kotlin之object,companion object,内部class和inner class之间的区别

最近开始使用kotlin写项目了,这几天一直对object,companion object,内部类和inner class搞得有点迷糊

现在抽空,理清一下他们之间区别

我定义了一个class A
然后再class A内部分别定义了class B,object C,companion object D,inner class E四中不同的类
每个类中包含一个字段和一个函数

/**
 * Copyright:AndroidInterview
 * Author: liyang <br>
 * Date:2019-11-12 14:29<br>
 * Desc: <br>
 */
class A  {
    var sex = "男"
    val age = 11
    val a1:String="nan"
    val a: String
        get() {
            return when (sex) {
                "男" -> "man"
                else -> "women"
            }
        }

    fun a() {
        //class B
        //在A类中调用class B的b方法
        val b = B()
        b.b()
        b.b
        
        //inner class E
        //在A类中调用inner class E的e变量和e()方法
        val e=E()
        e.e
        e.e()

        //object C
        //在A类中调用object C的c()方法和c变量
        C.c()
        C.c

        //companion object D
        //可以直接在A类中调用伴生类的d()方法和d变量
        d()
        d
    }

    class B {
        val b = "b"
        fun b() {
            println("b() is invoked")
        }
    }

    object C {
       const val c = "c"
        fun c() {
            println("c() is invoked")
        }
    }


    companion object D {
        val d = "d"

        fun d() {
            println("d() is invoked")
        }
    }

    inner class E {
        val e = "e"
        fun e() {
            println("e() is invoked")
        }
    }


}

接下来在main方法中调用它们,看看调用的时候有什么不同

/**
 * Copyright:AndroidInterview
 * Author: liyang <br>
 * Date:2019-11-12 14:29<br>
 * Desc: <br>
 */
open class Outer {
    companion object {
        @JvmStatic
        fun main(args: Array<String>) {
            val a = A()
            a.a
            a.a()

            //内部类
            val b = A.B()
            println(b.b)
            b.b()

            //inner class
            val e = a.E()
            println(e.e)
            e.e()

            //object 
            println(A.C.c)
            A.C.c()

            //companion object
            println(A.d)
            A.d()
        }
    }
}

内部类类型	在A类内部调用	在A类外部调用
object C	`C.c()`,`C.c`	`A.C.c`,`A.C.c()`
companion object D	可以直接使用 `d`,`d()`	`A.d`,`A.d()`
class B	实例化B对象`val b = B(); b.b() ; b.b`	实例化B对象`val b = A.B(); b.b() ; b.b`
inner class E	实例化E对象`val e = E(); e.e() ; e.e`	通过外部类的实例a来实例化E对象`val e = a.E(); e.e() ; e.e`

为什么这样?反编译看看源码就知道了

A.decompiled.java

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

@Metadata(
   mv = {1, 1, 15},
   bv = {1, 0, 3},
   k = 1,
   d1 = {"\u0000$\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\b\u0005\n\u0002\u0010\b\n\u0002\b\u0006\n\u0002\u0010\u0002\n\u0002\b\u0005\u0018\u0000 \u00142\u00020\u0001:\u0004\u0012\u0013\u0014\u0015B\u0005¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0011R\u0011\u0010\u0003\u001a\u00020\u00048F¢\u0006\u0006\u001a\u0004\b\u0005\u0010\u0006R\u0014\u0010\u0007\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\b\u0010\u0006R\u0014\u0010\t\u001a\u00020\nX\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u000b\u0010\fR\u001a\u0010\r\u001a\u00020\u0004X\u0086\u000e¢\u0006\u000e\n\u0000\u001a\u0004\b\u000e\u0010\u0006\"\u0004\b\u000f\u0010\u0010¨\u0006\u0016"},
   d2 = {"LA;", "", "()V", "a", "", "getA", "()Ljava/lang/String;", "a1", "getA1", "age", "", "getAge", "()I", "sex", "getSex", "setSex", "(Ljava/lang/String;)V", "", "B", "C", "D", "E", "baselib"}
)
public final class A {
   @NotNull
   private String sex = "男";
   private final int age = 11;
   @NotNull
   private final String a1 = "nan";
   @NotNull
   private static final String d = "d";
   public static final A.D D = new A.D((DefaultConstructorMarker)null);

