Does ArrayList's `contains()` Method Use Object Equality or Reference Equality?

Understanding Object Comparison in ArrayList's contains() Method

The contains() method in ArrayList evaluates whether a given object exists within the list. When determining object equality, ArrayList utilizes the equals() method defined within the object's class.

Consider the following scenario: you create a Thing object with an integer value and add it to an ArrayList. If you subsequently create another Thing object with an identical value, will the contains() method identify them as equivalent?

Implementation Considerations

For the contains() method to return true, the equals() method within the Thing class must be implemented appropriately. In this case, the equals() method correctly compares the value instance variable between the objects, returning true if they are identical.

Conclusion

The contains() method in ArrayList delegates object comparison to the equals() method defined in the object's class. By implementing equals() to compare relevant instance variables, you can ensure that objects are correctly identified as equal or not in ArrayList operations.

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