Consider the following Java code:

What is the output of running:

Question

Consider the following Java code:

What is the output of running:

BlackTom AI · Accepted Answer

The key concept is that Java calls an object's toString() when concatenating it with a String and that overridden methods are resolved at runtime (polymorphism). So each object's runtime class determines which toString() executes.

1) Relevant knowledge points
- String concatenation with a non-String object triggers the object's toString().
- Method overriding: the most-derived implementation is invoked at runtime.
- super.toString() calls the parent class's implementation.

2) Step-by-step reasoning
- Foo defines toString() -> "Foo".
- Boo extends Foo and overrides toString(): return super.toString() + "Boo".
  - For a Boo instance, super.toString() calls Foo.toString() => "Foo", then adds "Boo" => "FooBoo".
- Bar extends Boo and overrides toString(): return super.toString() + "Bar".
  - For a Bar instance, super.toString() calls Boo.toString() (because Boo is Bar's immediate parent). Boo.toString() calls Foo.toString() => "Foo", adds "Boo" => "FooBoo". Then Bar appends "Bar" => "FooBooBar".

3) Intermediate string evaluation (showing concatenation)
- foo is new Foo(): "" + foo => "" + foo.toString() => "" + "Foo" => "Foo"
- bar is new Bar(): bar.toString() => Foo -> Boo -> Bar yields "FooBooBar"
- boo is new Boo(): boo.toString() => Foo -> Boo yields "FooBoo"

4) Why the correct answer is correct
System.out.println("" + foo + ", " + bar + ", " + boo) builds the single output string by calling toString() on each object in order. That produces the three values computed above separated by commas and spaces.

5) Why other answers would be incorrect
- Any option that has the wrong order misunderstands the print argument order.
- Any option that omits the chained super calls (e.g., expecting "Bar" alone or "Boo" for bar) ignores method overriding and super.toString() chaining.

Conclusion: The printed output is exactly:
Foo, FooBooBar, FooBoo

This matches the provided answer.

Consider the following Java code: What is the output of running:简答题

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