[OCAJP]繼承的欄位

Given:

package p1;

public class Acc {

    int p;
    private int q;
    protected int r;
    public int s;
}

package p2;

import p1.Acc;

public class Test extends Acc {

    public static void main(String[] args) {
        Acc obj = new Test();
    }
}

Which statement is true?

A. Both p and s are accessible by obj.
B. Only s is accessible by obj.
C. Both r and s are accessible by obj.
D. p, r, and s are accessible by obj.

題解

在不同套件的繼承關係下，只有public和protected的可見度可以將變數繼承至底下的類別。因此在這題中，只有s變數和r變數可以用Test類別所實體化出來的Test物件存取。但是，obj變數是Acc型態，不是Test型態，因此沒有辦法存取到使用protected修飾的r變數。