来源:http://blog.csdn.net/lovehuangjiaju/article/details/47009607
本节主要内容
1 类定义、创建对象
2 主构造器
3 辅助构造器
类定义、创建对象
class Person { var name:String=null}
Person类在编译后会生成Person.class文件
利用javap -prviate Person命令查看字节码文件内容,可以看得到以下内容
D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private java.lang.String name; public java.lang.String name(); public void name_$eq(java.lang.String); public cn.scala.xtwy.Person();}
从字节码文件内容可以看到:虽然我们只在Person类中定义了一个类成员(域)name,类型为String,但Scala会默认帮我们生成name()与name_=()及构造函数Person()。其中name()对应Java中的getter方法,name_=()对应java中的setter方法(由于JVM中不允许出现=,所以用$eq代替。值得注意的是定义的是公有成员,但生成的字节码中却是以私有的方式实现的,生成的getter、setter方法是公有的
因此,可以直接new操作创建Person对象
//默认已经有构建函数,所以可以直接newscala> val p=new Person()p: Person = Person@84c504//直接调用getter和setter方法//setter方法scala> p.name_=("john")//getter方法scala> p.nameres2: String = john//直接修改,但其实调用的是p.name_=("jonh")scala> p.name="jonh"p.name: String = jonh//getter方法scala> p.nameres28: String = jonh
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你也可以定义自己的getter和setter方法
class Person{ private var privateName:String=null; def name=privateName def name_=(name:String){ this.privateName=name }}
D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private java.lang.String privateName; private java.lang.String privateName(); private void privateName_$eq(java.lang.String); public java.lang.String name(); public void name_$eq(java.lang.String); public cn.scala.xtwy.Person();}
从生成的字节码中可以看出:(1)定义成私有成员,其getter、setter方法也是私有的;(2)直接能访问的是我们自己定义的getter、setter方法。下面给出的是调用方式
scala> val p=new Person()p: Person = Person@12d0b54scala> p.nameres29: String = null//直接赋值法scala> p.name="john"p.name: String = johnscala> p.nameres30: String = john
从代码执行产生的结果,我们可以知道:通过p.name=“john”这种方式进行赋值,调用者并不需要知道是其通过方法调用还是字段访问来进行操作的,这便是著名的统一访问原则
如果类的成员域是val类型的变量,则只会生成getter方法
class Person { val name:String="john"}D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private final java.lang.String name; public java.lang.String name(); public cn.scala.xtwy.Person();}
从字节码文件中可以看出:val变量对应的是java中的final类型变量,只生成了getter方法
如果将成员域定义为private[this],则不会生成getter、setter方法
class Person { private[this] var name:String="john"}D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private java.lang.String name; public cn.scala.xtwy.Person();}
在java语言当中,在定义JavaBean的时候生成的都是setXxx()、getXxx()方法,但scala语言生成的getter方法和setter方法并不是这样的,如果也需要程序自动会生成getter方法和setter方法,则需要引入 scala.reflect.BeanProperty
然后采用注解的方式修饰变量
class Person { @BeanProperty var name:String="john"}D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private java.lang.String name; public java.lang.String name(); public void name_$eq(java.lang.String); public void setName(java.lang.String); public java.lang.String getName(); public cn.scala.xtwy.Person();}
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下图给出的是getter、setter方法产生的规则
来源:scala for the impatient
类主构造器
主构造器的定义与类的定义交织在一直,将构造器参数直接放在类名称之后,如下代码:
class Person(val name:String,val age:Int)D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private final java.lang.String name; private final int age; public java.lang.String name(); public int age(); public cn.scala.xtwy.Person(java.lang.String, int);}public class Person{ private final String name; private final int age; public Person(String name,int age){ this.name=name; this.age=age; } public String getName(){ return name} public int getAge() {return age}}scala> val p=new Person("john",29)p: Person = Person@abdc0fscala> p.nameres31: String = johnscala> p.ageres32: Int = 29
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主构造器会执行类定义中的所有语句,例如
class Person(val name:String,val age:Int){ println("constructing Person ........") override def toString()= name + ":"+ age}scala> val p=new Person("john",29)constructing Person ........p: Person = john:29
回过头来看的话,前面我们定义的Person类是一种无参主构建器
class Person { println("constructing Person....") val name:String="john"}scala> val p=new Person()constructing Person....p: Person = Person@79895f
主构建器还可以使用默认参数
class Person(val name:String="",val age:Int=18){ println("constructing Person ........") override def toString()= name + ":"+ age}scala> val p=new Personconstructing Person ........p: Person = :18scala> val p=new Person("john")constructing Person ........p: Person = john:18
主构造器中的参数还可以加访问控制符
class Person(val name:String="",private val age:Int=18){ println("constructing Person ........") override def toString()= name + ":"+ age}
当主构造器的参数不用var或val修饰的时候,参数会生成类的私有val成员,并且不会产生getter和setter方法
