Final classes and methods in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of final method call in subclass

What is the output of this PHP code?

PHP

<?php
class Base {
    final public function greet() {
        echo "Hello from Base";
    }
}

class Child extends Base {
    public function greet() {
        echo "Hello from Child";
    }
}

$child = new Child();
$child->greet();
?>

A Fatal error: Cannot override final method Base::greet()

B Hello from Child

C Hello from Base

D No output

💡 Hint

Remember that final methods cannot be overridden in subclasses.

The method greet() in Base is declared final, so Child cannot override it. Trying to do so causes a fatal error.

❓ Predict Output

intermediate

Output when instantiating a final class

What will this PHP code output?

PHP

<?php
final class FinalClass {
    public function sayHi() {
        return "Hi!";
    }
}

class SubClass extends FinalClass {
}

$obj = new SubClass();
echo $obj->sayHi();
?>

A Fatal error: Class SubClass may not inherit from final class (FinalClass)

B Hi!

C Parse error: syntax error

D No output

💡 Hint

Final classes cannot be extended.

FinalClass is declared final, so no class can extend it. Trying to extend causes a fatal error.

🔧 Debug

advanced

Identify the error in final method redeclaration

This PHP code causes an error. What is the error message?

PHP

<?php
class ParentClass {
    final public function doSomething() {
        echo "Doing something";
    }
}

class ChildClass extends ParentClass {
    public function doSomething() {
        echo "Doing something else";
    }
}
?>

A No error, code runs fine

B Fatal error: Cannot override final method ParentClass::doSomething()

C Parse error: syntax error, unexpected 'public' (T_PUBLIC)

D Warning: Method ChildClass::doSomething() should be compatible with ParentClass::doSomething()

💡 Hint

Final methods cannot be overridden in child classes.

The child class tries to override a final method from the parent, which causes a fatal error.

🧠 Conceptual

advanced

Why use final classes or methods?

Which of the following is the best reason to declare a class or method as final in PHP?

A To make the class or method private and inaccessible outside the class

B To allow multiple inheritance from the final class

C To prevent the class or method from being extended or overridden, ensuring consistent behavior

D To automatically optimize the code for faster execution

💡 Hint

Think about controlling how code can be changed by others.

Declaring a class or method final prevents other classes from changing its behavior by extending or overriding it, which helps maintain consistency and security.

❓ Predict Output

expert

Output of final method called via parent reference

What is the output of this PHP code?

PHP

<?php
class A {
    final public function show() {
        echo "A";
    }
}

class B extends A {
    public function show() {
        echo "B";
    }
}

function callShow(A $obj) {
    $obj->show();
}

$b = new B();
callShow($b);
?>

A Type error: Argument 1 passed to callShow() must be of type A, B given

B B

C A

D Fatal error: Cannot override final method A::show()

💡 Hint

Check if class B can override the final method show() from class A.

Class B tries to override the final method show() from class A, which causes a fatal error before the function callShow is executed.