继承为什么即使扩展了类，我也不能访问受保护的java方法？

Question

以下是受保护方法的文档：

/** Converts jmusic score data into a MIDI Sequence */
protected  javax.sound.midi.Sequence scoreToSeq(Score score)

我制作了这个小类来扩展scoreToSeq方法的类：

public class MidiSequence extends MidiSynth{

    public Sequence getSequence(Score score){
        MidiSynth synth = new MidiSynth();
        Sequence sequence = null;
        try
        {
                    // Here I get the error saying that the method has
                    // protected access in MidiSynth
            sequence = synth.scoreToSeq(score);

        }
        catch (InvalidMidiDataException e)
        {
            /*
             *  In case of an exception, we dump the exception
             *  including the stack trace to the console.
             *  Then, we exit the program.
             */
            e.printStackTrace();
            System.exit(1);
        }

        return sequence;

    }
}

Answer 1

通过这样做：

MidiSynth synth = new MidiSynth();
sequence = synth.scoreToSeq(score); 

实际上，您并没有利用扩展MidSynth类这一事实

如果你想试试的话

this.scoreToSec(score);

然后您会发现您可以访问受保护的函数

Answer 2

（编辑：theycallmemorty's answer给出了在您的案例中避免此问题的实用建议。此答案给出了您必须遵循该建议的原因，即语言为何以这种方式设计。）

您只能访问与访问代码（或子类）类型相同的另一个对象的受保护成员，即使该成员在超类型中声明为

从Java Language Specification, section 6.6.2开始：

Let C be the class in which a protected member m is declared. Access is permitted only within the body of a subclass S of C. In addition, if Id denotes an instance field or instance method, then:

• If the access is by a qualified name Q.Id, where Q is an ExpressionName, then the access is permitted if and only if the type of the expression Q is S or a subclass of S.
• If the access is by a field access expression E.Id, where E is a Primary expression, or by a method invocation expression E.Id(. . .), where E is a Primary expression, then the access is permitted if and only if the type of E is S or a subclass of S.

这是为了允许类型访问与其自身继承树相关的成员，而不会破坏其他类的封装。例如，假设我们有：

     A
    / \
   B   Other
  /
 C

和一个声明为受保护的成员x。如果没有规则的工作方式，您可以通过将一个成员放入Other来完成封装：

public int getX(A a)
{
    return a.x;
}

只需调用B或C实例的传递，该成员将有效地成为公共成员，因为您可以通过引入另一个类来解决此问题。。。这不是个好主意。使用当前规则，您必须将B或C子类化，而这可能是您首先无法实现的