I am trying to override equals method in Java. I have a class People which basically has 2 data fields name and age. Now I want to override equals method so that I can check between 2 People objects.
My code is as follows
public boolean equals(People other){
boolean result;
if((other == null) || (getClass() != other.getClass())){
result = false;
} // end if
else{
People otherPeople = (People)other;
result = name.equals(other.name) && age.equals(other.age);
} // end else
return result;
} // end equals
But when I write age.equals(other.age) it gives me error as equals method can only compare String and age is Integer.
Solution
I used == operator as suggested and my problem is solved.
//Written by K#stackoverflow
public class Main {
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
ArrayList<Person> people = new ArrayList<Person>();
people.add(new Person("Subash Adhikari", 28));
people.add(new Person("K", 28));
people.add(new Person("StackOverflow", 4));
people.add(new Person("Subash Adhikari", 28));
for (int i = 0; i < people.size() - 1; i++) {
for (int y = i + 1; y <= people.size() - 1; y++) {
boolean check = people.get(i).equals(people.get(y));
System.out.println("-- " + people.get(i).getName() + " - VS - " + people.get(y).getName());
System.out.println(check);
}
}
}
}
//written by K#stackoverflow
public class Person {
private String name;
private int age;
public Person(String name, int age){
this.name = name;
this.age = age;
}
#Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if (obj.getClass() != this.getClass()) {
return false;
}
final Person other = (Person) obj;
if ((this.name == null) ? (other.name != null) : !this.name.equals(other.name)) {
return false;
}
if (this.age != other.age) {
return false;
}
return true;
}
#Override
public int hashCode() {
int hash = 3;
hash = 53 * hash + (this.name != null ? this.name.hashCode() : 0);
hash = 53 * hash + this.age;
return hash;
}
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;
}
}
Output:
run:
-- Subash Adhikari - VS - K false
-- Subash Adhikari - VS - StackOverflow false
-- Subash Adhikari - VS - Subash Adhikari true
-- K - VS - StackOverflow false
-- K - VS - Subash Adhikari false
-- StackOverflow - VS - Subash Adhikari false
-- BUILD SUCCESSFUL (total time: 0 seconds)
Introducing a new method signature that changes the parameter types is called overloading:
public boolean equals(People other){
Here People is different than Object.
When a method signature remains the identical to that of its superclass, it is called overriding and the #Override annotation helps distinguish the two at compile-time:
#Override
public boolean equals(Object other){
Without seeing the actual declaration of age, it is difficult to say why the error appears.
I'm not sure of the details as you haven't posted the whole code, but:
remember to override hashCode() as well
the equals method should have Object, not People as its argument type. At the moment you are overloading, not overriding, the equals method, which probably isn't what you want, especially given that you check its type later.
you can use instanceof to check it is a People object e.g. if (!(other instanceof People)) { result = false;}
equals is used for all objects, but not primitives. I think you mean age is an int (primitive), in which case just use ==. Note that an Integer (with a capital 'I') is an Object which should be compared with equals.
See What issues should be considered when overriding equals and hashCode in Java? for more details.
Item 10: Obey the general contract when overriding equals
According to Effective Java, Overriding the equals method seems simple, but there are many ways to get it wrong, and consequences can be dire. The easiest way to avoid problems is not to override the equals method, in which case each instance of the class is equal only to itself. This is the right thing to do if any of the following conditions apply:
Each instance of the class is inherently unique. This is true for classes such as Thread that represent active entities rather than values. The equals implementation provided by Object has exactly the right behavior for these classes.
There is no need for the class to provide a “logical equality” test. For example, java.util.regex.Pattern could have overridden equals to check whether two Pattern instances represented exactly the same regular expression, but the designers didn’t think that clients would need or want this functionality. Under these circumstances, the equals implementation inherited from Object is ideal.
A superclass has already overridden equals, and the superclass behavior is appropriate for this class. For example, most Set implementations inherit their equals implementation from AbstractSet, List implementations from AbstractList, and Map implementations from AbstractMap.
The class is private or package-private, and you are certain that its equals method will never be invoked. If you are extremely risk-averse, you can override the equals method to ensure that it isn’t invoked accidentally:
The equals method implements an equivalence relation. It has these properties:
Reflexive: For any non-null reference value x, x.equals(x) must return true.
Symmetric: For any non-null reference values x and y, x.equals(y) must return true if and only if y.equals(x) returns true.
Transitive: For any non-null reference values x, y, z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) must return true.
Consistent: For any non-null reference values x and y, multiple invocations of x.equals(y) must consistently return true or consistently return false, provided no information used in equals comparisons is modified.
