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Why do two different HashSets with the same data have the same HashCode?

I recently ran across a problem on leetcode which I solved with a nested hashset. This is the problem, if you're interested: https://leetcode.com/problems/group-anagrams/.
My intuition was to add all of the letters of each word into a hashset, then put that hashset into another hashset. At each iteration, I would check if the hashset already existed, and if it did, add to the existing hashset.
Oddly enough, that seems to work. Why do 2 hashsets share the same hashcode if they are different objects? Would something like if(set1.hashCode() == set2.hashCode()) doStuff() be valid code?

This is expected. HashSet extends AbstractSet. The hashCode() method in AbstractSet says:
Returns the hash code value for this set. The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the hash code of a null element is defined to be zero. This ensures that s1.equals(s2) implies that s1.hashCode()==s2.hashCode() for any two sets s1 and s2, as required by the general contract of Object.hashCode.
This implementation iterates over the set, calling the hashCode method on each element in the set, and adding up the results.
Here's the code from AbstractSet:
public int hashCode() {
int h = 0;
Iterator<E> i = iterator();
while (i.hasNext()) {
E obj = i.next();
if (obj != null)
h += obj.hashCode();
}
return h;
}
Why do 2 hashsets share the same hashcode if they are different objects?
With HashSet, the hashCode is calculated using the contents of the set. Since it's just numeric addition, the order of addition doesn't matter – just add them all up. So it makes sense that you have two sets, each containing objects which are equivalent (and thus should have matching hashCode() values), and then the sum of hashCodes within each set is the same.
Would something like if(set1.hashCode() == set2.hashCode()) doStuff() be valid code?
Sure.
EDIT: The best way of comparing two sets for equality is to use equals(). In the case of AbstractSet, calling set1.equals(set2) would result in individual calls to equals() at the level of the objects within the set (as well as some other checks).

Why do two different HashSets with the same data have the same
HashCode?
Actually this is needed to fulfill another need that is specified in Java.
The equals method of Set is overridden to take in consideration that equals returns true (example a.equals(b)) if:
a is of type Set and b is of type Set.
both a and b have exactly the same size.
a contains all elements of b.
b contains all elements of a.
Since the default equals (which compares only the memory reference to be the same) is overridden for Set, according to java guidelines the hashCode method has to be overridden as well. So, this custom implementation of hashCode is provided in order to match with the custom implementation of equals.
In order to see why it is necessary to override hashCode method when the equals method is overridden, you can take a look at this previous answer of mine.
Why do 2 hashsets share the same hashcode if they are different
objects
Because as explained above this is needed so that Set can have the custom functionality for equals that it currently has.
If you want to just check if a and b are different instances of set you can still check this with operators == and !=.
a == b -> true means a and b point to the same instance of Set in memory
a != b -> true means a and b point to different instances of Set in memory

If the hashcode() creates hashcode based on the address of the object, how can two different objects with same contents create the same hashcode?

When you read the description of hashCode() in the Object class, it says that
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.
I've read an article and it says that an object's hashcode() can provide different integer values if it runs in different environments even though their content is the same.
It does not happen with String class's hashcode() because the String class's hashcode() creates integer value based on the content of the object. The same content always creates the same hash value.
However, it happens if the hash value of the object is calculated based on its address in the memory. And the author of the article says that the Object class calculates the hash value of the object based on its address in the memory.
In the example below, the objects, p1 and p2 have the same content so equals() on them returns true.
However, how come these two objects return the same hash value when their memory addresses are different?
Here is the example code: main()
Person p1 = new Person2("David", 10);
Person p2 = new Person2("David", 10);
boolean b = p1.equals(p2);
int hashCode1 = p1.hashCode();
int hashCode2 = p2.hashCode();
Here is the overriden hashcode()
public int hashCode(){
return Objects.hash(name, age);
}
Is the article's content wrong?
If there is a hashCode() that calculates a hash value based on the instance's address what is the purpose of them?
Also, if it really exists, it violates the condition that Object class's hashCode() specifies. How should we use the hashCode() then?

I think you have misunderstood this statement:
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.
The word “must” means that you, the programmer, are required to write a hashCode() method which always produces the same hashCode value for two objects which are equal to each other according to their equals(Object) method.
If you don’t, you have written a broken object that will not work with any class that uses hash codes, particularly unsorted Sets and Maps.
I've read an article and it says that an object's hashcode() can provide different integer values if it runs in different environments even though their content is the same.
Yes, hashCode() can provide different values in different runtimes, for a class which does not override the hashCode() method.
… how come these two objects return the same hash value when their memory addresses are different?
Because you told them to. You overrode the hashCode() method.
If there is a hashCode() that calculates a hash value based on the instance's address what is the purpose of them?
It doesn’t have a lot of use. That’s why programmers are strongly recommended to override the method, unless they don’t care about object identity.
Also, if it really exists, it violates the condition that Object class's hashCode() specifies. How should we use the hashCode() then?
No it does not. The contract states:
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.
As long as the hashCode method defined in the Object class returns the same value for the duration of the Java runtime, it is compliant.
The contract also says:
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.
Two objects whose equals method reports that they are effectively equal must return the same hashCode. Any instance of the Object class whose type is not a subclass of Object is only equal to itself and cannot be equal to any other object, so any value it returns is valid, as long as it is consistent throughout the life of the Java runtime.

