使用伴生对象创建计数器工具类

package excise.day02

/**
  * 单例模式，实现计数器
  */
class SingletonInstance private {
  //用 private 关键字修饰，这样就只有伴生对象一个对象了，有效防止外部实例化 SingletonInstance 类
  //只能供对应伴生对象使用
  override def toString = "This is a counter Class "
  protected var counter = 0  // protected  伴生对象可访问， private 伴生对象也访问不了
}

object SingletonInstance{
  private val singleton = new SingletonInstance
  def getInstance: SingletonInstance = singleton
  def f_counter_plus_one : Int= {
    singleton.counter += 1
    singleton.counter
  }
  def f_counter_minus_one  : Int= {
    singleton.counter -= 1
    singleton.counter
  }
  def f_show_current_counter : String = {
    // 这里不知道怎么消除黄色警告，返回类型用Unit也不行，只好借助返回一个空字符串
    println( "The current value of counter is :  "+singleton.counter)
    ""
  }
  def f_get_current_counter : Int = {
    singleton.counter
  }
}

object SingletonInstanceTest {
  def main(args: Array[String]): Unit = {
    SingletonInstance.f_show_current_counter
    SingletonInstance.f_counter_plus_one
    SingletonInstance.f_show_current_counter
    SingletonInstance.f_counter_plus_one
    SingletonInstance.f_show_current_counter
    SingletonInstance.f_counter_plus_one
    SingletonInstance.f_show_current_counter
    SingletonInstance.f_counter_minus_one
    SingletonInstance.f_show_current_counter
    SingletonInstance.f_counter_plus_one
    SingletonInstance.f_show_current_counter
  }
}

使用伴生对象创建计数器工具类

输出结果：

The current value of counter is :  0
The current value of counter is :  1
The current value of counter is :  2
The current value of counter is :  3
The current value of counter is :  2
The current value of counter is :  3