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
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."":()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 "":(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."":()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
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."":()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 "":(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 "":(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 "":(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."":()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。