Kotlin数据类深度解析与底层剖析

2019/07/23 22:31
阅读数 32

今天来学习一下全新关于Kotlin的概念---数据类【data class】,也是非常有用的东东,下面先来对其进行理论化的了解:

数据类其实跟java的实体类(model)很类似,像Java定义一个Person类,里面有各种属性然后再生成它的get和set方法,当然可以借助于IDE来生成,但是其实java的这种做法是挺冗长啰嗦的,当然其实有现成的解决方案来解决这个冗长了,比如用lombok库,利用注解就可以动态生成get和set,接下来咱们来看一下kotlin数据类是怎么使用的:

这样数据库就定义成了,也没任何额外的get和set方法,是不是特别的简洁,好,下面来使用一下:

如果将data关键字去掉,其输出就会是对象的toString()了,如下:

数据类的定义是不是很简单,其实数据类是有一些要求的,下面来瞅下:

1、主构造方法,至少要有一个参数。

2、所有的主构造方法参数都需要被标记为var或val。

3、数据类不能是抽象的、open的、sealed【密封类,还未学到】的以及inner的。

另外对于数据类,编译器会自动的生成如下内容:

1、equals/hashcode对。

2、toString()方法,形式为我们看到的:

3、针对属性的componentN方法,并且是按照属性的声明顺序来生成的。

啥意思,也就是针对咱们的这个类会生成component1()、component2()、component3(),而这三个方法的返回值是按照我们定义的name、age、address。

好,下面来反编译一下,来看一下数据类的底层细节:

xiongweideMacBook-Pro:kotlin_lecture xiongwei$ javap -c com/kotlin/test2/Person.class
Compiled from "HelloKotlin1.kt"
public final class com.kotlin.test2.Person {
  public final java.lang.String getName();
    Code:
       0: aload_0
       1: getfield      #11                 // Field name:Ljava/lang/String;
       4: areturn

  public final int getAge();
    Code:
       0: aload_0
       1: getfield      #19                 // Field age:I
       4: ireturn

  public final void setAge(int);
    Code:
       0: aload_0
       1: iload_1
       2: putfield      #19                 // Field age:I
       5: return

  public final java.lang.String getAddress();
    Code:
       0: aload_0
       1: getfield      #26                 // Field address:Ljava/lang/String;
       4: areturn

  public final void setAddress(java.lang.String);
    Code:
       0: aload_1
       1: ldc           #29                 // String <set-?>
       3: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: aload_0
       7: aload_1
       8: putfield      #26                 // Field address:Ljava/lang/String;
      11: return

  public com.kotlin.test2.Person(java.lang.String, int, java.lang.String);
    Code:
       0: aload_1
       1: ldc           #38                 // String name
       3: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: aload_3
       7: ldc           #39                 // String address
       9: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
      12: aload_0
      13: invokespecial #42                 // Method java/lang/Object."<init>":()V
      16: aload_0
      17: aload_1
      18: putfield      #11                 // Field name:Ljava/lang/String;
      21: aload_0
      22: iload_2
      23: putfield      #19                 // Field age:I
      26: aload_0
      27: aload_3
      28: putfield      #26                 // Field address:Ljava/lang/String;
      31: return

  public final java.lang.String component1();
    Code:
       0: aload_0
       1: getfield      #11                 // Field name:Ljava/lang/String;
       4: areturn

  public final int component2();
    Code:
       0: aload_0
       1: getfield      #19                 // Field age:I
       4: ireturn

  public final java.lang.String component3();
    Code:
       0: aload_0
       1: getfield      #26                 // Field address:Ljava/lang/String;
       4: areturn

  public final com.kotlin.test2.Person copy(java.lang.String, int, java.lang.String);
    Code:
       0: aload_1
       1: ldc           #38                 // String name
       3: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: aload_3
       7: ldc           #39                 // String address
       9: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
      12: new           #2                  // class com/kotlin/test2/Person
      15: dup
      16: aload_1
      17: iload_2
      18: aload_3
      19: invokespecial #49                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
      22: areturn

