创建数据类
上节我们了解的数据类的创建,复习一下也就是通过 data class
来定义一个类,然后添加对应的属性即可,我们还是以 Student 为例
data class Student(var age:Int,var height:Double,val sex:String)
这里的代码就不做多解释了,我们都知道 Kotlin 也是 基于 jvm 的语言,那么我们看下 Kotlin 的代码编译成的 Java 代码是什么样
反编译
通过 IDEA 的 Kotlin 插件,转换为字节码,然后再转换为 Java 代码,如图
生成的字节码如下,之后点击Decompile 按钮,即可生成 Java 代码:
转换后的代码:
public final class Student {
private int age;
private double height;
@NotNull
private final String sex;
public final int getAge() {
return this.age;
}
public final void setAge(int var1) {
this.age = var1;
}
public final double getHeight() {
return this.height;
}
public final void setHeight(double var1) {
this.height = var1;
}
@NotNull
public final String getSex() {
return this.sex;
}
public Student(int age, double height, @NotNull String sex) {
Intrinsics.checkNotNullParameter(sex, "sex");
super();
this.age = age;
this.height = height;
this.sex = sex;
}
public final int component1() {
return this.age;
}
public final double component2() {
return this.height;
}
@NotNull
public final String component3() {
return this.sex;
}
@NotNull
public final Student copy(int age, double height, @NotNull String sex) {
Intrinsics.checkNotNullParameter(sex, "sex");
return new Student(age, height, sex);
}
// $FF: synthetic method
public static Student copy$default(Student var0, int var1, double var2, String var4, int var5, Object var6) {
if ((var5 & 1) != 0) {
var1 = var0.age;
}
if ((var5 & 2) != 0) {
var2 = var0.height;
}
if ((var5 & 4) != 0) {
var4 = var0.sex;
}
return var0.copy(var1, var2, var4);
}
@NotNull
public String toString() {
return "Student(age=" + this.age + ", height=" + this.height + ", sex=" + this.sex + ")";
}
public int hashCode() {
int var10000 = (Integer.hashCode(this.age) * 31 + Double.hashCode(this.height)) * 31;
String var10001 = this.sex;
return var10000 + (var10001 != null ? var10001.hashCode() : 0);
}
public boolean equals(@Nullable Object var1) {
if (this != var1) {
if (var1 instanceof Student) {
Student var2 = (Student)var1;
if (this.age == var2.age && Double.compare(this.height, var2.height) == 0 && Intrinsics.areEqual(this.sex, var2.sex)) {
return true;
}
}
return false;
} else {
return true;
}
}
}
和我们之前了解到的差不多,和 JavaBean 一样,有 getter/setter 方法。重写了 equals/hashCode/toString 方法,同样有构造函数。看 equals 方法的定义,我们可以知道,通过 data class 创建的对象,也是可以通过 ==
来判断对象是否相等。
fun main() {
val student:Student = Student(12,145.00,"female")
val student2:Student = Student(12,145.00,"female")
println("equals : ${student.equals(student2)}")
println("== : ${student == student2}")
}
===========
equals : true
== : true
仔细观察我们还会发现有几个方法,在我们定义 JavaBean 的时候是没有遇到的copy/component1/component2…componentN
copy 方法
我们先看 copy:
@NotNull
public final Student copy(int age, double height, @NotNull String sex) {
Intrinsics.checkNotNullParameter(sex, "sex");
return new Student(age, height, sex);
}
// $FF: synthetic method
public static Student copy$default(Student var0, int var1, double var2, String var4, int var5, Object var6) {
if ((var5 & 1) != 0) {
var1 = var0.age;
}
if ((var5 & 2) != 0) {
var2 = var0.height;
}
if ((var5 & 4) != 0) {
var4 = var0.sex;
}
return var0.copy(var1, var2, var4);
}
通过代码我们发现,这两个方法中 copy 方法是根据传进来的参数创建新的 Student 对象,从方法名来看也就是复制了一个对象,而copy$default
方法复制的是传进来的 student 的属性(如果对应的属性没有设值)。这么做有什么好处呢?我们在 Java 中如果想要复制一个对象会这样写
Student s1 = new Student(12,145.0,"female");
Student s2 = s1;
s2.setAge(15);
这里的修改 s2,会导致 s1 的 age 也变了,因为他们的引用的地址是同一个。
如果是复制后创建一个新的对象是这样的
Student s1 = new Student(12,145.0,"female");
Student s2 = new Studetn(s1.getAge(),s2..getHeight(),s1.getSex());
s2.setAge(15);
在 Kotlin 中使用 copy 方法却不会修改 s1 的属性。
val s1:Student = Student(12,145.00,"female")
val s2 = s1.copy()
s2.age = 15
println("s1 is : ${s1} s2 is : ${s2}")
输出 ======
s1 is : Student(age=12, height=145.0, sex=female) s2 is : Student(age=15, height=145.0, sex=female)
同样的,因为 copy 函数也是可以传递参数的,我们也可以这么写
val s1:Student = Student(12,145.00,"female")
val s2 = s1.copy(sex = "male")
s2.age = 15
println("s1 is : ${s1} s2 is : ${s2}")
输出====
s1 is : Student(age=12, height=145.0, sex=female) s2 is : Student(age=15, height=145.0, sex=male)
componentN
接着看下 component 代码:
public final int component1() {
return this.age;
}
public final double component2() {
return this.height;
}
@NotNull
public final String component3() {
return this.sex;
}
可以看出,这边都是 Student 对象的属性值,而且是按照顺序编号的,所以 componentN 中的 N 也就是有多少个属性参数就会到几。这么做有什么用呢?我们简单想一下,在 Java 中我们拿出对象的值是不是如下操作:
Student student = new Student(12,145.0,"female");
int age = student.getAge();
double height = student.getHeight();
String sex = student.getSex();
在 Kotlin 中我们常规操作是不是这样:
val student = Student(12,145.00,"female")
val age = student.age
val height = student.height
val sex = student.sex
println("age : $age, height : $height, sex : $sex")
但是在在 Kotlin 的代码中,还有一种更简单的方式
val student = Student(12,145.00,"female")
val (age,height,sex) = student
println("age : $age, height : $height, sex : $sex")
这就是 Kotlin 的解构,而解构就依赖 componentN 方法。除了IDEA 自动生成的 component 方法外,我们也可以自己写 component 方法例如:
data class Student(var age: Int, var height: Double, val sex: String){
var name:String = "" //新增 name 属性
constructor(age: Int,height: Double,sex: String,name:String):this(age,height, sex){
this.name = name //name 属性赋值
}
operator fun component4():String{ //使用 operator 修饰
return this.name
}
}
我们的使用方式如下:
val student: Student = Student(12, 145.00, "female","lihua")
val (age, height, sex, name) = student //解构增加 name 属性
println("age : $age, height : $height, sex : $sex , name : $name")