The article is wrong, memory address is not involved (btw. the address can change during the lifecycle of objects, so it's abstracted away as a reference). You can look at the default hashCode as a method that returns a random integer, which is always the same for the same object.
Default equals (inherited from Object) works exactly like ==. Since there is a condition (required for classes like HashSet etc. to work) that states when a.equals(b) then a.hashCode == b.hashCode, then we need a default hashCode which only has to have one property: when we call it twice, it must return the same number.
The default hashCode exists exactly so that the condition you mention is upheld. The value returned is not important, it's only important that it never changes.

No p1 and p2 does not have the same content
If you do p1.equals(p2) that will be false, so not the same content.
If you want p1 and p2 to equal, you need to implement the equals methods from object, in a way that compare their content. And IF you implement the equals method, then you also MUST implement the hashCode method, so that if equals return true, then the objects have the same hashCode().

Here's the design decision every programmer needs to make for objects they define of (say) MyClass:
Do you want it to be possible for two different objects to be "equal"?
If you do, then firstly you have to implement MyClass.equals() so that it gives the correct notion of equality for your purposes. That's entirely in your hands.
Then you're supposed to implement hashCode such that, if A.equals(B), then A.hashCode() == B.hashCode(). You explicitly do not want to use Object.hashCode().
If you don't want different objects to ever be equal, then don't implement equals() or hashCode(), use the implementations that Object gives you. For Object A and Object B (different Objects, and not subclasses of Object), then it is never the case that A.equals(B), and so it's perfectly ok that A.hashCode() is never the same as B.hashCode().

Should I override hashCode() of Collections?

Given that I some class with various fields in it:
class MyClass {
private String s;
private MySecondClass c;
private Collection<someInterface> coll;
// ...
#Override public int hashCode() {
// ????
}
}
and of that, I do have various objects which I'd like to store in a HashMap. For that, I need to have the hashCode() of MyClass.
I'll have to go into all fields and respective parent classes recursively to make sure they all implement hashCode() properly, because otherwise hashCode() of MyClass might not take into consideration some values. Is this right?
What do I do with that Collection? Can I always rely on its hashCode() method? Will it take into consideration all child values that might exist in my someInterface object?
I OPENED A SECOND QUESTION regarding the actual problem of uniquely IDing an object here: How do I generate an (almost) unique hash ID for objects?
Clarification:
is there anything more or less unqiue in your class? The String s? Then only use that as hashcode.
MyClass hashCode() of two objects should definitely differ, if any of the values in the coll of one of the objects is changed. HashCode should only return the same value if all fields of two objects store the same values, resursively. Basically, there is some time-consuming calculation going on on a MyClass object. I want to spare this time, if the calculation had already been done with the exact same values some time ago. For this purpose, I'd like to look up in a HashMap, if the result is available already.
Would you be using MyClass in a HashMap as the key or as the value? If the key, you have to override both equals() and hashCode()
Thus, I'm using the hashCode OF MyClass as the key in a HashMap. The value (calculation result) will be something different, like an Integer (simplified).
What do you think equality should mean for multiple collections? Should it depend on element ordering? Should it only depend on the absolute elements that are present?
Wouldn't that depend on the kind of Collection that is stored in coll? Though I guess ordering not really important, no
The response you get from this site is gorgeous. Thank you all
#AlexWien that depends on whether that collection's items are part of the class's definition of equivalence or not.
Yes, yes they are.

I'll have to go into all fields and respective parent classes recursively to make sure they all implement hashCode() properly, because otherwise hashCode() of MyClass might not take into consideration some values. Is this right?
That's correct. It's not as onerous as it sounds because the rule of thumb is that you only need to override hashCode() if you override equals(). You don't have to worry about classes that use the default equals(); the default hashCode() will suffice for them.
Also, for your class, you only need to hash the fields that you compare in your equals() method. If one of those fields is a unique identifier, for instance, you could get away with just checking that field in equals() and hashing it in hashCode().
All of this is predicated upon you also overriding equals(). If you haven't overridden that, don't bother with hashCode() either.
What do I do with that Collection? Can I always rely on its hashCode() method? Will it take into consideration all child values that might exist in my someInterface object?
Yes, you can rely on any collection type in the Java standard library to implement hashCode() correctly. And yes, any List or Set will take into account its contents (it will mix together the items' hash codes).

