Can Subclasses Throw Broader Checked Exceptions When Overriding Methods in Java?

Exception Propagation Limitations in Method Overriding

In Java, method overriding grants subclasses the flexibility to implement a different behavior for a method inherited from a superclass. However, this process is governed by certain restrictions, one of which concerns the exception handling capabilities of overriding methods.

Specifically, overriding methods are prohibited from declaring checked exceptions that are broader than those declared by the overridden method. This means that if a superclass method throws a specific checked exception, a subclass implementation cannot declare an exception that is not a subclass of the superclass's exception.

Rationale Behind the Restriction

This restriction exists for reasons of polymorphism and type safety. When a subclass overrides a method and changes the exception behavior, it could unintentionally break the contract established by the superclass.

Consider the following example:

class Superclass {
    public void doSomething() throws IOException {
        // Implementation
    }
}

class Subclass extends Superclass {
    @Override
    public void doSomething() throws SQLException { // Not allowed
        // Implementation
    }
}

If we allowed the doSomething() method in Subclass to throw a broader exception (SQLException), it could potentially undermine the assumption made by code that calls the doSomething() method through a reference to the superclass.

In the above scenario, a try-catch block that handles IOException in the calling code would not catch the SQLException thrown by the overriding method. This could lead to unexpected errors and program crashes.

Implications for Exception Handling

The restriction on broader exceptions in overriding methods ensures that subclasses can only expand the set of potential exceptions thrown by the overridden method, not introduce new or broader exceptions.

Additionally, unchecked exceptions (such as NullPointerException or IllegalArgumentException) are not subject to this rule. Subclass methods are free to declare unchecked exceptions regardless of whether the overridden method declares them.

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