1Z0-811 Object-Oriented Programming Practice Question
Which statement about abstract classes and interfaces is true in Java?
⚠ Common exam trap
Candidates often confuse the capabilities of abstract classes and interfaces, mistakenly thinking abstract classes cannot have concrete methods or that interfaces can have constructors, leading them to select options like A, B, or D.
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
✓
An abstract class can have concrete methods
An abstract class in Java can contain concrete (non-abstract) methods with full implementations, which subclasses can inherit or override. This is a key distinction from interfaces (prior to Java 8), which could only declare abstract methods. Abstract classes cannot be instantiated directly, but they provide a partial implementation that concrete subclasses complete.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Both B and D
Why it's wrong here
Incorrect because D is false.
- ✗
An interface can have instance variables
Why it's wrong here
Incorrect; interface variables are implicitly public static final constants.
- ✗
An abstract class can be instantiated
Why it's wrong here
Incorrect; abstract classes cannot be instantiated directly.
- ✗
An interface can have constructor
Why it's wrong here
Incorrect; interfaces cannot have constructors.
- ✓
An abstract class can have concrete methods
Why this is correct
Correct. Abstract classes can have both abstract and concrete methods.
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