Something that isn't mentioned in any of the answers is the overhead of using companion objects. As you can read here, companion objects are in fact objects and creating them consumes resources. In addition, you may need to go through more than one getter function every time you use your constant. If all that you need is a few primitive constants on a few instances of your class, you'll probably just be better off using val to get a better performance and avoid the companion object. The trade off is higher memory consumption if you have many instances of your class so everyone should make their own decision.

TL,博士;文章:

使用伴侣对象实际上会将以下Kotlin代码:

class MyClass {

    companion object {
        private val TAG = "TAG"
    }

    fun helloWorld() {
        println(TAG)
    }
}

在这段Java代码中:

public final class MyClass {
    private static final String TAG = "TAG";
    public static final Companion companion = new Companion();

    // synthetic
    public static final String access$getTAG$cp() {
        return TAG;
    }

    public static final class Companion {
        private final String getTAG() {
            return MyClass.access$getTAG$cp();
        }

        // synthetic
        public static final String access$getTAG$p(Companion c) {
            return c.getTAG();
        }
    }

    public final void helloWorld() {
        System.out.println(Companion.access$getTAG$p(companion));
    }
}

class Myclass {

    companion object {
        const val MYCONSTANT = 479
    }
}

你有两个选择，你可以使用const关键字或使用@JvmField，这使它成为java的静态最终常量。

class Myclass {

    companion object {
        @JvmField val MYCONSTANT = 479
    }
}

如果您使用@JvmField注释，那么在它编译之后，常量就会以java中调用它的方式为您放入。 就像你在java中调用它一样，当你在代码中调用同伴常量时，编译器会为你替换它。

但是，如果使用const关键字，则常量的值将内联。内联是指在编译后使用实际值。

所以总结一下编译器将为你做什么:

//so for @JvmField:

Foo var1 = Constants.FOO;

//and for const:

Foo var1 = 479

Something that isn't mentioned in any of the answers is the overhead of using companion objects. As you can read here, companion objects are in fact objects and creating them consumes resources. In addition, you may need to go through more than one getter function every time you use your constant. If all that you need is a few primitive constants on a few instances of your class, you'll probably just be better off using val to get a better performance and avoid the companion object. The trade off is higher memory consumption if you have many instances of your class so everyone should make their own decision.

TL,博士;文章:

使用伴侣对象实际上会将以下Kotlin代码:

class MyClass {

    companion object {
        private val TAG = "TAG"
    }

    fun helloWorld() {
        println(TAG)
    }
}

在这段Java代码中:

public final class MyClass {
    private static final String TAG = "TAG";
    public static final Companion companion = new Companion();

    // synthetic
    public static final String access$getTAG$cp() {
        return TAG;
    }

    public static final class Companion {
        private final String getTAG() {
            return MyClass.access$getTAG$cp();
        }

        // synthetic
        public static final String access$getTAG$p(Companion c) {
            return c.getTAG();
        }
    }

    public final void helloWorld() {
        System.out.println(Companion.access$getTAG$p(companion));
    }
}

像val一样，用const关键字定义的变量是不可变的。这里的区别在于，const用于编译时已知的变量。

声明一个变量const很像在Java中使用static关键字。

让我们看看如何在Kotlin中声明一个const变量:

const val COMMUNITY_NAME = "wiki"

用Java编写的类似代码是:

final static String COMMUNITY_NAME = "wiki";

加上上面的答案-

@JvmField可以用来指示Kotlin编译器不生成 getter /setter，并将其公开为字段。

 @JvmField
 val COMMUNITY_NAME = "Wiki"

静态字段

在命名对象或伴生对象中声明的Kotlin属性 将有静态支持字段，无论是在该命名对象或 包含伴生对象的类。

通常这些字段是私有的，但它们可以通过以下方式之一公开:

@JvmField注释; lateinit修饰语; const修饰符。

更多详情请访问:https://kotlinlang.org/docs/reference/java-to-kotlin-interop.html#instance-fields

在编译时已知的值可以(并且在我看来应该)被标记为常量。

命名约定应该遵循Java约定，并且在从Java代码中使用时应该正确可见(这在某种程度上很难通过伴生对象实现，但无论如何)。

正确的常量声明是:

const val MY_CONST = "something"
const val MY_INT = 1

在Kotlin中有几种定义常量的方法，

使用伴随对象

    companion object {
        const val ITEM1 = "item1"
        const val ITEM2 = "item2"
    }

你可以在任何类中使用上面的同伴对象块，并在这个块中定义你的所有字段。但是这种方法有一个问题，文档说，

尽管伴随对象的成员看起来像其他语言中的静态成员，但在运行时，它们仍然是实际对象的实例成员，并且可以实现接口。

当你使用同伴对象创建常量，并看到反编译的字节码时，你会像下面这样:

  ClassName.Companion Companion = ClassName.Companion.$$INSTANCE;
  @NotNull
  String ITEM1 = "item1";
  @NotNull
  String ITEM2 = "item2";
 
  public static final class Companion {
     @NotNull
     private static final String ITEM1 = "item1";
     @NotNull
     public static final String ITEM2 = "item2";
     
     // $FF: synthetic field
     static final ClassName.Companion $$INSTANCE;

     private Companion() {
     }

     static {
        ClassName.Companion var0 = new ClassName.Companion();
        $$INSTANCE = var0;
     }
  }

从这里你可以很容易地看到文档说了什么，即使伴随对象的成员看起来像其他语言中的静态成员，在运行时，它们仍然是实际对象的实例成员。

现在有另一种方法，我们不需要像下面这样使用同伴对象，

object ApiConstants {
      val ITEM1: String = "item1"
 }

同样，如果你看到上述片段的字节代码的反编译版本，你会发现这样的东西，

public final class ApiConstants {
     private static final String ITEM1 = "item1";

     public static final ApiConstants INSTANCE;

     public final String getITEM1() {
           return ITEM1;
      }

     private ApiConstants() {
      }

     static {
         ApiConstants var0 = new ApiConstants();
         INSTANCE = var0;
         CONNECT_TIMEOUT = "item1";
      }
    }

现在，如果您看到上面的反编译代码，它正在为每个变量创建get方法。这个get方法根本不是必需的。

要摆脱这些get方法，你应该在val之前使用const，如下所示:

object ApiConstants {
     const val ITEM1: String = "item1"
 }

现在，如果您看到上述代码片段的反编译代码，您会发现它更容易阅读，因为它为您的代码进行了最少的后台转换。

public final class ApiConstants {
    public static final String ITEM1 = "item1";
    public static final ApiConstants INSTANCE;

    private ApiConstants() {
     }

    static {
        ApiConstants var0 = new ApiConstants();
        INSTANCE = var0;
      }
    }

所以这是创建常数的最好方法。