Question

我昨天陷入了一个令人感兴趣的问题,虽然固定点非常简单,但我仍然对“水”的怀疑。

我有一个在瞬时分配的私人成员变量的类别,但是,如果使用超级类别建筑商要求的抽象功能,则变量没有价值。问题的解决非常简单,我只得宣布变数为static,并正确地分配。说明问题的一些法典:

class Foo extends BaseClass
{
    private final String bar = "fooBar!";
    public Foo()
    {
        super();
    }

    @Override 
    public void initialize()
    {
        System.out.println(bar);
    }
}

基地类别:

abstract class BaseClass
{
    public BaseClass()
    {
        initialize();
    }

    public abstract void initialize();
}

In this example, when we call new Foo(); it will output (null) instead of the expected fooBar!

既然我们重新出现一种“Foo”的物体,那么其成员在打造者(及其超级阶级)之前是否没有得到分配和分配? 这一点在 Java语中是否具体指明了什么地方?

Thanks for any insight!

Answer 1

<代码bar = “foo Bar>;>:在编辑期间被指派到施工人员。

http://java.sun.com 二级建筑商因此,事后执行声明是自然的。

Generally though, it s bad practice to call overridable methods from a constructor.

Answer 2

定义是 Java语规格。在现实的世界形势下,改变这种状态几乎永远不会是可以接受的解决办法。

见and

Overall, it is bad practice to call a non-final method from a constructor. the reason being that it could (and if the method is abstract then definitely does) call method in the class that has not yet been initialized: When new Foo() is executed, the BaseClass initializer (constructor) gets called before the Foo constructor, so Foo.initialize is essentially working on an Object that has not been fully constructed yet.

Answer 3

我只想补充大家接受的答复,因为我不同意他的结论。

我们这样做。

class Engine {
  public Engine() {
    init();
  }

  void init() {
    lockDoors();
    releasePressure();
    tightenSeatbelts();
    launchRocket();
  }

  ...
}

现在的问题是:,可登陆Landifier。我们应在<条码>init(功能上添加。如果是私人的或受保护的。

make it private <-- keeps subclasses out
make it protected <-- allows subclasses in

Before you make a choice

首先,你应认识到,(几乎)<条码>Engine类别中的所有代码均可由一个子类取代。

code in a public function, can easily be overridden
code in a protected function, can easily be overridden
code in a private function, can be replaced by overriding all methods that call it.

只有一个例外:

you can never modify the code of a constructor
you can never avoid a private method being called from the constructor of a super class.
(and of course, you cannot replace a final method)

Protected init() - the wrong way

请说<代码>init()方法protected 确实存在陷阱。它试图推翻它,增加以下特征。这确实是一个错误。

class SubEngine extends Engine {
  int screws = 5;

  void init() {
    tightenScrews();
    super.init();
  }

  void tightenScrews() {
    // this won t print 5, but it will print 0.
    System.out.println("tightening " + screws + " screws"); 
  }
}

Protected init() - the right way

因此,基本上来说,你应当仅仅解除父母的罪责,而是把执行推迟到自己的建筑人。

class SubEngine extends Engine {
  int screws = 5;

  public SubEngine() {
    initSubEngine();
  }

  void init() {
    // disable parent code
  }

  void initSubEngine() {
    tightenScrews();
    super.init();
  }

  void tightenScrews() {
    // this will print 5 as expected
    System.out.println("tightening " + screws + " screws"); 
  }
}

Private init() - you may need a phonecall

现在,如果<代码>init()”方法是 private?

Like mentioned above, there is no way to disable the code of a parent constructor. And if init() is private you simply cannot disable it.
You ll end up copying the entire Engine class, perhaps just to add 1 line of code.
And that may not be the end of it. Even after copying your class, your copied object won t be an Engine meaning that you won t be able to use your EngineUtil#inspectEngine(Engine engine) function.
Perhaps somebody knew this in advance and made an IEngine interface. Then you can get away with it.
In practice it means you ll have to take your phone, and call to that other department that made the Engine class, and ask them to change their code a little to take away some restrictions.

Intelligent design

还有另一种方式。组成部分是确定变量。他们 anything弃任何东西。每当你们看到一个班级,在其建筑商(或采用私人方法)上,就应当有红色旗帜。

class Engine {
  public Engine() {
  }

  public void init() {
    lockDoors();
    releasePressure();
    tightenSeatbelts();
    launchRocket();
  }

  // and you probably also want one of these
  public void shutdown() { ... }

  ...
}

Intention

当然,你的意图很可能不是开放你的法典。也许你真的不想让他人延长你的班子。确实可以有这样的情况,即你想要把人锁住。

了解这也会使你法的测试更加困难。