Question at position 3 Consider the following class declarations. Which of the following best describes why an error occurs when the classes are compiled?The class Alpha does not have a defined constructor.The class Beta must be declared as a subclass of Alpha.The answer method cannot be accessed from a class other than Alpha.The result of the method call item.answer() cannot be assigned to a variable of type double.

Question

Question at position 3 Consider the following class declarations. Which of the following best describes why an error occurs when the classes are compiled?The class Alpha does not have a defined constructor.The class Beta must be declared as a subclass of Alpha.The answer method cannot be accessed from a class other than Alpha.The result of the method call item.answer() cannot be assigned to a variable of type double.

BlackTom AI · Accepted Answer

Private methods are only accessible within the class that declares them, so a call from another class will fail at compile time.

1) Relevant knowledge points and concepts
- Java access modifiers: private methods can only be invoked from within the same class; public (and package-private/protected) have broader visibility.
- If no constructor is defined, Java supplies a default public no-argument constructor.
- Primitive widening conversions: an int can be assigned to a double without an error.

2) Step-by-step reasoning through the code
- Alpha defines: private int answer() { return 10; } — this method has private access.
- Beta.sample() does: Alpha item = new Alpha(); — instantiation is valid because Alpha has an implicit public default constructor.
- Beta then does: double temp = item.answer(); — this attempts to invoke Alpha.answer() from a different class (Beta), which is not allowed because answer() is private.
- The compiler will reject this call before considering the assignment/widening conversion.

3) Intermediate steps (what the compiler checks)
- Check that class Alpha is instantiable: OK (default constructor provided).
- Resolve the method call item.answer(): compiler verifies visibility and finds answer() exists but with private access.
- Because access is illegal, compilation fails with an error such as: "answer() has private access in Alpha" (or similar).

4) Why the correct answer is correct
- The error arises specifically because Beta (a different class) is trying to call a private method of Alpha. Java’s access control prevents this, causing a compile-time error.

5) Why the other options are incorrect
- "Alpha does not have a defined constructor.": false — Java provides a default constructor automatically.
- "Beta must be declared as a subclass of Alpha.": false — subclassing is not required to call public/package methods; here the issue is access level, not inheritance.
- "The result ... cannot be assigned to a variable of type double.": false — int to double is a legal widening conversion.

6) Conclusion
The compile error is caused by trying to access Alpha.answer() from another class, so the correct answer is: "The answer method cannot be accessed from a class other than Alpha." This matches the provided answer.

类似问题

When using inheritance, fields of the superclass are typically declared as:

The protected access modifier allows access from:

A sub-class has no access to private instance variables of its super class.

____________ access modifier is only visible in parent and child class, but not in other packages.

____________ access modifier is only visible in parent and child class, but not in other packages.

Choose the correct option to be included for the <access-modifier>

_______ is the access specifier to make a class member accessible only within the same class.

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