1Z0-811 Object-Oriented Programming Practice Question
A developer wants to create a class that can be used to represent different types of vehicles (e.g., Car, Truck, Motorcycle) and each vehicle type should be able to start its own engine in a specific way. Which OOP concept should be used to allow the vehicle class to define a common interface while letting subclasses provide specific implementations?
⚠ Common exam trap
Oracle often tests the distinction between abstraction (defining the interface) and polymorphism (using that interface to invoke different implementations at runtime), so candidates mistakenly choose abstraction when the question emphasizes 'specific implementations' and runtime behavior.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Polymorphism
Polymorphism allows the Vehicle class to define a common method (e.g., startEngine()) that each subclass (Car, Truck, Motorcycle) overrides with its own specific implementation. When the method is called on a Vehicle reference, the JVM uses dynamic method dispatch at runtime to invoke the correct subclass version, enabling different engine-starting behaviors through a single interface.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Polymorphism
Why this is correct
Polymorphism allows a common interface to have different implementations, achieved via method overriding.
- ✗
Inheritance
Why it's wrong here
Inheritance enables code reuse but does not by itself allow runtime method selection.
- ✗
Encapsulation
Why it's wrong here
Encapsulation hides data, not behavior variation.
- ✗
Abstraction
Why it's wrong here
Abstraction reduces complexity by hiding implementation, but it does not provide polymorphic behavior.
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Same concept, more angles
2 more ways this is tested on 1Z0-811
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A developer writes a class 'Vehicle' with a method 'move()' that prints 'Vehicle moves'. A subclass 'Car' overrides 'move()' to print 'Car moves'. Given: Vehicle v = new Car(); v.move(); What is the output?
medium- A.Vehicle moves
- B.Runtime exception
- C.Compilation fails
- ✓ D.Car moves
Why D: Java uses dynamic method dispatch (runtime polymorphism). Even though the reference variable is of type 'Vehicle', the actual object is a 'Car' instance. At runtime, the JVM calls the overridden 'move()' method of the 'Car' class, printing 'Car moves'.
Variation 2. Refer to the exhibit. What is the output when the following code is executed? Vehicle v = new Car(); v.accelerate(); System.out.println(v.speed);
medium- A.10
- B.0
- C.Compilation error
- ✓ D.20
Why D: The code uses polymorphism: the reference variable v of type Vehicle points to a Car object. Since accelerate() is overridden in Car, the overridden method is invoked, adding 20 to the speed instance variable inherited from Vehicle. The protected field speed is accessible in the subclass. Therefore, after calling v.accelerate(), speed becomes 20. Option D is correct.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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