How to compare two java objects

Question

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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           amp  amp  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

User · Answer

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

[java] How to compare two java objects

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