SCJP(OCPJP) 考古題解析 第１６題

Given:

class Atom {
    Atom() {System.out.print("atom ");}
}
class Rock extends Atom {
    Rock(String type) {System.out.print(type);}
}
public class Mountain extends Rock {
    Mountain() {
        super("granite ");
        new Rock("granite ");
    }
    public static void main(String[] a) {new Mountain();}
}

What is the result?


A. Compilation fails.
B. atom granite
C. granite granite
D. atom granite granite
E. An exception is thrown at runtime.
F. atom granite atom granite


答案：Ｆ


解析：


子代的建構子在運作之前

必定會先運行父代的建構子

父代的建構子又會運行父代的父代的建構子

直到最上層Object 代為止...



若建構子中未寫this或super的呼叫敘述

編譯器會在建構子的首行加上super()

以此例來說

建立Mountain物件時

會先叫用super("granite")

呼叫其父類別Rock的建構子

Rock建構子會再呼叫其父類別Atom的建構子

因此會照順序印出atom→granite→atom→granite