1Z0-811 Polymorphism Practice Question
Exhibit
public class Vehicle { protected int speed; public Vehicle() { speed = 0; } public void accelerate() { speed += 10; } } public class Car extends Vehicle { public void accelerate() { speed += 20; } }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);
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
✓
20
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
10
Why it's wrong here
Incorrect because the Vehicle's accelerate method is not called due to dynamic binding; Car's overridden method adds 20, not 10.
- ✗
0
Why it's wrong here
Incorrect because speed is modified by the accelerate method; it does not remain 0.
- ✗
Compilation error
Why it's wrong here
Incorrect because the code compiles successfully: Vehicle has the accelerate method and speed is accessible.
- ✓
20
Why this is correct
Correct because the Car's overridden accelerate method adds 20 to speed, resulting in 20.
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