?
Let's consider the following code:
public abstract class Item {
private int id;
private float price;
public Item(int id, String name, float price, String category) {
this.id = id;
this.name = name;
this.price = price;
this.category = category;
}
public int getID() {
return id;
}
public float getPrice() {
return price;
}
#Override
public boolean equals(Object object){
if(object instanceof Item){
Item item = (Item) object;
if( id == item.getID()
&& price == item.getPrice())
{ return true; }
}
return false;
}
}
And the subclass DiscountedItem:
public class DiscountedItem extends Item {
// discount stored in %
private int discount;
#Override
public boolean equals(Object object) {
if(object instanceof DiscountedItem){
DiscountedItem item = (DiscountedItem) object;
return (super.equals(item)
&& discount == item.getDiscount()
);
}
return false;
}
public int getDiscount() {
return discount;
}
#Override
public float getPrice() {
return super.getPrice()*(100 - discount);
}
}
I've been just re-reading Angelika Langer's secrets of equals(), where she even states:
There is agreement that super.equals() should be invoked if the class has a superclass other than Object.
But I think it's highly unpredictable when the subclass will override some of the methods. For instance when I compare 2 DiscountedItem objects using equals, the super method is called and item.getPrice() is dynamically dispatched to the correct method in the subclass DiscountedItem, whereas the other price value is accessed directly using variable.
So, is it really up to me (as I should implement the method correctly) or is there a way around it?
Compare instance variables directly rather than comparing an instance variable to its related getter method.
For example, change
&& price == item.getPrice())
to
&& this.price == item.price)
The getter method is unnecessary since private instance variables are only inaccessible outside the class structure.
Note:
I previously recommended the following:
&& this.getPrice() == item.getPrice())
While it will work in the question's example, it is not well suited for all cases. Consider if the subclass DiscountedItem declared the method getPrice as such:
#Override
public float getPrice() {
return Math.floor(super.getPrice());
}
This would result in the false equivalence:
DiscountedItem firstItem = DiscountedItem(1, "", 1.1, "");
DiscountedItem secondItem = DiscountedItem(1, "", 1.0, "");
firstItem.equals(secondItem); // Returns true despite different prices.
The implementation of equals should depend on state and type, but not functionality. You went wrong in your base class:
#Override
public boolean equals(Object object){
if(object instanceof Item){ // Type check- good!
Item item = (Item) object;
if( id == item.getID() // Depends on functionality- bad!
&& price == item.getPrice()) // Depends on functionality- bad!
{ return true; }
}
return false;
}
item.getID() and item.getPrice(), as you've noticed, can be overwritten to break the contract of Item.equals().
#Override
public boolean equals(Object object){
if(object instanceof Item){ // Type check- good!
Item item = (Item) object;
if( id == item.id // Depends on state- good!
&& price == item.price) // Depends on state- good!
{ return true; }
}
return false;
}
This will never be broken by a child class. Further, it enables the child to meaningfully delegate to it.
#Override
public boolean equals(Object object) {
if(object instanceof DiscountedItem){
DiscountedItem item = (DiscountedItem) object;
return (super.equals(item)
&& this.discount == item.discount
);
}
return false;
}
The child only needs to worry about comparing the data that it owns.
Oh my, I guess I just had to post the question to get it..
To get rid of the problem, instead of directly accessing variables - call the getters!
#Override
public boolean equals(Object object){
if(object instanceof Item){
Item item = (Item) object;
if( this.getID() == item.getID()
&& this.getPrice() == item.getPrice())
{ return true; }
}
return false;
}
This code no longer has the problem when overriding methods.
If you call the equals of the DiscountedItem when can you have a problem?
If something is DiscountedItem and something else is Item these two
are never equal as per equals. Right? So I don't see a problem if you
always call the getters.
Also, you need to override hashCode if you override equals.
public class Car
{
private String name;
public int id;
public Car(String name, int id)
{
this.name = name;
this.id = id;
}
#Override
public boolean equals(Object ob)
{
if (!(ob instanceof Car))
{
return false;
}
Car that = (Car)ob;
return this.id == that.id;
}
#Override
public int hashCode()
{
return id;
}
// this class also got getters and setters
Then I got another class
public class CarList
{
private Collection<Car> cars;
public CarList()
{
cars = new HashSet<>();
}
public boolean insertCar(Car car)
{
return cars.add(car);
}
My question is: How to properly override equals() and hashCode() method, where I consider 'id' and 'name' attribute for object comparsion and hashCode calculation ( so there is no possibility to have 2 objects with the same name and ID - because in this code as it is - it only takes 'id' attribute for object comparsion)?
As of Java 7, there are static methods on Objects that makes implementing hashCode and equals easier. This should work well, assuming you don't want to use getClass() instead of instanceof to determine type compatibility. That depends on how subclasses of Car should compare to Cars.
#Override
public boolean equals(Object ob)
{
if (!(ob instanceof Car))
{
return false;
}
Car that = (Car)ob;
return Objects.equals(this.id, that.id) && Objects.equals(this.name, that.name);
}
#Override
public int hashCode()
{
return Objects.hash(id, name);
}
Instead of using
if (!(ob instanceof Car))
{
return false;
}
You should think about using
if (getClass() != obj.getClass())
return false;
Lets assume you have ForWdCar extends Car and TwoWdCar extends Car with equal name and id.
Do you want them to be equal? 1st solution,
Do you want them to be unequal? 2nd solution
You don't care, such cases don't happen? Second solution, it's faster.