[OCAJP][OCPJP]建構子的可視範圍

Given:

package p1;

public interface DoInterface {

    void m1(int n); // line n1

    public void m2(int n);
}

package p3;
public class DoClass implements DoInterface{
    int x1, x2;
    DoClass(){
        this.x1 = 0;
        this.x2 = 10;
    }
    public void m1(int p1) { x1+=p1; System.out.println(x1); } // line n2
    public void m2(int p1) { x2+=p1; System.out.println(x2); }
}

package p2;
import p1.*;
import p3.*;

class Test {

    public static void main(String[] args) {
        DoInterface doi = new DoClass(); // line n3
        doi.m1(100);
        doi.m2(200);
    }
}

What is the result?

A.

B. Compilation fails due to an error in line n1
C. Compilation fails due to an error at line n2
D. Compilation fails due to an error at line n3

題解

line n3實體化DoClass類別的物件實體，但是由於DoClass的建構子並沒有使用public修飾字宣告，因此它只能在同一個套件(package)之內被實體化。若在不同的套件下實體化DoClass物件，會發生編譯錯誤。