1Z0-811 Object-Oriented Programming Practice Question
Exhibit
public class Singleton { private static Singleton instance; private Singleton() {} public static Singleton getInstance() { if (instance == null) { instance = new Singleton(); } return instance; } }Refer to the exhibit. What is the potential issue with this singleton implementation in a multithreaded environment?
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
✓
Not thread-safe; two threads could simultaneously create different instances
The if-check and instantiation are not atomic. Two threads could both see instance == null and create separate instances, violating the singleton guarantee.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The getInstance() method should be non-static
Why it's wrong here
A static method is appropriate for a singleton to provide global access without an instance.
- ✗
Compilation error because the constructor is private
Why it's wrong here
Private constructor is intended and does not cause a compile error.
- ✗
Memory leak because instances are never garbage collected
Why it's wrong here
The singleton holds a reference, so it is not garbage collected, but this is not a leak; it's intentional.
- ✓
Not thread-safe; two threads could simultaneously create different instances
Why this is correct
Correct. The check-then-act sequence is not synchronized.
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