What are the Java Exception Handling Restrictions When Overriding Methods?

Restrictions on Exception Handling in Overridden Methods

The Java programming language enforces certain rules regarding exceptions that can be thrown by overridden methods. These rules ensure adherence to the Liskov Substitution Principle and maintain the integrity of polymorphism.

Rule: The overriding method cannot declare checked exceptions that are broader than those declared by the overridden method.

Explanation:

An overridden method can declare to throw the same checked exception as the overridden method or a subclass of that exception. If the overriding method declares a broader exception, it violates the Liskov Substitution Principle, which states that objects should be substitutable within the same type hierarchy without breaking the program.

Example:

class Parent {
    public void method() throws IOException {
        // ...
    }
}

class Child extends Parent {
    @Override
    public void method() throws SQLException {
        // ...
    }
}

In this example, the overriding method method cannot throw SQLException because it is a broader checked exception than IOException declared by the overridden method.

Justification:

If the overriding method were allowed to throw a broader exception, client code calling the overridden method would not be forced to catch the broader exception. However, when the overriding method is called through a reference to the superclass, the client code may not be prepared to handle the broader exception. This could lead to unchecked exceptions being thrown during runtime, violating the assumptions of the caller.

Unrestricted Unchecked Exceptions:

Unchecked exceptions, on the other hand, are not subject to these rules. The overriding method is free to throw any unchecked exception regardless of what is declared by the overridden method. This provides greater flexibility in handling runtime errors.

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