Why Does Overriding `equals()` with a Specific Parameter (e.g., `Book`) Break Java's Equality Checks?

Why Overriding Java's equals() Method with a Book Parameter Causes Issues

Overriding the equals() method is essential for implementing custom equality checks in Java. However, using a custom equals() method with a Book parameter instead of the standard equals(Object) method can lead to unexpected behavior.

Understanding the equals() Method

By default, Java defines the equals() method in the Object class that takes an Object parameter. If you want to override equals() in your custom class to compare objects of a specific type, you must adhere to the same signature, i.e., equals(Object).

Overriding vs. Overloading

Overriding is defined as providing a new implementation for a method inherited from a superclass. Overloading, on the other hand, is reusing a method name with different parameter types.

In the provided code, the public boolean equals(Book b) method constitutes overloading, not overriding. This is because it uses a different parameter type compared to the original equals(Object) method.

The Issue with Overloading

When you overload the equals() method, it breaks compatibility with the standard library classes that use equals(Object) for equality checks. For example, the ArrayList uses equals(Object) to determine whether an element is present in the list.

Therefore, using the overloaded equals(Book) method in the hasBook() method will cause contains() to fail when comparing Book objects instantiated with only an ID.

Solution

To resolve this issue, the equals() method should be overridden correctly by following the equals(Object) signature. This ensures compatibility with the standard library classes that rely on equals(Object) for equality checks.

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