For any non-null reference value x, x.equals(null) must return false.
Here’s a recipe for a high-quality equals method:
Use the == operator to check if the argument is a reference to this object. If so, return true. This is just a performance optimization but one that is worth doing if the comparison is potentially expensive.
Use the instanceof operator to check if the argument has the correct type. If not, return false. Typically, the correct type is the class in which the method occurs. Occasionally, it is some interface implemented by this class. Use an interface if the class implements an interface that refines the equals contract to permit comparisons across classes that implement the interface. Collection interfaces such as Set, List, Map, and Map.Entry have this property.
Cast the argument to the correct type. Because this cast was preceded by an instanceof test, it is guaranteed to succeed.
For each “significant” field in the class, check if that field of the argument matches the corresponding field of this object. If all these tests succeed, return true; otherwise, return false. If the type in Step 2 is an interface, you must access the argument’s fields via interface methods; if the type is a class, you may be able to access the fields directly, depending on their accessibility.
For primitive fields whose type is not float or double, use the == operator for comparisons; for object reference fields, call the equals method recursively; for float fields, use the static Float.compare(float, float) method; and for double fields, use Double.compare(double, double). The special treatment of float and double fields is made necessary by the existence of Float.NaN, -0.0f and the analogous double values; While you could compare float and double fields with the static methods Float.equals and Double.equals, this would entail autoboxing on every comparison, which would have poor performance. For array fields, apply these guidelines to each element. If every element in an array field is significant, use one of the Arrays.equals methods.
Some object reference fields may legitimately contain null. To avoid the possibility of a NullPointerException, check such fields for equality using the static method Objects.equals(Object, Object).
// Class with a typical equals method
public final class PhoneNumber {
private final short areaCode, prefix, lineNum;
public PhoneNumber(int areaCode, int prefix, int lineNum) {
this.areaCode = rangeCheck(areaCode, 999, "area code");
this.prefix = rangeCheck(prefix, 999, "prefix");
this.lineNum = rangeCheck(lineNum, 9999, "line num");
}
private static short rangeCheck(int val, int max, String arg) {
if (val < 0 || val > max)
throw new IllegalArgumentException(arg + ": " + val);
return (short) val;
}
#Override public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof PhoneNumber))
return false;
PhoneNumber pn = (PhoneNumber)o;
return pn.lineNum == lineNum && pn.prefix == prefix
&& pn.areaCode == areaCode;
}
... // Remainder omitted
}
#Override
public boolean equals(Object that){
if(this == that) return true;//if both of them points the same address in memory
if(!(that instanceof People)) return false; // if "that" is not a People or a childclass
People thatPeople = (People)that; // than we can cast it to People safely
return this.name.equals(thatPeople.name) && this.age == thatPeople.age;// if they have the same name and same age, then the 2 objects are equal unless they're pointing to different memory adresses
}
When comparing objects in Java, you make a semantic check, comparing the type and identifying state of the objects to:
itself (same instance)
itself (clone, or reconstructed copy)
other objects of different types
other objects of the same type
null
Rules:
Symmetry: a.equals(b) == b.equals(a)
equals() always yields true or false, but never a NullpointerException, ClassCastException or any other throwable
Comparison:
Type check: both instances need to be of the same type, meaning you have to compare the actual classes for equality. This is often not correctly implemented, when developers use instanceof for type comparison (which only works as long as there are no subclasses, and violates the symmetry rule when A extends B -> a instanceof b != b instanceof a).
Semantic check of identifying state: Make sure you understand by which state the instances are identified. Persons may be identified by their social security number, but not by hair color (can be dyed), name (can be changed) or age (changes all the time). Only with value objects should you compare the full state (all non-transient fields), otherwise check only what identifies the instance.
For your Person class:
public boolean equals(Object obj) {
// same instance
if (obj == this) {
return true;
}
// null
if (obj == null) {
return false;
}
// type
if (!getClass().equals(obj.getClass())) {
return false;
}
// cast and compare state
Person other = (Person) obj;
return Objects.equals(name, other.name) && Objects.equals(age, other.age);
}
Reusable, generic utility class:
public final class Equals {
private Equals() {
// private constructor, no instances allowed
}
/**
* Convenience equals implementation, does the object equality, null and type checking, and comparison of the identifying state
*
* #param instance object instance (where the equals() is implemented)
* #param other other instance to compare to
* #param stateAccessors stateAccessors for state to compare, optional
* #param <T> instance type
* #return true when equals, false otherwise
*/
public static <T> boolean as(T instance, Object other, Function<? super T, Object>... stateAccessors) {
if (instance == null) {
return other == null;
}
if (instance == other) {
return true;
}
if (other == null) {
return false;
}
if (!instance.getClass().equals(other.getClass())) {
return false;
}
if (stateAccessors == null) {
return true;
}
return Stream.of(stateAccessors).allMatch(s -> Objects.equals(s.apply(instance), s.apply((T) other)));
}
}
For your Person class, using this utility class:
public boolean equals(Object obj) {
return Equals.as(this, obj, t -> t.name, t -> t.age);
}
Since I'm guessing age is of type int:
public boolean equals(Object other){
boolean result;
if((other == null) || (getClass() != other.getClass())){
result = false;
} // end if
else{
People otherPeople = (People)other;
result = name.equals(otherPeople.name) && age == otherPeople.age;
} // end else
return result;
} // end equals
if age is int you should use == if it is Integer object then you can use equals().
