This question already has answers here:
Why do I need to override the equals and hashCode methods in Java?
(31 answers)
Closed 9 years ago.
Why we should always override hashCode() method whenever we override equals() method? As per my understanding they both seem to fulfill different purpose. hashCode() method is used in hashtables to determine the equality of keys. However equals() method is used to determine the equality of two objects.
If two objects are equal according to the equals method, their hashcodes must be equal as well. Otherwise, lookup in the hash table would fail.
Related
This question already has answers here:
How is hashCode() calculated in Java
(11 answers)
Closed 3 years ago.
The hashCode() method return a serie of numbers, but two different objects can have the same result.
So how this function is really calculating that value,internally ?
Is that related to memory case, and what's the element included in th calculation?
According to the Object javadoc, hashCode method may returns the object's address in memory at some point in time
The hashCode may or may not be implemented as some function of an
object's memory address at some point in time
https://docs.oracle.com/javase/10/docs/api/java/lang/Object.html#hashCode()
Remember that this method is useful to compare whether two or more or object are the same together with equals method,but you can override this method to return the same hashcode if some attributes (e.g PKs) are the same although them are different objects in memory
This question already has answers here:
Why do I need to override the equals and hashCode methods in Java?
(31 answers)
Closed 7 years ago.
I know there are lots of similar questions out there but I have not satisfied by the answers I have read. I tried to figure it out but I still did not get the idea.
What I know is these two are important while using set or map especially HashSet, HashMap or Hash objects in general which use hash mechanism for storing element objects.
Both methods are used to test if two Objects are equal or not.
For two objects A and B to be equal first they need to have the same hash value( have to be in the same bucket) and second we have to get true while executing A.equals(B).
What I do not understand is, WHY is it necessary to override both of these methods.
WHAT if we do not override hashcode. IS IT A MUST TO OVERRIDE BOTH.If it is not what is the disadvantage of overriding equals and not hashcode and vice versa.
Properly implementing hashCode is necessary for your object to be a key in hash-based containers. It is not necessary for anything else.
Here's why it is important for hash-based containers such as HashMap, HashSet, ConcurrentHashMap etc.
At a high level, a HashMap is an array, indexed by the hashCode of the key, whose entries are "chains" - lists of (key, value) pairs where all keys in a particular chain have the same hash code. For a refresher on hashtables, see Wikipedia.
Consider what happens if two keys A, B are equal, but have a different hash code - for example, a.hashCode() == 42 and b.hashCode() == 37. Suppose you write:
hashTable.put(a, "foo");
hashTable.get(b);
Since the keys are equal, you would like the result to be "foo", right?
However, get(b) will look into the chain corresponding to hash 37, while the pair (a, "foo") is located in the chain corresponding to hash 42, so the lookup will fail and you'll get null.
This is why it is important that equal objects have equal hash codes if you intend to use the object as a key in a hash-based container.
Note that if you use a non-hash based container, such as TreeMap, then you don't have to implement hashCode because the container doesn't use it. Instead, in case of TreeMap, you should implement compareTo - other types of containers may have their own requirements.
Yes it's correct when you override equals method you have to override hashcode method as well. The reason behind is that in hash base elements two objects are equal if their equals method return true and their hashcode method return same integer value. In hash base elements (hash map) when you make the equal check for two objects first their hashcode method is get called, if it return same value for both then only equals method is get called. If hashcode don't return same value for both then it simplity consider both objects as not equal. By default the hashcode method return some random value, so if you are making two objects equal for some specific condition by overriding equals method, they still won't equal because their hashcode value is different, so in order to make their hascode value equal you have to override it. Otherwise you won't be able to make this object as a key to your hash map.
This question already has answers here:
What issues should be considered when overriding equals and hashCode in Java?
(11 answers)
Closed 9 years ago.
I created a HashMap having Student as key and String as value.
Now everywhere i have read It is necessary to override equals and hashcode method if using as a key for hashmap.
But I did not override it. And insert multiple key value pairs in hashmap.
I am also able to fetch it back.
So why it is necessary?
They are required when you want to use your objects as useful hashing keys into collections that use hashing.
Have your tried to add the Student objects which are meaningfully equal?
