如何定义枚举项目的属性 [英] How to define properties for Enum items

问题描述

我已经阅读了 java和C ++之间的枚举差异？ a>但我还是很困惑。

I have read the question Difference of Enum between java and C++? but I'm still confused.

我希望以下内容返回相关的字符串：

I would like the following to return the related String:

public enum Checker {
    EMPTY ("Empty"),
    RED ("Red"),
    YELLOW ("Yellow");
}

从我所看到的，这应该是可能的。

From what I have read, this should be possible. Just would like you to shed some light on it on how to implement it.

推荐答案

简单答案

你需要一个构造函数，一个字段和一个getter。

Short Answer

You need a constructor, a field and a getter.

枚举类型可以具有构造函数，前提是它们的访问级别是私有或默认（包私有）。您不能直接调用这些构造函数，除了在枚举声明本身之外。类似于类，当您定义不带参数的枚举常量时，您实际调用编译器生成的默认构造函数。例如

Enum types can have constructors, provided that their access level is either private or default (package-private). You can not directly call these constructors, except in the enum declaration itself. Similar to classes, when you define an enum constant without parameters, you actually call the default constructor generated by the compiler. E.g.

public enum King{
    ELVIS
}

相当于

public enum King{
    ELVIS() // the compiler will happily accept this
}

只是就像在类中一样，如果你定义一个显式的构造函数，编译器将不会插入一个默认的构造函数，所以不会编译：

And just like in classes, if you define an explicit constructor, the compiler will not insert a default constructor, so this will not compile:

public enum King{
    ELVIS, // error, default constructor is not defined
    MICHAEL_JACKSON(true)
    ;
    private boolean kingOfPop;
    King(boolean kingOfPop){this.kingOfPop = kingOfPop;}
}

这是枚举的相当好的参考，这也解释了构造函数问题。

This is a pretty good reference on enums that also explains the constructor issues.

枚举是常量，是不可变的。然而，他们可以定义可以具有状态的字段。这是一个可怕的做法，因为开发人员会期望枚举及其关联的值是常量，但是您可以仍然可以在带有getter和setter的枚举中定义非final字段。

Enums are constants and are immutable as such. They can however define fields, that can have state. This is an awful practice, because developers will expect enums and their associated values to be constants, but you can still define a non-final field in an enum with getters and setters.

这是合法的java代码：

This is legal java code:

public enum Color{
    RED("FF0000"),
    GREEN("00FF00"),
    BLUE("0000FF");
    private String code;
    public String getCode(){return code;}
    public void setCode(String code){this.code = code;}
    private Color(String code){this.code = code;}
}

但是它能够像这样的恶意代码：

But it enables evil code like this:

String oldBlue = Color.BLUE.getCode();
Color.BLUE.setCode(Color.RED.getCode());
Color.RED.setCode(oldBlue);

所以在99.99％的情况下：如果你的枚举中有字段，仅提供吸气剂。如果这些字段不是不可变的，请提供防御副本：

So in 99.99 % of cases: if you have fields in your enums, you should make them final and provide getters only. If the fields are not immutable themselves, provide defensive copies:

public enum Band{
    THE_BEATLES("John","Paul","George","Ringo");
    private final List<String> members;
    public List<String> getMembers(){
        // defensive copy, because the original list is mutable
        return new ArrayList<String>(members);
    }
    private Band(String... members){
        this.members=Arrays.asList(members);
    }
}

解决方案

在你的情况下，这很简单：你只需要一个类型为string（immutable）的单个字段，因此在构造函数中初始化它并提供一个getter是完全可以的：

Solution

In your case it's very simple: you just need a single field of type string (immutable), so initializing it in the constructor and providing a getter is perfectly ok:

public enum Checker {

    EMPTY ("Empty"),
    RED ("Red"),
    YELLOW ("Yellow");

    private final String value;

    private Checker(final String value) {
        this.value = value;
    }

    public String getValue() { return value; }
}

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