class Person(name:String,age:Int){ println("constructing Person ........") override def toString()= name + ":"+ age}D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private final java.lang.String name; private final int age; public java.lang.String toString(); public cn.scala.xtwy.Person(java.lang.String, int);}class Person(private[this] val name:String,private[this] val age:Int){ println("constructing Person ........") override def toString()= name + ":"+ age}D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private final java.lang.String name; private final int age; public java.lang.String toString(); public cn.scala.xtwy.Person(java.lang.String, int);}
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值得注意的是,将上述Person类中的toString()方法去掉,则类中无任何地方使用了主构造器的参数,此时主构造器参数不会生成类成员
即将class Person(name:String,age:Int){ println("constructing Person ........") override def toString()= name + ":"+ age}改成:class Person( val name:String,age:Int){ println("constructing Person ........")}其字节码文件如下:D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { public cn.scala.xtwy.Person(java.lang.String, int);}
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下面图给出了Scala中主构建器参数生成类成员和方法时的规则
来源:scala for the impatient
在某些情况下,可能需要禁用主构建器,代码如下:
class Person private(var name:String,var age:Int){ println("constructing Person ........")}D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private java.lang.String name; private int age; public java.lang.String name(); public void name_$eq(java.lang.String); public int age(); public void age_$eq(int); private cn.scala.xtwy.Person(java.lang.String, int);}scala> val p=new Person("john",19)<console>:9: error: constructor Person in class Person cannot be accessed in object $iw val p=new Person("john",19) ^
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辅助构造函数
前面讲了,如果禁用掉了主构建器,则必须使用辅助构造函数来创建对象。辅助构造函数具有两个特点:(1)辅助构建器的名称为this,java中的辅助构造函数与类名相同,这常常会导致修改类名时出现不少问题,scala语言避免了这样的问题;(2)调用辅助构造函数时,必须先调用主构造函数或其它已经定义好的构造函数。
3.1 我们首先看一下只有辅助构造函数的Person类
class Person{ private var name:String=null private var age:Int=18 private var sex:Int=0 def this(name:String){ this() this.name=name } def this(name:String,age:Int){ this(name) this.age=age } def this(name:String,age:Int,sex:Int){ this(name,age) this.sex=sex }}D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private java.lang.String name; private int age; private int sex; private java.lang.String name(); private void name_$eq(java.lang.String); private int age(); private void age_$eq(int); private int sex(); private void sex_$eq(int); public cn.scala.xtwy.Person(); public cn.scala.xtwy.Person(java.lang.String); public cn.scala.xtwy.Person(java.lang.String, int); public cn.scala.xtwy.Person(java.lang.String, int, int);}class Person{ private var name:String=null private var age:Int=18 private var sex:Int=0 def this(name:String,age:Int,sex:Int){ this(name,age) this.sex=sex } def this(name:String){ this() this.name=name } def this(name:String,age:Int){ this(name) this.age=age }}
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3.2 带主构造函数、辅助构造函数的Person类
class Person(var name:String,var age:Int){ private var sex:Int=0 def this(name:String,age:Int,sex:Int){ this(name,age) this.sex=sex }}生成的字节码文件如下:D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private java.lang.String name; private int age; private int sex; public java.lang.String name(); public void name_$eq(java.lang.String); public int age(); public void age_$eq(int); private int sex(); private void sex_$eq(int); public cn.scala.xtwy.Person(java.lang.String, int); public cn.scala.xtwy.Person(java.lang.String, int, int);}
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在主构造函数小节当中我们提到,有时候可能会禁用掉主构造函数,此时只能通过辅助构造函数来创建对象
class Person private(var name:String,var age:Int){ private var sex:Int=0 def this(name:String,age:Int,sex:Int){ this(name,age) this.sex=sex }}D:\ScalaWorkspace\ScalaChapter06\bin\cn\scala\xtwy>javap -private Person警告: 二进制文件Person包含cn.scala.xtwy.PersonCompiled from "Person.scala"public class cn.scala.xtwy.Person { private java.lang.String name; private int age; private int sex; public java.lang.String name(); public void name_$eq(java.lang.String); public int age(); public void age_$eq(int); private int sex(); private void sex_$eq(int); private cn.scala.xtwy.Person(java.lang.String, int); public cn.scala.xtwy.Person(java.lang.String, int, int);}
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