java - 一个枚举问题

发布于 2024-11-17 14:30:09 字数 628 浏览 2 评论 0原文

我在我的应用程序（java）中遇到了一个奇怪的问题。

我有一个枚举。类似的东西

public enum myEnum implement myIntrface{
   valueA(1),valueb(2),valuec(3),valued(4)
   private int i;
   // and then - a constructor 
   public MyEnum(int number){
        i = number;
   }       


   private MyObj obj = new MyObj;
   // getter and setter for obj
}

，在另一个类中，我

   MyEnum.valueA.setObj(new Obj(...))

简要介绍了这一点 - 我有一个带有私有实例成员的枚举，该实例成员有一个集合和一个获取。

到目前为止一切顺利 -

唯一让我惊讶的是，稍后我查看 MyEnum.valueA().obj 的值是 null。

没有任何东西可以将值更新为 null，我什至在构造函数中给了它一个默认值，但稍后我仍然看到它为 null。
有什么建议吗？

原文

I have encountered a weird problem in my app (java).

I have an enum. Something like that

public enum myEnum implement myIntrface{
   valueA(1),valueb(2),valuec(3),valued(4)
   private int i;
   // and then - a constructor 
   public MyEnum(int number){
        i = number;
   }       


   private MyObj obj = new MyObj;
   // getter and setter for obj
}

and in another class I have this

   MyEnum.valueA.setObj(new Obj(...))

in briefe - I have an enum with a private instance member that has a set and a get.

So far so good -

The only thing that amazes me is that later on I look at the value of the MyEnum.valueA().obj is null.

there is nothing that updates the value to null, I have even gave it a default value in the constructor and I still see it null later.
any suggestions?

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深巷少女 2024-11-24 14:30:09

枚举应该是不可修改的类，因此您不应该真正这样做。如果您希望修改基于类型的对象（例如枚举）的状态，您应该使用带有嵌入常量的最终类方法。下面是一个基于类的方法的示例，具有可修改的名称和不可修改的名称......

public final class Connection {

    public static final Connection EMAIL = new Connection("email");
    public static final Connection PHONE = new Connection("phone");
    public static final Connection FAX = new Connection("fax");
    /**/
    private final String unmodifiableName; //<-- it's final
    private String modifiableName;

    /*
     * The constructor is private so no new connections can be created outside.
     */
    private Connection(String name) { 
        this.unmodifiableName = name;
    }

    public String getUnmodifiableName() {
        return unmodifiableName;
    }

    public String getModifiableName() {
        return modifiableName;
    }

    public void setModifiableName(String modifiableName) {
        this.modifiableName = modifiableName;
    }

}

Enums should be un-modifiable classes so you shouldn't really be doing this. If your looking to modify the state of a type based object like an enum you should use an final class approach with embedded constants. Below is an example of a class based approach with a modifiable name an a un-modifiable name...

public final class Connection {

    public static final Connection EMAIL = new Connection("email");
    public static final Connection PHONE = new Connection("phone");
    public static final Connection FAX = new Connection("fax");
    /**/
    private final String unmodifiableName; //<-- it's final
    private String modifiableName;

    /*
     * The constructor is private so no new connections can be created outside.
     */
    private Connection(String name) { 
        this.unmodifiableName = name;
    }

    public String getUnmodifiableName() {
        return unmodifiableName;
    }

    public String getModifiableName() {
        return modifiableName;
    }

    public void setModifiableName(String modifiableName) {
        this.modifiableName = modifiableName;
    }

}

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奢华的一滴泪 2024-11-24 14:30:09

枚举的目的是表示常量值。将字段设置为常量值没有任何意义。

您应该将字段声明为 final，并使用构造函数来初始化所有字段。

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且行且努力 2024-11-24 14:30:09

作为参考，以下代码按预期工作：

public class Test {

    public static enum MyEnum {
        valueA(1),valueb(2),valuec(3),valued(4);
        private int i;
        private Object o;

        private MyEnum(int number) {
             i = number;
        }

        public void set(Object o) {
            this.o = o;
        }

        public Object get() {
            return o;
        }


     } 

    public static void main(String[] args) {
        System.out.println(MyEnum.valueA.get());  // prints "null"
        MyEnum.valueA.set(new Integer(42));
        System.out.println(MyEnum.valueA.get());  // prints "42"
    }
}

For reference, the following code works as expected:

public class Test {

    public static enum MyEnum {
        valueA(1),valueb(2),valuec(3),valued(4);
        private int i;
        private Object o;

        private MyEnum(int number) {
             i = number;
        }

        public void set(Object o) {
            this.o = o;
        }

        public Object get() {
            return o;
        }


     } 

    public static void main(String[] args) {
        System.out.println(MyEnum.valueA.get());  // prints "null"
        MyEnum.valueA.set(new Integer(42));
        System.out.println(MyEnum.valueA.get());  // prints "42"
    }
}

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