Java中的抽象变量？ [英] Abstract variables in Java?

问题描述

我来自c＃，这很容易，可能。

我有这个代码：

  public abstract class clsAbstractTable {
 
 public abstract String TAG; 
 public abstract void init（）; 
 
} 
  

但Eclipse告诉我使用非法修饰符。

我有这个类：

  public class clsContactGroups extends clsAbstractTable { 
 
 
} 
  

我想要定义的变量和方法这样，Eclipse 提示我，我没有实现抽象变量和方法。

要定义我的抽象类，以便提示您实现抽象？

编辑1

我将为不同的数据库表创建不同的类。每个类都应该有自己的TABLENAME变量，没有异常。

然后在抽象类中，我将有一个方法eg： init（）;

如果在这个init（）方法中我调用TABLENAME，它应该从子类中取值。

这样的东西也应该工作。

  String tablename =（clsAbstract）objItem.TABLENAME; 
 //其中objItem可以是任何扩展clsAbstract的类; 
  

EDIT 2

我想要一个常量（静态）定义在每个类的名称定义在抽象。

• 我以抽象的方式定义变量TABLENAME，但未指定值。


• 我创建了一个clsContactGroups，我应该提示实现TABLENAME，这是在哪里获取一些数据。例如：TABLENAME =contactgroups;


• 我创建了第二个类clsContacts，我应该提示实现TABLENAME，这是获取一些数据的地方。例如：TABLENAME =contacts;
  
  etc ...

解决方案

我想你的困惑是C＃属性与字段/变量。在C＃中，您不能定义抽象字段，即使在抽象类中。然而，您可以定义抽象属性，因为这些是有效的方法（例如编译为 get_TAG（）和 set_TAG（...））。

有些人已经提醒过，你不应该在你的类中有公共字段/变量，即使在C＃。几个答案暗示了我会推荐什么，但没有说明。您应该使用getTAG（）将您的想法转换为Java作为JavaBean属性。然后你的子类将必须实现这个（我也写了一个项目与表类，这样做）。

所以你可以有一个抽象类定义...

  public abstract Class AbstractTable {
 
 public abstract String getTag（）; 
 public abstract void init（）; 
 
 ... 
} 
  

子类，你需要定义一个静态的最终变量（常量），并返回从 getTag（），类似这样：

  public class SalesTable extends AbstractTable {
 
 private static final String TABLE_NAME =Sales; 
 
 public String getTag（）{
 return TABLE_NAME; 
} 
 
 public void init（）{
 ... 
 String tableName = getTag（）; 
 ... 
} 
 
} 
  

：

您不能覆盖继承的字段（C＃或Java）。也不能覆盖静态成员，无论它们是字段还是方法。所以这也是最好的解决方案。我改变了我的init方法示例，显示如何使用 - 再次，认为getXXX方法作为属性。

I am coming from c# where this was easy, and possible.

I have this code:

public abstract class clsAbstractTable {

    public abstract String TAG;
    public abstract void init();

}

but Eclipse tells me I use illegal modifier.

I have this class:

public class clsContactGroups extends clsAbstractTable {


}

I want the variable and method defined in such way, that Eclipse to prompt me, I have unimplemented abstract variables and methods.

How do I need to define my abstract class so I should be prompted to implement the abstracts?

EDIT 1

I will create different classes for different db tables. Each class should have it's own TABLENAME variable, no exception. I have to make sure this variable is static each time when I create a new class that extends the abstract class.

Then in the abstract class I will have a method eg: init();

If in this init() method I call TABLENAME, it should take the value from the sub-class.

something like this should also work out

String tablename=(clsAbstract)objItem.TABLENAME;
// where objItem can be any class that extended clsAbstract;

EDIT 2

I want a constant(static) defined in each class having it's name defined in abstract.

• I define variable TABLENAME in abstract, but no value given.
• I create a clsContactGroups, I should be prompted to implement TABLENAME, this is where gets some data. eg: TABLENAME="contactgroups";
• I create a second class clsContacts, I should be prompted to implement TABLENAME, this is where gets some data. eg: TABLENAME="contacts";
  etc...

解决方案

I think your confusion is with C# properties vs. fields/variables. In C# you cannot define abstract fields, even in an abstract class. You can, however, define abstract properties as these are effectively methods (e.g. compiled to get_TAG() and set_TAG(...)).

As some have reminded, you should never have public fields/variables in your classes, even in C#. Several answers have hinted at what I would recommend, but have not made it clear. You should translate your idea into Java as a JavaBean property, using getTAG(). Then your sub-classes will have to implement this (I also have written a project with table classes that do this).

So you can have an abstract class defined like this...

public abstract class AbstractTable {

    public abstract String getTag();
    public abstract void init();

    ...
}

Then, in any concrete subclasses you would need to define a static final variable (constant) and return that from the getTag(), something like this:

public class SalesTable extends AbstractTable {

    private static final String TABLE_NAME = "Sales";

    public String getTag() {
        return TABLE_NAME;
    }

    public void init() {
        ...
        String tableName = getTag();
        ...
    }

}

EDIT:

You cannot override inherited fields (in either C# or Java). Nor can you override static members, whether they are fields or methods. So this also is the best solution for that. I changed my init method example above to show how this would be used - again, think of the getXXX method as a property.

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