kotlin-sealed classes

sealed classes是一堆存在继承关系的类的集合。

类似于java中的枚举。不同的是，每个枚举类型只能存在一个对象实例，
而sealed class的子类可以拥有多个对象实例。

sealed关键字用来修饰sealed class。
sealed class的子类必须声明在同一个文件中。
但是sealed class的子类的子类可以定义在任何位置。

sealed class Expr
data class Const(val number: Double) : Expr()
data class Sum(val e1: Expr, val e2: Expr) : Expr()
object NotANumber : Expr()

类Expr是抽象类，可以声明抽象变量、方法等，所以不能被实例化。
Expr的构造方法默认是private修饰的，并且不能修改。

当我们使用when表达式的时候，传入的参数如果是Expr类型，则可以智能推断出是否覆盖了所有的条件。

fun eval(expr: Expr): Double = when(expr) {
    is Const -> expr.number
    is Sum -> eval(expr.e1) + eval(expr.e2)
    NotANumber -> Double.NaN
}

上述例子不再需要else语句了，因为编译器推断出expr只能为Const、Sum或者NotANumber。

我们反编译sealed class来查看一下真面目：

// Sum.java
import kotlin.Metadata;
import kotlin.jvm.internal.DefaultConstructorMarker;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;

@Metadata(
   mv = {1, 1, 10},
   bv = {1, 0, 2},
   k = 1,
   d1 = {"\u0000$\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\b\n\n\u0002\u0010\u000b\n\u0000\n\u0002\u0010\u0000\n\u0000\n\u0002\u0010\b\n\u0000\n\u0002\u0010\u000e\n\u0000\b\u0086\b\u0018\u00002\u00020\u0001B\u0015\u0012\u0006\u0010\u0002\u001a\u00020\u0001\u0012\u0006\u0010\u0003\u001a\u00020\u0001¢\u0006\u0002\u0010\u0004J\t\u0010\b\u001a\u00020\u0001HÆ\u0003J\t\u0010\t\u001a\u00020\u0001HÆ\u0003J\u001d\u0010\n\u001a\u00020\u00002\b\b\u0002\u0010\u0002\u001a\u00020\u00012\b\b\u0002\u0010\u0003\u001a\u00020\u0001HÆ\u0001J\u0013\u0010\u000b\u001a\u00020\f2\b\u0010\r\u001a\u0004\u0018\u00010\u000eHÖ\u0003J\t\u0010\u000f\u001a\u00020\u0010HÖ\u0001J\t\u0010\u0011\u001a\u00020\u0012HÖ\u0001R\u0011\u0010\u0002\u001a\u00020\u0001¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006R\u0011\u0010\u0003\u001a\u00020\u0001¢\u0006\b\n\u0000\u001a\u0004\b\u0007\u0010\u0006¨\u0006\u0013"},
   d2 = {"LSum;", "LExpr;", "e1", "e2", "(LExpr;LExpr;)V", "getE1", "()LExpr;", "getE2", "component1", "component2", "copy", "equals", "", "other", "", "hashCode", "", "toString", "", "production sources for module app"}
)
public final class Sum extends Expr {
   @NotNull
   private final Expr e1;
   @NotNull
   private final Expr e2;

   @NotNull
   public final Expr getE1() {
      return this.e1;
   }

   @NotNull
   public final Expr getE2() {
      return this.e2;
   }

   public Sum(@NotNull Expr e1, @NotNull Expr e2) {
      Intrinsics.checkParameterIsNotNull(e1, "e1");
      Intrinsics.checkParameterIsNotNull(e2, "e2");
      super((DefaultConstructorMarker)null);
      this.e1 = e1;
      this.e2 = e2;
   }

   @NotNull
   public final Expr component1() {
      return this.e1;
   }

   @NotNull
   public final Expr component2() {
      return this.e2;
   }

   @NotNull
   public final Sum copy(@NotNull Expr e1, @NotNull Expr e2) {
      Intrinsics.checkParameterIsNotNull(e1, "e1");
      Intrinsics.checkParameterIsNotNull(e2, "e2");
      return new Sum(e1, e2);
   }

