How to compare two objects in java example

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

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This post will discuss how to compare two objects in Java.

You should never use the == operator for comparing two objects, since the == operator performs a reference comparison and it simply checks if the two objects refer to the same instance or not. The recommended option to compare two objects is using the equals() method. However, simply calling the equals() method won’t work as expected, as shown below:

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The above program will call the equals() method of the Object class, which behaves similar to the == operator as it is not overridden by a class. Therefore, every class should override the equals (and hashCode) method of the Object class and specify the equivalence relation on objects, such that it evaluates the comparison of values in the object irrespective of whether two objects refer to the same instance or not. See this post for more details.

Here’s a simple program that demonstrates the working of the equals() method.

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You must also override the hashCode() method, since equal objects must have equal hash codes. If you prefer not to override the equals() and hashCode() method, you can create a custom function to check for equality. This is demonstrated below:

Download  Run Code

That’s all about comparing two objects in Java.

Thanks for reading.

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