java 私有变量访问,从超类（JAVA）访问私有变量

清疚 2022-10-04 12:52 122阅读 0赞

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Ok so I have studied java all semester and thought I had a clear understanding about inheritance and super/sub classes. Today we were given as assignment for making a superclass called enemy, with sub classes of different types of enemies. I did everything fine and all of my subclasses are working, but when I went back to read the guidelines we must follow, I found this sentence:

"All member variables of the super class must be private. Any access to a variable must be done through protected methods in the subclasses."

From what I have learned, this makes no sense to me. If a variable is private within the superclass, doesn't that disallow access even from a subclass? The last part that talks about protected methods in the subclasses also doesn't make any sense to me. How does this help and/or allow any access whatsoever to the super class?

From what I've learned about inheritance, Below is what i thought was true:

Access Levels

Modifier Class Package Subclass World

public Y Y Y Y

protected Y Y Y N

no modifier Y Y N N

private Y N N N

If I'm understanding something wrong here please do explain! I don't want to confront the instructor about giving us faulty instructions, if I'm the one not understanding it correctly!

解决方案

The part

Any access to an a variable must be done through protected methods in the sub classes.

... just means that the subclasses have to call protected methods that are defined in the superclass. Since these methods are protected they can be accessed by the subclasses.

First you would define a base class like this:

public class Base \{

private int x; // field is private

protected int getX() \{ // define getter

return x;

protected void setX(int x) \{ // define setter

this.x = x;

Then you would use it in your child class like this:

class Child extends Base\{

void foo() \{

int x = getX(); // we can access the method since it is protected.

setX(42); // this works too.