Understanding Why the contains Method Works While equals Fails in Java

Explore the intricacies of Java's `contains` and `equals` methods. Understand how default behavior impacts equality checks, particularly with custom classes like Employee.
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Understanding Why the contains Method Works While equals Fails in Java

In the world of Java programming, encountering unexpected behaviors in method outputs can be a source of confusion and frustration, especially when working with collections. A commonly discussed issue is related to the contains method in lists, particularly when dealing with custom objects such as instances of a class defined by the user.

The Problem

You might be wondering, why does the contains method return true while the equals method returns false in certain scenarios? Let's take a look at a practical example using an Employee class:

[[See Video to Reveal this Text or Code Snippet]]

In the example above, we see an interesting behavior:

The Explanation

To understand why this occurs, we need to dive into how Java's List and its methods work under the hood.

Default Equals Method

Reference Equality: The default implementation of the equals method in Java checks for reference equality. This means it compares whether two references point to the exact same object in memory.

Behavior in contains: When you call contains on an ArrayList, it iterates over the elements and checks if any element is considered equal to the provided argument by calling equals on each element.

Example Walkthrough:

Customization Needed for Value Equality

If you want to check for logical equality (i.e., whether two different Employee instances are considered equal based on their properties rather than their memory addresses), you must override both the equals and hashCode methods:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

In summary, the contains method works as expected because it simply checks for reference equality by default. If you want both equals and contains to work in a way that considers the properties of the object instead of the memory references, you need to provide custom implementations for these methods.

By overriding equals and hashCode, you enable logical equality checks, ensuring that methods like contains function as one might intuitively expect.

This adjustment not only helps clarify behavior but also ensures that collections operate correctly with your custom objects. Happy coding!