   @NotNull
   public final String getSex() {
      return this.sex;
   }

   public final void setSex(@NotNull String var1) {
      Intrinsics.checkParameterIsNotNull(var1, "<set-?>");
      this.sex = var1;
   }

   public final int getAge() {
      return this.age;
   }

   @NotNull
   public final String getA1() {
      return this.a1;
   }

   @NotNull
   public final String getA() {
      String var1 = this.sex;
      String var10000;
      switch(var1.hashCode()) {
      case 30007:
         if (var1.equals("男")) {
            var10000 = "man";
            break;
         }
      default:
         var10000 = "women";
      }

      return var10000;
   }

   public final void a() {
      A.B b = new A.B();
      b.b();
      b.getB();
      A.C.INSTANCE.c();
      D.d();
      String var10000 = d;
   }

   @Metadata(
      mv = {1, 1, 15},
      bv = {1, 0, 3},
      k = 1,
      d1 = {"\u0000\u001a\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\u0018\u00002\u00020\u0001B\u0005¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0007R\u0014\u0010\u0003\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\b"},
      d2 = {"LA$B;", "", "()V", "b", "", "getB", "()Ljava/lang/String;", "", "baselib"}
   )
   public static final class B {
      @NotNull
      private final String b = "b";

      @NotNull
      public final String getB() {
         return this.b;
      }

      public final void b() {
         String var1 = "b() is invoked";
         boolean var2 = false;
         System.out.println(var1);
      }
   }

   @Metadata(
      mv = {1, 1, 15},
      bv = {1, 0, 3},
      k = 1,
      d1 = {"\u0000\u0016\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\u0010\u0002\n\u0000\bÆ\u0002\u0018\u00002\u00020\u0001B\u0007\b\u0002¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0005R\u000e\u0010\u0003\u001a\u00020\u0004X\u0086T¢\u0006\u0002\n\u0000¨\u0006\u0006"},
      d2 = {"LA$C;", "", "()V", "c", "", "", "baselib"}
   )
   public static final class C {
      @NotNull
      public static final String c = "c";
      public static final A.C INSTANCE;

      public final void c() {
         String var1 = "c() is invoked";
         boolean var2 = false;
         System.out.println(var1);
      }

      private C() {
      }

      static {
         A.C var0 = new A.C();
         INSTANCE = var0;
      }
   }

   @Metadata(
      mv = {1, 1, 15},
      bv = {1, 0, 3},
      k = 1,
      d1 = {"\u0000\u001a\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\b\u0086\u0004\u0018\u00002\u00020\u0001B\u0005¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0007R\u0014\u0010\u0003\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\b"},
      d2 = {"LA$E;", "", "(LA;)V", "e", "", "getE", "()Ljava/lang/String;", "", "baselib"}
   )
   public final class E {
      @NotNull
      private final String e = "e";

      @NotNull
      public final String getE() {
         return this.e;
      }

      public final void e() {
         String var1 = "e() is invoked";
         boolean var2 = false;
         System.out.println(var1);
      }
   }

   @Metadata(
      mv = {1, 1, 15},
      bv = {1, 0, 3},
      k = 1,
      d1 = {"\u0000\u001a\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\b\u0086\u0003\u0018\u00002\u00020\u0001B\u0007\b\u0002¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0007R\u0014\u0010\u0003\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\b"},
      d2 = {"LA$D;", "", "()V", "d", "", "getD", "()Ljava/lang/String;", "", "baselib"}
   )
   public static final class D {
      @NotNull
      public final String getD() {
         return A.d;
      }

      public final void d() {
         String var1 = "d() is invoked";
         boolean var2 = false;
         System.out.println(var1);
      }

      private D() {
      }

      // $FF: synthetic method
      public D(DefaultConstructorMarker $constructor_marker) {
         this();
      }
   }
}

通过查看decompile A.kt文件之后得到的A.decompiled.java源文件
可以得到以下几个重要信息

kotlin中只要不是被inner修饰的内部类,都是静态的!所以class B,object C,companion object D都是静态内部类
inner class E是非静态内部类,所以它会持有A的引用
object C和companion object D的构造函数都是private的
无论是object还是companion object中的变量都是静态常量

梳理下

内部类类型	是否可以定义多个	是否是单例	是否是静态内部类
class	是	否	是
inner class	是	否	否
object	是	是	是
companion object	否	是	是