  public static com.kotlin.test2.Person copy$default(com.kotlin.test2.Person, java.lang.String, int, java.lang.String, int, java.lang.Object);
    Code:
       0: iload         4
       2: iconst_1
       3: iand
       4: ifeq          12
       7: aload_0
       8: getfield      #11                 // Field name:Ljava/lang/String;
      11: astore_1
      12: iload         4
      14: iconst_2
      15: iand
      16: ifeq          24
      19: aload_0
      20: getfield      #19                 // Field age:I
      23: istore_2
      24: iload         4
      26: iconst_4
      27: iand
      28: ifeq          36
      31: aload_0
      32: getfield      #26                 // Field address:Ljava/lang/String;
      35: astore_3
      36: aload_0
      37: aload_1
      38: iload_2
      39: aload_3
      40: invokevirtual #53                 // Method copy:(Ljava/lang/String;ILjava/lang/String;)Lcom/kotlin/test2/Person;
      43: areturn

  public java.lang.String toString();
    Code:
       0: new           #56                 // class java/lang/StringBuilder
       3: dup
       4: invokespecial #57                 // Method java/lang/StringBuilder."<init>":()V
       7: ldc           #59                 // String Person(name=
       9: invokevirtual #63                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      12: aload_0
      13: getfield      #11                 // Field name:Ljava/lang/String;
      16: invokevirtual #63                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      19: ldc           #65                 // String , age=
      21: invokevirtual #63                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: aload_0
      25: getfield      #19                 // Field age:I
      28: invokevirtual #68                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      31: ldc           #70                 // String , address=
      33: invokevirtual #63                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      36: aload_0
      37: getfield      #26                 // Field address:Ljava/lang/String;
      40: invokevirtual #63                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      43: ldc           #72                 // String )
      45: invokevirtual #63                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      48: invokevirtual #74                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      51: areturn

  public int hashCode();
    Code:
       0: aload_0
       1: getfield      #11                 // Field name:Ljava/lang/String;
       4: dup
       5: ifnull        14
       8: invokevirtual #77                 // Method java/lang/Object.hashCode:()I
      11: goto          16
      14: pop
      15: iconst_0
      16: bipush        31
      18: imul
      19: aload_0
      20: getfield      #19                 // Field age:I
      23: invokestatic  #82                 // Method java/lang/Integer.hashCode:(I)I
      26: iadd
      27: bipush        31
      29: imul
      30: aload_0
      31: getfield      #26                 // Field address:Ljava/lang/String;
      34: dup
      35: ifnull        44
      38: invokevirtual #77                 // Method java/lang/Object.hashCode:()I
      41: goto          46
      44: pop
      45: iconst_0
      46: iadd
      47: ireturn

  public boolean equals(java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: if_acmpeq     64
       5: aload_1
       6: instanceof    #2                  // class com/kotlin/test2/Person
       9: ifeq          66
      12: aload_1
      13: checkcast     #2                  // class com/kotlin/test2/Person
      16: astore_2
      17: aload_0
      18: getfield      #11                 // Field name:Ljava/lang/String;
      21: aload_2
      22: getfield      #11                 // Field name:Ljava/lang/String;
      25: invokestatic  #91                 // Method kotlin/jvm/internal/Intrinsics.areEqual:(Ljava/lang/Object;Ljava/lang/Object;)Z
      28: ifeq          66
      31: aload_0
      32: getfield      #19                 // Field age:I
      35: aload_2
      36: getfield      #19                 // Field age:I
      39: if_icmpne     46
      42: iconst_1
      43: goto          47
      46: iconst_0
      47: ifeq          66
      50: aload_0
      51: getfield      #26                 // Field address:Ljava/lang/String;
      54: aload_2
      55: getfield      #26                 // Field address:Ljava/lang/String;
      58: invokestatic  #91                 // Method kotlin/jvm/internal/Intrinsics.areEqual:(Ljava/lang/Object;Ljava/lang/Object;)Z
      61: ifeq          66
      64: iconst_1
      65: ireturn
      66: iconst_0
      67: ireturn
}
xiongweideMacBook-Pro:kotlin_lecture xiongwei$

其中是不是可以看到自动为其生成属性的set和get方法了,除了name之外,因为它声明的是一个常量:

接着生成了两个参数的构造方法:

再接着就看到我们第三点说的componentN了,如下:

再往下看:

还帮我们生成了toString()方法:

最后就是hashCode()和equals():

下面继续来对数据类成员的继承要点进行说明:

1、如果数据类中显示定义了equals,hashCode或者是toString方法,或者是在数据类的父类中将这些方法声明为final,那么这些方法就不会再生成,转而使用已有的。

2、如果父类拥有componentN方法,并且是open的以及返回兼容的类型,那么编译器就会在数据类中生成相应的componentN方法,并且重写父类的这些方法;如果父类中的这些方法由于不兼容的签名或者被定义为final的,那么编译器就会报错。

