I have a Functional Interface in the code below which has one abstract method and one object method override. So when I write Lambda expression for that , how can I implement my equals method.
import static java.lang.System.out;
public class Test {
public static void main(String[] args) {
AddToString test = a -> (a + " End");
out.println(test.stringManipulation("some string"));
out.println(test.increment(5));
out.println(test.equals(null));
}
}
#FunctionalInterface
interface AddToString {
String stringManipulation(String a);
default int increment(int a) { return a+1; }
#Override
public boolean equals(Object obj);
}
One way to do that is to create Anonymous class like given below, but is there a better method using lambda expressions -
public class Test {
public static void main(String[] args) {
AddToString test = new AddToString() {
public String stringManipulation(String a) {
return a + " End";
}
#Override
public boolean equals(Object Obj) {
//Just testing whether it overrides
return 5==5;
}
};
out.println(test.stringManipulation("some string"));
out.println(test.increment(5));
out.println(test.equals(null));
}
}
You can't. If you need to override equals, you'll need to create a class (anonymous or otherwise), you can't do it with a lambda.
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.
I have the code below:
class Animal {
int i;
public Animal(){
i=13;
}
public String toString(){
return "Animal "+i;
}
}
//
public class NewMain {
public static void main(String[] args) {
HashSet hs=new HashSet();
boolean b=hs.add(new Animal());
boolean b1=hs.add(new Animal());
System.out.println(b+" "+b1);
for(Object o:hs)
{
System.out.println(hs);
}
}
}
I know that Set does not allow duplicates, but why does the above code seem to allow two identical values?
Because Java has no way of knowing that your two instances are the same.
You need to override equals() and hashcode()
.
As has already been pointed out, you need to override hashCode and equals. You can compare on the int i with something like,
class Animal {
int i;
public Animal() {
i = 13;
}
#Override
public String toString() {
return "Animal " + i;
}
#Override
public int hashCode() {
return Integer.hashCode(i);
}
#Override
public boolean equals(Object obj) {
if (obj instanceof Animal) {
return ((Animal) obj).i == i;
}
return false;
}
}
Then I would suggest you use generics with your Collection. Also, you should print the instance (not the Collection) in your loop. Something like,
public static void main(String[] args) {
Set<Animal> hs = new HashSet<>();
boolean b = hs.add(new Animal());
boolean b1 = hs.add(new Animal());
System.out.println(b + " " + b1);
for (Animal a : hs) {
System.out.println(a);
}
}
And I get (the expected)
true false
Animal 13
You haven't overridden hashcode and equals in the Animal class. Therefore, these methods are inherited from java.lang.Object.
The inherited behaviour (from Object) is that objects are only equal if they are the same object.
In this case, a suitable pair of overrides for equals and hashcode would be something like this:
#Override
public boolean equals(Object other) {
return other instanceof Animal &&
other != null &&
((Animal) other).i == this.i;
}
#Override
public int hashCode() {
return i;
}
(Note that if you start creating subclasses of Animal, you need to think carefully on how best to implement equals(Object). By code above doesn't attempt to deal with that ...)
I tried to override equals and hashcode methods in a class. It is a subclass of another class which does not implement the equals method and hashCode methods.
Eclipse gave the below warning .
The super class ABC does not implement equals() and hashCode() methods.
The resulting code may not work correctly.
Why is the above warning given ? Under what circumstances it may not work correctly ?
If you say a.equals(b) versus b.equals(a) it is reasonable to expect the behaviour to be the same. But if they are of corresponding types B and A related by inheritance and only one of them properly implements equals then the behaviour will be different in those two examples.
Here, A is the superclass and does not implement equals at all (so it inherits java.lang.Object.equals). Subclass B overrides equals to depend on the name field.
class A {
String name;
public A() {
this.name = "Fred";
}
}
class B extends A {
public boolean equals(Object o) {
A a = (A)o;
return a != null && a.name.equals(this.name);
}
}
public class Test {
public static void main(String[] args) {
A a = new A();
B b = new B();
System.out.println(a.equals(b) == b.equals(a));
}
}
Unsurprisingly, the output is false, thus breaking symmetry.
Have you tried super class override the equals ... and then auto generate subclass override implementation...
I am sure it will be differnt. it will have call to super.equals()
in current auto generated implementation it is only checking values in child class..
Consider below scenario and you will understand why warning.
abstract Class A{
private int a;
public void setA(int a){
this.a=a;
}
}
Class B extends A{
private int x;
public void setX(int x){
this.x=x;
}
#Override
public boolean equals(Object obj) { // This does not call Super.equals
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
B other = (B) obj;
if (x != other.x)
return false;
return true;
}
}
and in main Method try
B b1= new B();
b1.setA(10);
b1.setX(20);
B b2= new B();
b2.setA(20);
b2.setX(20);
if(b1.equals(b2)){
System.out.println("Warning was Right");
}
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.