Java：在使用整型常量声明枚举时遇到麻烦 [英] Java: Having trouble declaring an enum with integer constants

问题描述

Urg，我对Java中的枚举如何工作感到困惑。在C＃和C ++（我正常使用的东西）中，这似乎还可以，但Java想要让我生气。>

 枚举方向
 {
 NORTH_WEST = 0x0C，
 NORTH = 0x10，
 NORTH_EAST = 0x14，
 WEST = 0x18，
 NONE = 0x20，
 EAST = 0x28，
 SOUTH_WEST = 0x24，
 SOUTH = 0x30，
 SOUTH_EAST = 0x3C 
} 
  

有人可以告诉我我做错了什么吗？
谢谢

以下是错误：

  --- jGRASP exec：javac -g Test.java 
 
 Test.java:79：'，'，'}'或';'预计
 NORTH_WEST = 0x0C，
 ^ 
 Test.java:79：'}'expected 
 NORTH_WEST = 0x0C，
 ^ 
 Test.java:80：< identifier>预期
 NORTH = 0x10，
 ^ 
 Test.java:87：';'expected 
 SOUTH_EAST = 0x3C 
 ^ 
  

解决方案

对于这种情况，您可以简单地使用实例字段 。

  public enum Direction {
 NORTH（0x10），WEST（0x18），...; 
 
 private final int code; 
 Direction（int code）{this.code = code; } 
 public int getCode（）{return code; 
} 
  

Java 枚举被实现为对象。他们可以有领域和方法。您还可以选择声明构造函数，该构造函数接受一些参数，并在常量声明中为这些参数提供值。

另请参见

• Java语言指南/枚举 - 一个快速但全面的指南Java 枚举

---

附录： EnumSet 和 EnumMap

请注意，这些值可能比实例字段更好。也就是说，如果您尝试设置位字段的值，则应该使用 EnumSet 。

两个常量（例如C ++）与按位操作结合使用，作为一个紧凑型

  //beforeimplementation，with bitwise operations 
 
 public static final int BUTTON_A = 0x01; 
 public static final int BUTTON_B = 0x02; 
 public static final int BUTTON_X = 0x04; 
 public static final int BUTTON_Y = 0x08; 
 
 int buttonState = BUTTON_A | BUTTON_X; // A& X被按下！ 
 
 if（（buttonState& BUTTON_B）！= 0）... // B被按下... 
  

使用枚举和 EnumSet ，可以看起来像这样：枚举和枚举

枚举按钮{A，B，X， Ÿ; }

设置< Button> buttonState = EnumSet.of（Button.A，Button.X）; // A& X被按下！

if（buttonState.contains（Button.B））... // B被按下...

还有您可能想要使用的 EnumMap 。这是一个 地图 ，其关键是枚举常量。

所以，如下所示：

  //before，使用int常量和数组索引
 
 public static final int JANUARY = 0; ... 
 
员工[] employeeOfTheMonth = ... 
 
 employeeOfTheMonth [JANUARY] = jamesBond; 
  

现在您可以：

  //after，带有枚举和枚举地图
 
枚举月份{JANUARY，...} 
 
地图< Month，Employee> employeeOfTheMonth = ... 
 
 employeeOfTheMonth.put（Month.JANUARY，jamesBond）; 
  

在Java中，枚举是一个非常强大的抽象也适用于Java Collections Framework。

另请参见

• Java教程/集合框架


• 有效的Java第2版
  
  
  
  
  • 项目30：使用枚举项目31：使用实例字段而不是ordinals
  
  
  • 项目32： int 常量
  
  <使用 EnumSet 而不是位字段
  
  • 项目33：使用 EnumMap 而不是顺序索引
  
  

相关问题

枚举：为什么？什么时候？ - 例如 EnumSet 和 EnumMap usage




Urgh, I'm kind of confused on how enums work in Java. In C# and C++ (what I use normally), this seems okay, but Java wants to get mad at me >.>

   enum Direction
   {
      NORTH_WEST = 0x0C,
      NORTH      = 0x10,
      NORTH_EAST = 0x14,
      WEST       = 0x18,
      NONE       = 0x20,
      EAST       = 0x28,
      SOUTH_WEST = 0x24,
      SOUTH      = 0x30,
      SOUTH_EAST = 0x3C
   }

Could someone tell me what I'm doing wrong? Thanks

Here are the errors:

 ----jGRASP exec: javac -g Test.java

Test.java:79: ',', '}', or ';' expected
      NORTH_WEST = 0x0C,
                 ^
Test.java:79: '}' expected
      NORTH_WEST = 0x0C,
                  ^
Test.java:80: <identifier> expected
      NORTH      = 0x10,
           ^
Test.java:87: ';' expected
      SOUTH_EAST = 0x3C
                       ^

解决方案

For this scenario, it looks like you can simply use an instance field.

public enum Direction {
   NORTH(0x10), WEST(0x18), ...;

   private final int code;
   Direction(int code)  { this.code = code; }
   public int getCode() { return code; }
}

Java enum are implemented as objects. They can have fields and methods. You also have the option of declaring a constructor that takes some arguments, and providing values for those arguments in your constant declaration. You can use these values to initialize any declared fields.

See also

• Java Language Guide/Enums - a quick but comprehensive guide to Java enum

---

Appendix: EnumSet and EnumMap

Note that depending on what these values are, you may have an even better option than instance fields. That is, if you're trying to set up values for bit fields, you should just use an EnumSet instead.

It is common to see powers of two constants in, say, C++, to be used in conjunction with bitwise operations as a compact representation of a set.

// "before" implementation, with bitwise operations

public static final int BUTTON_A = 0x01;
public static final int BUTTON_B = 0x02;
public static final int BUTTON_X = 0x04;
public static final int BUTTON_Y = 0x08;

int buttonState = BUTTON_A | BUTTON_X; // A & X are pressed!

if ((buttonState & BUTTON_B) != 0) ...   // B is pressed...

With enum and EnumSet, this can look something like this:

// "after" implementation, with enum and EnumSet

enum Button { A, B, X, Y; }

Set<Button> buttonState = EnumSet.of(Button.A, Button.X); // A & X are pressed!

if (buttonState.contains(Button.B)) ... // B is pressed...

There is also EnumMap that you may want to use. It's a Map whose keys are enum constants.

So, where as before you may have something like this:

// "before", with int constants and array indexing

public static final int JANUARY = 0; ...

Employee[] employeeOfTheMonth = ...

employeeOfTheMonth[JANUARY] = jamesBond;

Now you can have:

// "after", with enum and EnumMap

enum Month { JANUARY, ... }

Map<Month, Employee> employeeOfTheMonth = ...

employeeOfTheMonth.put(Month.JANUARY, jamesBond);

In Java, enum is a very powerful abstraction which also works well with the Java Collections Framework.

See also

• Java Tutorials/Collections Framework
• Effective Java 2nd Edition
  • Item 30: Use enum instead of int constants
  • Item 31: Use instance fields instead of ordinals
  • Item 32: Use EnumSet instead of bit fields
  • Item 33: Use EnumMap instead of ordinal indexing

Related questions

• Enumerations: why? when? - with examples of EnumSet and EnumMap usage

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