1Z0-811 Object-Oriented Programming Practice Question
Which TWO statements are true about interfaces in Java?
⚠ Common exam trap
The trap here is that candidates often forget that interfaces can have private methods (Java 9+) and default methods (Java 8), and mistakenly think interfaces are purely abstract with only public abstract methods and constants.
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
✓
Interfaces can contain private methods.
Since Java 9, interfaces can contain private methods to share common code between default methods or static methods within the interface, without exposing that logic to implementing classes. Option D is correct because all variables declared in an interface are implicitly public, static, and final, meaning they are constants that cannot be changed once assigned.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Interfaces can contain private methods.
Why this is correct
Since Java 9, interfaces can have private methods for code reuse.
- ✗
Interfaces can be instantiated.
Why it's wrong here
Interfaces cannot be instantiated.
- ✗
Interfaces can contain constructors.
Why it's wrong here
Interfaces cannot have constructors.
- ✓
All interface variables are implicitly public static final.
Why this is correct
Interface fields are constants.
- ✗
Interfaces cannot have default methods.
Why it's wrong here
Default methods are allowed since Java 8.
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