Can Java Overridden Methods Have Covariant Return Types, and What Are the Restrictions on Exception Handling?

Covariant Return Types in Java

In Java, the return type of an overridden method can be covariant, meaning it can return a more derived type than the overridden method. However, this rule does not apply to exceptions thrown by an overridden method.

Exception Throwing Restrictions in Overriding Methods

The compiler enforces the following restrictions on overridden methods regarding exception handling:

• The overridden method cannot throw new or broader checked exceptions than those declared by the overridden method.
• The overridden method can throw narrower or new unchecked exceptions.

Explanation

These restrictions ensure that subclasses cannot introduce new or more severe checked exceptions to the interface of the overridden method. This is necessary for polymorphism, as it allows code written against the superclass to handle exceptions appropriately.

Example

Consider the following example:

class A {
    public void foo() throws FileNotFoundException {
        // ...
    }
}

class B extends A {
    @Override
    public void foo() throws SQLException { // ERROR: broader checked exception
        // ...
    }
}

In this example, class A defines the method foo that throws a FileNotFoundException. Class B, which extends A, attempts to override foo by declaring a broader checked exception, SQLException. This is not allowed because it violates the exception throwing restrictions.

Rationale

The compiler enforces these restrictions to:

• Preserve the contract: The overridden method's interface should not be modified in a way that breaks existing code that relies on it.
• Ensure object substitution: Objects of subclasses must be substitutable for objects of the superclass, and this includes handling exceptions appropriately.

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