You also need to implement hashcode method if you override equals. Details of the contract is available in the javadoc of Object and also at various pages in web.
tl;dr
record Person ( String name , int age ) {}
if(
new Person( "Carol" , 27 ) // Compiler auto-generates implicitly the constructor.
.equals( // Compiler auto-generates implicitly the `equals` method.
new Person( "Carol" , 42 )
)
) // Returns `false`, as the name matches but the age differs.
{ … }
Details
While your specific problem is solved (using == for equality test between int primitive values), there is an alternative that eliminates the need to write that code.
record
Java 16 brings the record feature.
A record is a brief way to write a class whose main purpose is to transparently and immutably carry data. The compiler implicitly creates the constructor, getters, equals & hashCode, and toString.
equals method provided automatically
The default implicit equals method compares each and every member field that you declared for the record. The members can be objects or primitives, both types are automatically compared in the default equals method.
For example, if you have a Person record carrying two fields, name & age, both of those fields are automatically compared to determine equality between a pair of Person objects.
public record Person ( String name , int age ) {}
Try it.
Person alice = new Person( "Alice" , 23 ) ;
Person alice2 = new Person( "Alice" , 23 ) ;
Person bob = new Person( "Bob" , 19 ) ;
boolean samePerson1 = alice.equals( alice2 ) ; // true.
boolean samePerson2 = alice.equals( bob ) ; // false.
You can override the equals method on a record, if you want a behavior other than the default. But if you do override equals, be sure to override hashCode for consistent logic, as you would for a conventional Java class. And, think twice: Whenever adding methods to a record, reconsider if a record structure is really appropriate to that problem domain.
Tip: A record can be defined within another class, and even locally within a method.
Here is the solution that I recently used:
public class Test {
public String a;
public long b;
public Date c;
public String d;
#Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof Test)) {
return false;
}
Test testOther = (Test) obj;
return (a != null ? a.equals(testOther.a) : testOther.a == null)
&& (b == testOther.b)
&& (c != null ? c.equals(testOther.c) : testOther.c == null)
&& (d != null ? d.equals(testOther.d) : testOther.d == null);
}
}
For lazy programmers: lombok library is very easy and time saving. please have a look at this link
instead of writing lines of codes and rules, you just need to apply this library in your IDE and then just #Data and it is Done.
import lombok.Data;
#Data // this is the magic word :D
public class pojo {
int price;
String currency;
String productName;
}
in fact in the above code, #Data is a shortcut for
import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.Getter;
import lombok.Setter;
import lombok.ToString;
#Getter
#Setter
#EqualsAndHashCode
#ToString
//or instead of all above #Data
public class pojo {
int price;
String currency;
String productName;
}
import java.util.HashMap;
import java.util.Map;
class Geek
{
public String name;
public int id;
Geek(String name, int id)
{
this.name = name;
this.id = id;
}
#Override
public boolean equals(Object obj)
{
// checking if both the object references are
// referring to the same object.
if(this == obj)
return true;
// it checks if the argument is of the
// type Geek by comparing the classes
// of the passed argument and this object.
// if(!(obj instanceof Geek)) return false; ---> avoid.
if(obj == null || obj.getClass()!= this.getClass())
return false;
// type casting of the argument.
Geek geek = (Geek) obj;
// comparing the state of argument with
// the state of 'this' Object.
System.out.println("equals method ....."+(geek.name == this.name && geek.id == this.id));
return (geek.name == this.name && geek.id == this.id);
}
int counter = 0;
#Override
public int hashCode()
{
// We are returning the Geek_id
// as a hashcode value.
// we can also return some
// other calculated value or may
// be memory address of the
// Object on which it is invoked.
// it depends on how you implement
// hashCode() method.
++counter;
System.out.println("counter ::>>> "+counter);
return counter;
}
Driver code:
public static void main (String[] args)
{
Map<Geek, Integer> map = new HashMap<>();
// creating the Objects of Geek class.
