为枚举字符串值定义常量“值”

Question

对于如下定义的枚举

public enum Cars {

    SWIFT("Maruti"),
    DZIRE("Maruti"), 
    CIAZ("Maruti"), 
    I10("Hyundai"), 
    CRETA("Hyundai");

    String company;
    Cars(String company){
        this.company = company;
    }   
}

有声纳错误

Define a constant instead of duplicating this literal "Maruti" 3 times.

如果我们定义静态字符串，编译失败

Cannot reference a field before it is defined

解决方案 1：

public enum Cars {

    SWIFT(CarsString.MARUTI.companyName),DZIRE("Maruti"), I10("Hyundai"), CRETA("Hyundai");

    static String MARUTI = "Maruti";
    String company;
    Cars(String company){
        this.company = company;
    }

    enum CarsString {
        MARUTI("maruti"),
        HYUNDAI("Hyundai");
        String companyName;
        CarsString(String companyName){
            this.companyName = companyName;
        }
    }
}

解决方案 2：

不确定这是否是我们的做法。当 Cars.MARUTI 没有引用它时，它会出错Cannot reference a field before it is defined。

public enum Cars {

    SWIFT(Cars.MARUTI),DZIRE(Cars.MARUTI), I10("Hyundai"), CRETA("Hyundai");

    public static final String MARUTI = "Maruti";
    String company;
    Cars(String company){
        this.company = company;
    }
 }

我有 10 个处于相似状态的枚举，而上述解决方案 1 并没有给我太多鼓励。方案2解决了目的，

但是我们为什么要直接引用Cars.MARUTI 而不是MARUTI？这是限制吗？

我们如何在枚举中引用一个常量？

Answer 1

如何在自己的 Enum 中定义公司？例如：

public enum Car {
    SWIFT(Company.MARUTI),
    DZIRE(Company.MARUTI),
    CIAZ(Company.MARUTI),
    I10(Company.HYUNDAI),
    CRETA(Company.HYUNDAI);

    Company company;

    Cars(Company company) {
        this.company = company;
    }
}

public enum Company {
    HYUNDAI("Hyundai"),
    MARUTI("Maruti");

    String name;

    Company(String name) {
        this.name = name;
    }
}

更新：根据@Basil Bourque 的评论改进了命名约定，谢谢！

Answer 2

您需要知道枚举在内部是如何工作的。如果您查看枚举的字节码，您将得到答案。

 6: iconst_0
   7: ldc           #19                 // String Maruti
   9: invokespecial #21                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
  12: putstatic     #25                 // Field SWIFT:Labc/Cars;
  15: new           #1                  // class abc/Cars
  18: dup
  19: ldc           #27                 // String DZIRE
  21: iconst_1
  22: ldc           #19                 // String Maruti
  24: invokespecial #21                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
  27: putstatic     #28                 // Field DZIRE:Labc/Cars;
  30: new           #1                  // class abc/Cars
  33: dup
  34: ldc           #30                 // String CIAZ
  36: iconst_2:

从上面的byte code（由 Cars 枚举生成），我们可以有一个类似于Cars enum 的类

class Foo {
    public static Cars SWIFT = new Cars("Maruti");
    public static Cars DZIRE = new Cars("Maruti");
    public static Cars CIAZ = new Cars("Maruti");
}

现在，如果您将代码更改为：

class Cars {

    public static Cars SWIFT = new Cars(MARUTI); //statement-1
    public static Cars DZIRE = new Cars(MARUTI); //statement-2
    public static Cars CIAZ = new Cars(MARUTI); //statement-3

    public static String MARUTI = "Maruti"; // statement-4


    private String name;

    public Cars(String name) {
        this.name = name;
    }
}

它会给你同样的错误信息，因为你使用的是variable before it is defined。如果我们在statement-1 之前向上移动statement-4，它将起作用。但它是only possible in class but enum does not allow you as the first 声明。所以在你的情况下，你可以像这样解决：

enum Cars {

    SWIFT(Constants.MARUTI), DZIRE(Constants.MARUTI), I10("Hyundai"), CRETA("Hyundai");

    String company;

    Cars(String company) {
        this.company = company;
    }

    private static class Constants {

        public static String MARUTI = "Maruti";
    }
}