   // $FF: synthetic method
   // $FF: bridge method
   @NotNull
   public static Sum copy$default(Sum var0, Expr var1, Expr var2, int var3, Object var4) {
      if ((var3 & 1) != 0) {
         var1 = var0.e1;
      }

      if ((var3 & 2) != 0) {
         var2 = var0.e2;
      }

      return var0.copy(var1, var2);
   }

   public String toString() {
      return "Sum(e1=" + this.e1 + ", e2=" + this.e2 + ")";
   }

   public int hashCode() {
      return (this.e1 != null ? this.e1.hashCode() : 0) * 31 + (this.e2 != null ? this.e2.hashCode() : 0);
   }

   public boolean equals(Object var1) {
      if (this != var1) {
         if (var1 instanceof Sum) {
            Sum var2 = (Sum)var1;
            if (Intrinsics.areEqual(this.e1, var2.e1) && Intrinsics.areEqual(this.e2, var2.e2)) {
               return true;
            }
         }

         return false;
      } else {
         return true;
      }
   }
}
// NotANumber.java
import kotlin.Metadata;
import kotlin.jvm.internal.DefaultConstructorMarker;

@Metadata(
   mv = {1, 1, 10},
   bv = {1, 0, 2},
   k = 1,
   d1 = {"\u0000\f\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\b\u0002\bÆ\u0002\u0018\u00002\u00020\u0001B\u0007\b\u0002¢\u0006\u0002\u0010\u0002¨\u0006\u0003"},
   d2 = {"LNotANumber;", "LExpr;", "()V", "production sources for module app"}
)
public final class NotANumber extends Expr {
   public static final NotANumber INSTANCE;

   private NotANumber() {
      super((DefaultConstructorMarker)null);
   }

   static {
      NotANumber var0 = new NotANumber();
      INSTANCE = var0;
   }
}
// Expr.java
import kotlin.Metadata;
import kotlin.jvm.internal.DefaultConstructorMarker;

@Metadata(
   mv = {1, 1, 10},
   bv = {1, 0, 2},
   k = 1,
   d1 = {"\u0000\u0018\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0000\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0000\b6\u0018\u00002\u00020\u0001B\u0007\b\u0002¢\u0006\u0002\u0010\u0002\u0082\u0001\u0003\u0003\u0004\u0005¨\u0006\u0006"},
   d2 = {"LExpr;", "", "()V", "LConst;", "LSum;", "LNotANumber;", "production sources for module app"}
)
public abstract class Expr {
   private Expr() {
   }

   // $FF: synthetic method
   public Expr(DefaultConstructorMarker $constructor_marker) {
      this();
   }
}
// Const.java
import kotlin.Metadata;
import kotlin.jvm.internal.DefaultConstructorMarker;
import org.jetbrains.annotations.NotNull;

@Metadata(
   mv = {1, 1, 10},
   bv = {1, 0, 2},
   k = 1,
   d1 = {"\u0000*\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0000\n\u0002\u0010\u0006\n\u0002\b\u0006\n\u0002\u0010\u000b\n\u0000\n\u0002\u0010\u0000\n\u0000\n\u0002\u0010\b\n\u0000\n\u0002\u0010\u000e\n\u0000\b\u0086\b\u0018\u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u00020\u0003¢\u0006\u0002\u0010\u0004J\t\u0010\u0007\u001a\u00020\u0003HÆ\u0003J\u0013\u0010\b\u001a\u00020\u00002\b\b\u0002\u0010\u0002\u001a\u00020\u0003HÆ\u0001J\u0013\u0010\t\u001a\u00020\n2\b\u0010\u000b\u001a\u0004\u0018\u00010\fHÖ\u0003J\t\u0010\r\u001a\u00020\u000eHÖ\u0001J\t\u0010\u000f\u001a\u00020\u0010HÖ\u0001R\u0011\u0010\u0002\u001a\u00020\u0003¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\u0011"},
   d2 = {"LConst;", "LExpr;", "number", "", "(D)V", "getNumber", "()D", "component1", "copy", "equals", "", "other", "", "hashCode", "", "toString", "", "production sources for module app"}
)
public final class Const extends Expr {
   private final double number;

   public final double getNumber() {
      return this.number;
   }

   public Const(double number) {
      super((DefaultConstructorMarker)null);
      this.number = number;
   }

   public final double component1() {
      return this.number;
   }

   @NotNull
   public final Const copy(double number) {
      return new Const(number);
   }

   // $FF: synthetic method
   // $FF: bridge method
   @NotNull
   public static Const copy$default(Const var0, double var1, int var3, Object var4) {
      if ((var3 & 1) != 0) {
         var1 = var0.number;
      }

      return var0.copy(var1);
   }

   public String toString() {
      return "Const(number=" + this.number + ")";
   }

   public int hashCode() {
      long var10000 = Double.doubleToLongBits(this.number);
      return (int)(var10000 ^ var10000 >>> 32);
   }

   public boolean equals(Object var1) {
      if (this != var1) {
         if (var1 instanceof Const) {
            Const var2 = (Const)var1;
            if (Double.compare(this.number, var2.number) == 0) {
               return true;
            }
         }

         return false;
      } else {
         return true;
      }
   }
}