So you want to do a calculation on the contents of your object that will give you a unique key you'll be able to check in a HashMap whether the "heavy" calculation that you don't want to do twice has already been done for a given deep combination of fields.
Using hashCode alone:
I believe hashCode is not the appropriate thing to use in the scenario you are describing.
hashCode should always be used in association with equals(). It's part of its contract, and it's an important part, because hashCode() returns an integer, and although one may try to make hashCode() as well-distributed as possible, it is not going to be unique for every possible object of the same class, except for very specific cases (It's easy for Integer, Byte and Character, for example...).
If you want to see for yourself, try generating strings of up to 4 letters (lower and upper case), and see how many of them have identical hash codes.
HashMap therefore uses both the hashCode() and equals() method when it looks for things in the hash table. There will be elements that have the same hashCode() and you can only tell if it's the same element or not by testing all of them using equals() against your class.
Using hashCode and equals together
In this approach, you use the object itself as the key in the hash map, and give it an appropriate equals method.
To implement the equals method you need to go deeply into all your fields. All of their classes must have equals() that matches what you think of as equal for the sake of your big calculation. Special care needs to be be taken when your objects implement an interface. If the calculation is based on calls to that interface, and different objects that implement the interface return the same value in those calls, then they should implement equals in a way that reflects that.
And their hashCode is supposed to match the equals - when the values are equal, the hashCode must be equal.
You then build your equals and hashCode based on all those items. You may use Objects.equals(Object, Object) and Objects.hashCode( Object...) to save yourself a lot of boilerplate code.
But is this a good approach?
While you can cache the result of hashCode() in the object and re-use it without calculation as long as you don't mutate it, you can't do that for equals. This means that calculation of equals is going to be lengthy.
So depending on how many times the equals() method is going to be called for each object, this is going to be exacerbated.
If, for example, you are going to have 30 objects in the hashMap, but 300,000 objects are going to come along and be compared to them only to realize that they are equal to them, you'll be making 300,000 heavy comparisons.
If you're only going to have very few instances in which an object is going to have the same hashCode or fall in the same bucket in the HashMap, requiring comparison, then going the equals() way may work well.
If you decide to go this way, you'll need to remember:
If the object is a key in a HashMap, it should not be mutated as long as it's there. If you need to mutate it, you may need to make a deep copy of it and keep the copy in the hash map. Deep copying again requires consideration of all the objects and interfaces inside to see if they are copyable at all.
Creating a unique key for each object
Back to your original idea, we have established that hashCode is not a good candidate for a key in a hash map. A better candidate for that would be a hash function such as md5 or sha1 (or more advanced hashes, like sha256, but you don't need cryptographic strength in your case), where collisions are a lot rarer than a mere int. You could take all the values in your class, transform them into a byte array, hash it with such a hash function, and take its hexadecimal string value as your map key.
Naturally, this is not a trivial calculation. So you need to think if it's really saving you much time over the calculation you are trying to avoid. It is probably going to be faster than repeatedly calling equals() to compare objects, as you do it only once per instance, with the values it had at the time of the "big calculation".
For a given instance, you could cache the result and not calculate it again unless you mutate the object. Or you could just calculate it again only just before doing the "big calculation".
However, you'll need the "cooperation" of all the objects you have inside your class. That is, they will all need to be reasonably convertible into a byte array in such a way that two equivalent objects produce the same bytes (including the same issue with the interface objects that I mentioned above).
You should also beware of situations in which you have, for example, two strings "AB" and "CD" which will give you the same result as "A" and "BCD", and then you'll end up with the same hash for two different objects.

For future readers.
Yes, equals and hashCode go hand in hand.
Below shows a typical implementation using a helper library, but it really shows the "hand in hand" nature. And the helper library from apache keeps things simpler IMHO:
#Override
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null || getClass() != o.getClass()) {
return false;
}
MyCustomObject castInput = (MyCustomObject) o;
boolean returnValue = new org.apache.commons.lang3.builder.EqualsBuilder()
.append(this.getPropertyOne(), castInput.getPropertyOne())
.append(this.getPropertyTwo(), castInput.getPropertyTwo())
.append(this.getPropertyThree(), castInput.getPropertyThree())
.append(this.getPropertyN(), castInput.getPropertyN())
.isEquals();
return returnValue;
}
#Override
public int hashCode() {
return new org.apache.commons.lang3.builder.HashCodeBuilder(17, 37)
.append(this.getPropertyOne())
.append(this.getPropertyTwo())
.append(this.getPropertyThree())
.append(this.getPropertyN())
.toHashCode();
}
17, 37 .. those you can pick your own values.