Geek g1 = new Geek("aa", 1);
Geek g2 = new Geek("aa", 1);
map.put(g1, g1.id);
map.put(g2, g2.id);
map.forEach((k,v) -> {
System.out.println("key = "+k + "\n value = "+v);
});
/* else
System.out.println("Both Objects are not equal. "); */
}
Here, I am overriding the hashCode() method but still the map contains only one object which is g2. Why didn't the HashMap store two objects, given that my hashcode returns a different integer every time?
Even though my equals() method returns true for the same object, why is the HashMap not storing two objects? Can someone please guide me in this regard?
Your counter variable is an instance variable, so it's initialized to 0 for each Geek instance. Therefore, both g1 and g2 have the same hashCode() of 1 when you put them in the Map, and are considered identical by the HashMap, since they are equal to each other based on your equals implementation.
If you change counter to be static, you will get different hashCode() for the 2 instances of Geek, and they would be stored in separate map entries.
That said, your hashCode() implementation is very bad. If you call hashCode() for the same instance multiple times, you'll get a different result each time! This means that if you attempt to put g1 twice in the Map, it will probably put it twice, since the second put will see a different hashCode(), and will therefore search for the key in a different bucket.
hashCode() function must not change when you call it on the same object instance multiple times. You can't generate a new value each time you call it, right now you are doing it by incrementing counter.
As per Object.hashCode() javadoc:
Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
I'm making a 2D game that has a stars in it. I decided to create constructor in class named Star that gives random coordinates.
public Star(){
super(0,0);
x = randomX.nextInt(maxX - minX + 1);
y = randomY.nextInt(maxX - minY + 1);
}
Then, in other class I put them in HashSet
Set<Star> star = new HashSet<>();
public Set<Star> generateStars(){
while (star.size() < numberOfStars){
star.add(new Star());
}
return star;
}
Of course, I have render and tick methods but I think it's not worth to paste them. My lecturer told me that there can be same stars and to prevent that I should use identity function using hashcodes. Can someone help me figure that out ? I imagine that this function should check if the hashcodes are the same and if it's the case it should return only one value that way we will add 1 object instead of 2 into the HashSet. Am I right ?
Overriding the hashCode() method alone in your Star class will not work you will have to override the equals() method.
See the following code where we don't override the equals() method:
class Star {
int x, y;
public Star(int x, int y) {
this.x = x;
this.y = y;
}
#Override
public int hashCode() {
return Objects.hash(x, y);
}
}
public class Main {
public static void main(String[] args) {
Star s1 = new Star(0, 0);
Star s3 = new Star(0, 0);
Star s2 = new Star(31, -31*31);
Set<Star> set = new HashSet<>();
set.add(s1);
set.add(s2);
System.out.println(set.size());
}
}
This will print 3 (instead of 2 what you might expect).
The reason for this is that the add method of java.util.Set compares 2 objects based on equals() method and not based on hashCode() method.
In the above code for the Star class, if you add the equals() method the output will be 2 now. For your reference the you can override the equals() method as follows:
#Override
public boolean equals(Object startObject) {
if (this == startObject) return true;
if (startObject == null || getClass() != startObject.getClass()) return false;
Star star = (Star) startObject;
return x == star.x &&
y == star.y;
}
So why do you need to add hashCode()?
As you are using HashSet the add method behind the scene will call the equals() method and will also call hashCode() to decide the bucket in which the new object should be put. To maintain the contract of hashCode() and equals() Both should be overridden.
When ever you override equals(), it is recommended to override hashCode() also. (Vice-versa is also true). See this link for details.
Contract for hashCode() and equals(): If for two objects say o1 and o2, o1.equals(o2) is true then hash of o1 and o2 should be same.
Make sure that you understand this properly, from the above statement it is not implied that if the hash of 2 objects are same, then o1.equals(o2) should return true. It can be possible that for 2 objects, their hash is same but the o1.equals(o2) returns false
See here, what Object's hashCode() method guarantees.
See this link to get more detailed information about this topic.
In java, when adding an object to a HashSet, the add method uses the 'equals' method, which is part of the Object class (however you can override it) in order to determine if the set already contains the object you are trying to add, see : https://docs.oracle.com/javase/7/docs/api/java/util/HashSet.html
However, if you are overriding the equals method then you should also override the hashCode method, this is very well explained in the following post : Why do I need to override the equals and hashCode methods in Java?