3、在数据类中显示提供componentN方法以及copy方法实现是不允许的。下面来试一下:

其错误提示为:

接下来来对生成的copy方法进行一个说明,它的重用主要是根据已有的属性来创建一个新的对象,比如:

如果咱们这样写:

接下来想一个问题:既然Kotlin编译主动给属性生成了set和get方法了,那。。是不是可以直接像Java那样来操作属性呢,下面瞅下:

不通这样写,只能:

解构声明:

在主构造方法中有多少个参数,就会依次生成相应的component1、component2、component3...,这些方法返回的就是对应字段的值,componentN方法是用来实现解构声明的。如何理解,看代码:

那,如果只写两个变量呢?

另外,我们通过反编译数据类可以发现,貌似生成的是带参的构造方法,如下:

那,如何声明不带参的构造方法呢,其实在JVM平台上,如果生成的类需要拥有无参构造方法,那么就需要为所有属性指定默认值,如下:

再次反编译论证一下:

xiongweideMacBook-Pro:kotlin_lecture xiongwei$ javap -c com/kotlin/test2/Person2.class
Compiled from "HelloKotlin1.kt"
public final class com.kotlin.test2.Person2 {
  public final java.lang.String getName();
    Code:
       0: aload_0
       1: getfield      #11                 // Field name:Ljava/lang/String;
       4: areturn

  public final int getAge();
    Code:
       0: aload_0
       1: getfield      #19                 // Field age:I
       4: ireturn

  public final void setAge(int);
    Code:
       0: aload_0
       1: iload_1
       2: putfield      #19                 // Field age:I
       5: return

  public final java.lang.String getAddress();
    Code:
       0: aload_0
       1: getfield      #26                 // Field address:Ljava/lang/String;
       4: areturn

  public final void setAddress(java.lang.String);
    Code:
       0: aload_1
       1: ldc           #29                 // String <set-?>
       3: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: aload_0
       7: aload_1
       8: putfield      #26                 // Field address:Ljava/lang/String;
      11: return

  public com.kotlin.test2.Person2(java.lang.String, int, java.lang.String);
    Code:
       0: aload_1
       1: ldc           #38                 // String name
       3: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: aload_3
       7: ldc           #39                 // String address
       9: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
      12: aload_0
      13: invokespecial #42                 // Method java/lang/Object."<init>":()V
      16: aload_0
      17: aload_1
      18: putfield      #11                 // Field name:Ljava/lang/String;
      21: aload_0
      22: iload_2
      23: putfield      #19                 // Field age:I
      26: aload_0
      27: aload_3
      28: putfield      #26                 // Field address:Ljava/lang/String;
      31: return

  public com.kotlin.test2.Person2(java.lang.String, int, java.lang.String, int, kotlin.jvm.internal.DefaultConstructorMarker);
    Code:
       0: iload         4
       2: iconst_1
       3: iand
       4: ifeq          10
       7: ldc           #45                 // String
       9: astore_1
      10: iload         4
      12: iconst_2
      13: iand
      14: ifeq          20
      17: bipush        20
      19: istore_2
      20: iload         4
      22: iconst_4
      23: iand
      24: ifeq          30
      27: ldc           #47                 // String henan
      29: astore_3
      30: aload_0
      31: aload_1
      32: iload_2
      33: aload_3
      34: invokespecial #49                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
      37: return

  public com.kotlin.test2.Person2();
    Code:
       0: aload_0
       1: aconst_null
       2: iconst_0
       3: aconst_null
       4: bipush        7
       6: aconst_null
       7: invokespecial #51                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
      10: return

  public final java.lang.String component1();
    Code:
       0: aload_0
       1: getfield      #11                 // Field name:Ljava/lang/String;
       4: areturn

  public final int component2();
    Code:
       0: aload_0
       1: getfield      #19                 // Field age:I
       4: ireturn

  public final java.lang.String component3();
    Code:
       0: aload_0
       1: getfield      #26                 // Field address:Ljava/lang/String;
       4: areturn

  public final com.kotlin.test2.Person2 copy(java.lang.String, int, java.lang.String);
    Code:
       0: aload_1
       1: ldc           #38                 // String name
       3: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: aload_3
       7: ldc           #39                 // String address
       9: invokestatic  #35                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
      12: new           #2                  // class com/kotlin/test2/Person2
      15: dup
      16: aload_1
      17: iload_2
      18: aload_3
      19: invokespecial #49                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
      22: areturn

