Scala Fields in Classes
Scala Fields in Classes
private var定义field
class Student(name: String, age: Int) {
private var _id: Int = name.hashCode
def id: Int = _id //reader method
def id_=(newId: Int): Unit = _id = newId //writer method
def sayHi(): String = {
"my name is " + name + ",my age is " + age
}
}
在类的内部定义一个这样的属性字段private var _id: Int = name.hashCode。反编译如下，
C:\WorkSpace5-gitosc\scala-sample\out\production\scala-sample\com\usoft>javap -p Student.class
Compiled from "Student.scala"
public class com.usoft.Student {
private final java.lang.String name;
private final int age;
private int _id;
private int _id();
private void _id_$eq(int);
public int id();
public void id_$eq(int);
public java.lang.String sayHi();
public com.usoft.Student(java.lang.String, int);
}
_id 作为Stuent类的一个属性字段，显式的定义了reader method 和 writer method
object Main0 {
def main(args: Array[String]) {
println("hello world!!")
val s = new Student("xxggy", 12)
println(s.id)
s.id = "xxggyy".hashCode
println(s.id)
}
}
通过reader method 和 writer method读写_id 的值。同时还要注意到因为该field是用private定义的，
scala 还隐式的生成了这两个方法，这两个方法都是private私有的。
private int _id(); private void _id_$eq(int);
private val 定义field
class Student0(name: String, age: Int) {
private val _id: Int = name.hashCode
def id: Int = _id
def sayHi(): String = {
"my name is " + name + ",my age is " + age
}
}
在类中 使用 private val声明 _id ,由于val是不可变的，所以只能有reader method。
val s0 = new Student0("xx", 23)
println(s0.id)
反编译结果，
C:\WorkSpace5-gitosc\scala-sample\out\production\scala-sample\com\usoft>javap -p Student0.class
Compiled from "Student.scala"
public class com.usoft.Student0 {
private final java.lang.String name;
private final int age;
private final int _id;
private int _id();
public int id();
public java.lang.String sayHi();
public com.usoft.Student0(java.lang.String, int);
}
val定义的不可变的变量是final类型的 private final int _id;。还可以看到scala隐式的生成的private int _id()方法，这个方法不是我们定义的。
val 和 var 定义field
class Student1(name: String, age: Int) {
val _id: Int = name.hashCode
def id: Int = _id
def sayHi(): String = {
"my name is " + name + ",my age is " + age
}
}
class Student2(name: String, age: Int) {
var _id: Int = name.hashCode
def id: Int = _id //相当于getter方法
def id_=(newId: Int): Unit = _id = newId //相当于setter方法
def sayHi(): String = {
"my name is " + name + ",my age is " + age
}
}
上面两个类都是直接用var和val直接定义的，没有加访问修饰符，通过反编译我们来看一下不加访问修饰符有什么效果。
C:\WorkSpace5-gitosc\scala-sample\out\production\scala-sample\com\usoft>javap -p Student1.class
Compiled from "Student.scala"
public class com.usoft.Student1 {
private final java.lang.String name;
private final int age;
private final int _id;
public int _id();//scala隐式的生成的reader方法
public int id();//定义的reader方法
public java.lang.String sayHi();
public com.usoft.Student1(java.lang.String, int);
}
C:\WorkSpace5-gitosc\scala-sample\out\production\scala-sample\com\usoft>javap -p Student2.class
Compiled from "Student.scala"
public class com.usoft.Student2 {
private final java.lang.String name;
private final int age;
private int _id;
public int _id();//scala隐式的生成的reader方法
public void _id_$eq(int); // scala隐式的生成的
public int id();//定义的reader方法
public void id_$eq(int);//定义的writer方法
public java.lang.String sayHi();
public com.usoft.Student2(java.lang.String, int);
}
这样的话 效果就很明显了。
val 和 private val定义field
class Student3(name: String, age: Int) {
val _id: Int = name.hashCode
def sayHi(): String = {
"my name is " + name + ",my age is " + age
}
}
class Student4(name: String, age: Int) {
private val _id: Int = name.hashCode
def sayHi(): String = {
"my name is " + name + ",my age is " + age
}
}
反编译比较一下异同，
C:\WorkSpace5-gitosc\scala-sample\out\production\scala-sample\com\usoft>javap -p Student3.class
Compiled from "Student.scala"
public class com.usoft.Student3 {
private final java.lang.String name;
private final int age;
private final int _id;
public int _id();
public java.lang.String sayHi();
public com.usoft.Student3(java.lang.String, int);
}
C:\WorkSpace5-gitosc\scala-sample\out\production\scala-sample\com\usoft>javap -p Student4.class
Compiled from "Student.scala"
public class com.usoft.Student4 {
private final java.lang.String name;
private final int age;
private final int _id;
private int _id();
public java.lang.String sayHi();
public com.usoft.Student4(java.lang.String, int);
}
用val定义的 reader method 是：
public int _id();
用private val定义的 reader method 是：
private int _id();
差别就在这里。其实我认为 在field的定义上加的访问修饰符，只能加 private 和 protected 访问修饰符，表示该field的自动生成的reader method的访问修饰符是 private。
加 protected访问修饰符，生成的reader method还是public的，可以进行验证，
class Student5(name: String, age: Int) {
protected val _id: Int = name.hashCode
def sayHi(): String = {
"my name is " + name + ",my age is " + age
}
}
反编译，
C:\WorkSpace5-gitosc\scala-sample\out\production\scala-sample\com\usoft>javap -p Student5.class
Compiled from "Student.scala"
public class com.usoft.Student5 {
private final java.lang.String name;
private final int age;
private final int _id;
public int _id();
public java.lang.String sayHi();
public com.usoft.Student5(java.lang.String, int);
}
你看 public int _id();这个reader method还是public权限的。
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