If you are going to follow this advice, I would also advise using ApacheCommonsLang EqualsBuilder and HashCodeBuilder if your lecturer allows it as they provide a solid implementation based on the rules laid out in the book 'Effective Java' by Joshua Bloch
To override the equals method in your Star class, you want to think about the criteria that make two star objects equal. From your example this might be that both of them have the same x and y co-ordinates.
you can override hashCode() method from Object. So, in your Star class, add:
#Override
public int hashCode() {
in hash = .... //make your hash code about the coordinates of your star.
return hash;
}
so, when you place a new star with the same co-ordiante in the hash map, it will overwrite the previous start with the same co-ordiantes if already present in the map.
You have to implement hashCode and equals.
the code should be like this:
public static class Star {
int x, y;
public Star(int x, int y) {
this.x = x;
this.y = y;
}
#Override
public int hashCode() {
return x * 1000 + y;
}
#Override
public boolean equals(Object obj) {
if (obj instanceof Star) {
Star s = (Star) obj;
return this.x == s.x && this.y == s.y;
}
return false;
}
}
public static void main(String args[]) throws Exception {
HashSet<Star> set = new HashSet<Star>();
set.add(new Star(1, 1));
set.add(new Star(1, 1));
System.out.println(set.size());
}
note: choose the suitable hashcode function for you. I assumed here that y should always be less than 1000.
If you are using eclipse as your editor just right-click on the editor pane, go-to source then generate hashcode() and equals() pick the parameters you would like to consider. You will get an autogenerated function.
I have a hashset of Car objects eg Set<Car> cars = new HashSet<Car>.
The Car class is briefly given below
public abstract class Car {
protected Piece current;
protected Piece;
....
....
....
#Override
public booleanll) return false;
if ((obj.getClass() == this.getClass())) {
Car o = (Carrent.equals(o.current) && other.equals(o.other)) || (current.equals) && other.et)));
}
return false;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((current == null) ? 0 : current.hashCode());
result = prime =pokmob result;
}
As you can see above, Car is an abstract class so I have two other classes BigCar and SmallCar extending the Car abstract class. And my set contains BigCar and SmallCar objects.
Also note that these subclasses do not override the equals and hashcode method.
In your hashCode method, the two fields are not treated symmetrically - so two cars with current and other swapped will NOT have the same hashcode.
For example you could use this instead (also note the use of Objects::hash to avoid the null checks):
#Override public int hashCode() {
return Objects.hash(current) * Objects.hash(other);
}
Well, in your hashCode current and other are not interchangeable.
Lets denote current's value as x and other's value as y for some instance.
The value of hashCode is 31*(31+x)+y.
Now, for another instance, if other is x and current is y (which means that second instance should be equal to the first instance), hashCode is now 31*(31+y)+x which is not equal to 31*(31+x)+y for most values of x and y.
Hence you are breaking the contract of hashCode, since if a.equals(b) == true, a.hashCode() must be equal to b.hashCode().
According to the official documentation for the Java Hashtable class (https://docs.oracle.com/javase/7/docs/api/java/util/Hashtable.html), the get() opperation will return one of it's recorded values, if said value has a key that returns true when the parameter is fed into that key's equals() opperation.
So, in theory, the following code should return "Hello!" for both of the Hashtable's get() queries:
public static class Coordinates implements Serializable {
private int ex;
private int why;
public Coordinates(int xCo, int yCo) {
ex = xCo;
why = yCo;
}
public final int x() {
return ex;
}
public final int y() {
return why;
}
public boolean equals(Object o) {
if(o == null) {
return false;
} else if(o instanceof Coordinates) {
Coordinates c = (Coordinates) o;
return this.x() == c.x() && this.y() == c.y();
} else {
return false;
}
}
}
Hashtable<Coordinates, String> testTable = new Hashtable<Coordinates, String>();
Coordinates testKey = new Coordinates(3, 1);
testTable.put(testKey, "Hello!");
testTable.get(testKey); //This will return the "Hello" String as expected.
testTable.get(new Coordinates(3, 1)); //This will only return a null value.
However, get() doesn't work as it's supposed to. It seems to only work if you litterally feed it the exact same object as whatever was the original key.
Is there any way to correct this and get the Hashtable to function the way it's described in the documentation? Do I need to make any adjustments to the custom equals() opperation in the Coordinates class?
To be able to store and retrieve objects from hash-based collections you should implement/oeverride the equals() as well as hashCode() methods of the Object class. In your case, you have overridden the equals() and left the hashCode() method to its default implementation inherited from the Object.
Here is the general contract of the hashCode() method you must consider while implementing it:
Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.
And here is an example implementation that is generated from my IDE (as alread mentioned by #Peter in the comment area) which you can modify to suit your requirements:
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ex;
result = prime * result + why;
return result;
}