From your clarifications:
You want to store MyClass in an HashMap as key.
This means the hashCode() is not allowed to change after adding the object.
So if your collections may change after object instantiation, they should not be part of the hashcode().
From http://docs.oracle.com/javase/8/docs/api/java/util/Map.html
Note: great care must be exercised if mutable objects are used as map
keys. The behavior of a map is not specified if the value of an object
is changed in a manner that affects equals comparisons while the
object is a key in the map.
For 20-100 objects it is not worth that you enter the risk of an inconsistent hash() or equals() implementation.
There is no need to override hahsCode() and equals() in your case.
If you don't overide it, java takes the unique object identity for equals and hashcode() (and that works, epsecially because you stated that you don't need an equals() considering the values of the object fields).
When using the default implementation, you are on the safe side.
Making an error like using a custom hashcode() as key in the HashMap when the hashcode changes after insertion, because you used the hashcode() of the collections as part of your object hashcode may result in an extremly hard to find bug.
If you need to find out whether the heavy calculation is finished, I would not absue equals(). Just write an own method objectStateValue() and call hashcode() on the collection, too. This then does not interfere with the objects hashcode and equals().
public int objectStateValue() {
// TODO make sure the fields are not null;
return 31 * s.hashCode() + coll.hashCode();
}
Another simpler possibility: The code that does the time consuming calculation can raise an calculationCounter by one as soon as the calculation is ready. You then just check whether or not the counter has changed. this is much cheaper and simpler.

Logical Operators exhibiting strange behavior

if ((File_1[i].ID.equals(File_2[j].ID))
&& (File_1[i].Relation.equals(File_2[j].Relation)))
The first condition here, (File_1[i].ID.equals(File_2[j].ID) shows false when it is supposed to be true. They both have equal values.
Am I doing something wrong? This is unusual.
ID: fa001
ID: fa001
These are the values of the first 2 variables that are compared, but it shows up as false.

The original poster has said the ID is a String type and there are whitespaces around the String. In order to remove the whitespaces of String s:
s = s.trim();
However, if ID is a type you have created, make sure you implement the .hashCode() method. From the Javadocs:
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.
Also make sure that .equals() is overridden from class Object to allow for your definition of whether the values of your new object type are equal.

Depending on the TYPE of ID (int, string, char[], etc) the .equals method can do some weird things. Assuming they are strings, try comparing ID.trim(), which will remove any witespace around the ID.

Java equals and hashmaps problem

So I'm learning Java and I'm trying to understand equals, but I really don't understand it. So my question is: how does equals works?
Thanks

equals is a method you use to discover if two given objects are the same. The default implementation of equals for an object is: They are the same if they have the exact same reference.
Sometimes, you don't want this. Say you have a ComplexNumber class, and two instances with value 1+i. You don't want them not to be equal just because they are different instances. In essence, they represent the same number. In this case, you should override equals to make sure it behaves as intended.
HashMaps use info from equals to know if the key you passed is already there.
From Effective Java book:
Always override hashcode when you
override equals
I'd also add to that: specially if you're using a Hashmap =)
Hashmaps uses also hashcode() to search faster for the keys, and the result for hashcode() must be consistent with the equals result. In other words, if x.equals(y), then x.hashcode() == y.hashcode() (or you may have undefined behavior for your hashmap). You may have x and y with x.hashcode() == y.hashcode() and !x.equals(y)
If you want a more specific answer, please make a more specific question =).

The equals method will compare values for equality.
the method equals is defined in the object class so which means that every other class can use this method to compare how it works: it will first check if its referring to its self then the haschode's of the object and if those are equal if so, it will then check each field in that object against the fields of the object you are comaparing to why you might ask co's the haschode could be the same but it can still contain other value's in the fields the odds are low but its needed to compare more in depth then.

equals() means "meaningfully equivalent." It's not the same as ==, which means "these are the same object." All classes have the equals() method, inheirited from the Object class. For example, say you write a Car class that stores make, model and owner:
Car carOne = new Car("Buick", "LeSabre", "John Doe");
Car carTwo = carOne;
Here, equals() and == will both return true, because both references point to the same car. But, for
Car carOne = new Car("Buick", "LeSabre", "John Doe");
Car carTwo = new Car("Buick", "LeSabre", "John Doe");
there are two distinct objects, so == returns false. However, since both cars are Buick LeSabres owned by John Doe, your equals() should be written return true (assuming for this example that nobody owns more than one car of the same type).
Also, as Samuel pointed out, if you override equals(), you should also override hashCode(); the reasons for that are outside the scope of this question, and well-documented elsewhere.