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 · Accepted 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)
        && 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

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 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

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

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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