First understand the significance of equals method in java
Read the significance of equals and hashcode
This question already has answers here:
What issues should be considered when overriding equals and hashCode in Java?
(11 answers)
Closed 9 years ago.
Should the equals (Object) method be overridden when overriding hashCode() in Java?
I have read the contract that overriding equals, you should override hashCode. Is the vice versa true?
I was thinking of a scenario where I don't compare objects, no equals method.
Yes, it should be overridden. If you think you need to override hashCode(), then you need to override equals() and vice versa. The general contract of hashCode() is:
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 hashtables.
Joshua Bloch makes it clear in chapter 3 of his "Effective Java" - these must be in lockstep. They should always be overridden together.
This question already has answers here:
Compare two objects with .equals() and == operator
(16 answers)
Closed 9 years ago.
I have two java objects that are instantiated from the same class.
MyClass myClass1 = new MyClass();
MyClass myClass2 = new MyClass();
If I set both of their properties to the exact same values and then verify that they are the same
if(myClass1 == myClass2){
// objects match
...
}
if(myClass1.equals(myClass2)){
// objects match
...
}
However, neither of these approaches return a true value. I have checked the properties of each and they match.
How do I compare these two objects to verify that they are identical?
You need to provide your own implementation of equals() in MyClass.
#Override
public boolean equals(Object other) {
if (!(other instanceof MyClass)) {
return false;
}
MyClass that = (MyClass) other;
// Custom equality check here.
return this.field1.equals(that.field1)
&& this.field2.equals(that.field2);
}
You should also override hashCode() if there's any chance of your objects being used in a hash table. A reasonable implementation would be to combine the hash codes of the object's fields with something like:
#Override
public int hashCode() {
int hashCode = 1;
hashCode = hashCode * 37 + this.field1.hashCode();
hashCode = hashCode * 37 + this.field2.hashCode();
return hashCode;
}
See this question for more details on implementing a hash function.
You need to Override equals and hashCode.
equals will compare the objects for equality according to the properties you need and hashCode is mandatory in order for your objects to be used correctly in Collections and Maps
You need to implement the equals() method in your MyClass.
The reason that == didn't work is this is checking that they refer to the same instance. Since you did new for each, each one is a different instance.
The reason that equals() didn't work is because you didn't implement it yourself yet. I believe it's default behavior is the same thing as ==.
Note that you should also implement hashcode() if you're going to implement equals() because a lot of java.util Collections expect that.
You have to correctly override method equals() from class Object
Edit: I think that my first response was misunderstood probably because I was not too precise. So I decided to to add more explanations.
Why do you have to override equals()? Well, because this is in the domain of a developer to decide what does it mean for two objects to be equal. Reference equality is not enough for most of the cases.
For example, imagine that you have a HashMap whose keys are of type Person. Each person has name and address. Now, you want to find detailed bean using the key. The problem is, that you usually are not able to create an instance with the same reference as the one in the map. What you do is to create another instance of class Person. Clearly, operator == will not work here and you have to use equals().
But now, we come to another problem. Let's imagine that your collection is very large and you want to execute a search. The naive implementation would compare your key object with every instance in a map using equals(). That, however, would be very expansive. And here comes the hashCode(). As others pointed out, hashcode is a single number that does not have to be unique. The important requirement is that whenever equals() gives true for two objects, hashCode() must return the same value for both of them. The inverse implication does not hold, which is a good thing, because hashcode separates our keys into kind of buckets. We have a small number of instances of class Person in a single bucket. When we execute a search, the algorithm can jump right away to a correct bucket and only now execute equals for each instance. The implementation for hashCode() therefore must distribute objects as evenly as possible across buckets.
There is one more point. Some collections require a proper implementation of a hashCode() method in classes that are used as keys not only for performance reasons. The examples are: HashSet and LinkedHashSet. If they don’t override hashCode(), the default Object
hashCode() method will allow multiple objects that you might consider "meaningfully
equal" to be added to your "no duplicates allowed" set.
Some of the collections that use hashCode()
HashSet
LinkedHashSet
HashMap
Have a look at those two classes from apache commons that will allow you to implement equals() and hashCode() easily
EqualsBuilder
HashCodeBuilder
1) == evaluates reference equality in this case
2) im not too sure about the equals, but why not simply overriding the compare method and plant it inside MyClass?