Why Does Incorrectly Overriding Java's `equals()` Method Lead to Unexpected Behavior?

Overriding the Java equals() Method: Understanding the Object Requirement

In Java, the equals() method plays a crucial role in determining object equality. As described, the equals() method was overridden to compare objects of type Book. However, an issue arose when a Book object was initialized with only the ID, leading to unexpected behavior.

To understand the discrepancy, it's important to note that Java defines the equals() method as:

public boolean equals(Object other);

This means that the parameter of the overridden method should be of type Object. In the provided code:

public boolean equals(Book b) {
    ... // More code here
}

The equals() method was overloaded rather than overridden, as it had a parameter of type Book instead of Object. This difference led to unexpected behavior because the ArrayList, which performs equality comparisons using the overridden equals() method, was using the overloaded method instead.

Correctly overriding equals() requires using the Object parameter type:

@Override
public boolean equals(Object o) {
    Book b = (Book) o;
    ... // The rest goes here   
}

With this modification, the equals() method properly compares Book objects by converting the Object parameter to type Book.

In summary, when overriding methods, it's essential to adhere to the parameter types specified in the method signature of the superclass or interface. Failing to do so can lead to unexpected behavior or errors, highlighting the importance of testing and debugging code thoroughly.

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