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.

Practice

1. What does the final keyword do when used with a class in PHP?

easy

A. Prevents the class from being extended by other classes

B. Allows the class to be extended multiple times

C. Makes the class abstract

D. Allows the class methods to be overridden

Solution

1. Step 1: Understand the role of final with classes

   The final keyword before a class means no other class can extend it.

2. Step 2: Compare with other options

   Options A, B, and D describe behaviors that are opposite or unrelated to final classes.

3. Final Answer:

   Prevents the class from being extended by other classes -> Option A

4. Quick Check:

   final class = no extension allowed [OK]

Hint: Final class means no child classes allowed [OK]

Common Mistakes:

• Thinking final class can be extended
• Confusing final with abstract
• Assuming final allows method overrides

2. Which of the following is the correct syntax to declare a final method in a PHP class?

easy

A. function final myMethod() {}

B. final function myMethod() {}

C. function myMethod() final {}

D. final myMethod function() {}

Solution

1. Step 1: Recall correct final method syntax

   In PHP, the final keyword comes before the function keyword when declaring a final method.

2. Step 2: Check each option

   final function myMethod() {} matches the correct syntax. Options A, C, and D have incorrect order or placement of keywords.

3. Final Answer:

   final function myMethod() {} -> Option B

4. Quick Check:

   final before function keyword = correct syntax [OK]

Hint: final keyword always before function name in declaration [OK]

Common Mistakes:

• Placing final after function name
• Writing final after function keyword incorrectly
• Mixing order of keywords

3. What will be the output of the following PHP code?

final class Car {
    final public function start() {
        return "Engine started";
    }
}

class SportsCar extends Car {
    public function start() {
        return "Sports engine started";
    }
}

$car = new SportsCar();
echo $car->start();

medium

A. Engine started

B. Sports engine started

C. Fatal error: Cannot override final method Car::start()

D. Fatal error: Cannot extend final class Car

Solution

1. Step 1: Analyze class and method declarations

   The class Car is declared as final, so it cannot be extended. But the code tries to extend it with SportsCar.

2. Step 2: Identify the error

   Since Car is final, extending it causes a fatal error. But the question is about overriding a final method. Actually, the class is final, so extending is not allowed at all.

3. Step 3: Check options carefully

   Fatal error: Cannot extend final class Car says cannot extend final class, which matches the error. Fatal error: Cannot override final method Car::start() says cannot override final method, which would be error if class was not final but method was final.

4. Final Answer:

   Fatal error: Cannot extend final class Car -> Option D

5. Quick Check:

   final class cannot be extended = fatal error [OK]

Hint: Final class cannot be extended, causes fatal error [OK]

Common Mistakes:

• Confusing final method error with final class error
• Assuming method override works despite final class
• Ignoring that class is final and cannot be extended

4. Identify the error in the following PHP code:

class Vehicle {
    final public function move() {
        return "Moving";
    }
}

class Bike extends Vehicle {
    public function move() {
        return "Bike moving";
    }
}

medium

A. No error, code runs fine

B. Cannot extend class Vehicle

C. Cannot override final method move() in class Bike

D. Syntax error in method declaration

Solution

1. Step 1: Understand final method behavior

   The method move() in Vehicle is declared as final, so child classes cannot override it.

2. Step 2: Check the child class method

   The Bike class tries to override move(), which is not allowed and causes a fatal error.

3. Final Answer:

   Cannot override final method move() in class Bike -> Option C

4. Quick Check:

   final method cannot be overridden [OK]

Hint: Final method cannot be changed in child classes [OK]

Common Mistakes:

• Thinking final method can be overridden
• Confusing final method with abstract method
• Assuming no error when overriding final method

5. You want to create a base class Document that cannot be extended, but you want to allow extending a method print() in child classes. Which is the correct approach?

hard

A. Do not declare class Document as final, and declare method print() normally

B. Declare class Document as final, and method print() normally (not final)

C. Do not declare class Document as final, but declare method print() as final

D. Declare class Document as final, and method print() as final

Solution

1. Step 1: Understand final class effect

   Declaring a class as final prevents any extension, so no child classes can exist to override methods.

2. Step 2: Understand method overriding

   To allow overriding print(), the class must be extendable and the method must not be final.

3. Step 3: Choose correct option

   Do not declare class Document as final, and declare method print() normally allows extending the class and overriding the method. Options B and D prevent extension. Do not declare class Document as final, but declare method print() as final prevents method overriding.

4. Final Answer:

   Do not declare class Document as final, and declare method print() normally -> Option A

5. Quick Check:

   Allow extension and method override = no final on class or method [OK]

Hint: Final class blocks extension; omit final to allow overrides [OK]

Common Mistakes:

• Declaring class final but expecting method override
• Declaring method final when override is needed
• Confusing final class and final method effects