Consider the following Java program. What is the output of the program? class A { int x = 10; }
class B extends A { int x = 20; public static void main(String[] args) {
A obj = new B(); System.out.print(obj.x); }
}

Question

BlackTom AI · Accepted Answer

Field access in Java is resolved at compile time according to the reference (declared) type, not the actual runtime (object) type. This is called field hiding (not polymorphic overriding).

Relevant concepts:
- Instance methods are polymorphic (virtual): overridden methods are chosen at runtime based on the object's actual type.
- Fields are not polymorphic: if a subclass declares a field with the same name, it hides the superclass field. Access to a field is determined by the compile-time type of the reference.

Step-by-step reasoning:
1. class A declares int x = 10;
2. class B extends A and declares int x = 20; — this hides A.x in B.
3. In main: A obj = new B(); — declared type of obj is A, actual object is a B instance.
4. System.out.print(obj.x); — compiler looks at the declared type of obj (A) and binds the field access to A.x.
5. A.x has value 10, so the printed output is 10.

No intermediate numeric calculations are needed; the key “step” is resolving which field is accessed based on the reference type.

Why the correct answer is correct:
Because obj is declared as type A, obj.x refers to A’s field x (10). Field hiding means B’s x (20) does not affect which field is accessed when the reference type is A.

Why other options are incorrect:
- A. Compilation error: incorrect — the code is syntactically valid; hiding fields is allowed.
- C. Runtime error: incorrect — no exception occurs; object creation and field access are valid.
- D. 20: incorrect — that would be true only if the field access used a B reference (e.g., ((B)obj).x) or if x were accessed via an overridden method; fields do not use runtime polymorphism.

Conclusion: The program prints 10, so the correct choice is B. 10, which matches the provided answer.

Consider the following Java program. What is the output of the program? class A { int x = 10; } class B extends A { int x = 20; public static void main(String[] args) { A obj = new B(); System.out.print(obj.x); } }单项选择题

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