  public static com.kotlin.test2.Person2 copy$default(com.kotlin.test2.Person2, java.lang.String, int, java.lang.String, int, java.lang.Object);
    Code:
       0: iload         4
       2: iconst_1
       3: iand
       4: ifeq          12
       7: aload_0
       8: getfield      #11                 // Field name:Ljava/lang/String;
      11: astore_1
      12: iload         4
      14: iconst_2
      15: iand
      16: ifeq          24
      19: aload_0
      20: getfield      #19                 // Field age:I
      23: istore_2
      24: iload         4
      26: iconst_4
      27: iand
      28: ifeq          36
      31: aload_0
      32: getfield      #26                 // Field address:Ljava/lang/String;
      35: astore_3
      36: aload_0
      37: aload_1
      38: iload_2
      39: aload_3
      40: invokevirtual #60                 // Method copy:(Ljava/lang/String;ILjava/lang/String;)Lcom/kotlin/test2/Person2;
      43: areturn

  public java.lang.String toString();
    Code:
       0: new           #63                 // class java/lang/StringBuilder
       3: dup
       4: invokespecial #64                 // Method java/lang/StringBuilder."<init>":()V
       7: ldc           #66                 // String Person2(name=
       9: invokevirtual #70                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      12: aload_0
      13: getfield      #11                 // Field name:Ljava/lang/String;
      16: invokevirtual #70                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      19: ldc           #72                 // String , age=
      21: invokevirtual #70                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: aload_0
      25: getfield      #19                 // Field age:I
      28: invokevirtual #75                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      31: ldc           #77                 // String , address=
      33: invokevirtual #70                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      36: aload_0
      37: getfield      #26                 // Field address:Ljava/lang/String;
      40: invokevirtual #70                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      43: ldc           #79                 // String )
      45: invokevirtual #70                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      48: invokevirtual #81                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      51: areturn

  public int hashCode();
    Code:
       0: aload_0
       1: getfield      #11                 // Field name:Ljava/lang/String;
       4: dup
       5: ifnull        14
       8: invokevirtual #84                 // Method java/lang/Object.hashCode:()I
      11: goto          16
      14: pop
      15: iconst_0
      16: bipush        31
      18: imul
      19: aload_0
      20: getfield      #19                 // Field age:I
      23: invokestatic  #89                 // Method java/lang/Integer.hashCode:(I)I
      26: iadd
      27: bipush        31
      29: imul
      30: aload_0
      31: getfield      #26                 // Field address:Ljava/lang/String;
      34: dup
      35: ifnull        44
      38: invokevirtual #84                 // Method java/lang/Object.hashCode:()I
      41: goto          46
      44: pop
      45: iconst_0
      46: iadd
      47: ireturn

  public boolean equals(java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: if_acmpeq     64
       5: aload_1
       6: instanceof    #2                  // class com/kotlin/test2/Person2
       9: ifeq          66
      12: aload_1
      13: checkcast     #2                  // class com/kotlin/test2/Person2
      16: astore_2
      17: aload_0
      18: getfield      #11                 // Field name:Ljava/lang/String;
      21: aload_2
      22: getfield      #11                 // Field name:Ljava/lang/String;
      25: invokestatic  #98                 // Method kotlin/jvm/internal/Intrinsics.areEqual:(Ljava/lang/Object;Ljava/lang/Object;)Z
      28: ifeq          66
      31: aload_0
      32: getfield      #19                 // Field age:I
      35: aload_2
      36: getfield      #19                 // Field age:I
      39: if_icmpne     46
      42: iconst_1
      43: goto          47
      46: iconst_0
      47: ifeq          66
      50: aload_0
      51: getfield      #26                 // Field address:Ljava/lang/String;
      54: aload_2
      55: getfield      #26                 // Field address:Ljava/lang/String;
      58: invokestatic  #98                 // Method kotlin/jvm/internal/Intrinsics.areEqual:(Ljava/lang/Object;Ljava/lang/Object;)Z
      61: ifeq          66
      64: iconst_1
      65: ireturn
      66: iconst_0
      67: ireturn
}

果真如此,跟Java不同的是,如果Java中的属性没有赋值会有默认值,而在Kotlin中默认是不会自动给默认值给属性进行赋值的,其实这样做更加的安全,这样既便我们在使用Kotlin的无参构造方法时,它里面的属性也能保证都有初始值。

关于数据类在实际中使用是非常多的,可见也比Java要简洁好使得多,可以多多体会。

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