Scala方法与函数使用和定义详解

def max(a: Int, b: Int) = {
  if (a > b) {
    "哈哈"
  } else {
    b
  }
}
// 语法糖: 方法定义
def max0(a: Int, b: Int) = if (a > b) a else b

def fun2(num: Int): Int = {
  if (num == 1)
    num
  else
    num * fun2(num - 1)
}
// 语法糖
def fun2(num:Int) : Int = if (num == 1) num else num * fun2(num - 1)

def fun3(a: Int = 10, b: Int) = {
  println(a + b)
}

def fun4(elements: Int*) = {
  println(elements)
  elements.foreach(i => println(i))
  // 当匿名函数的元素只用到一次的时候, 可以用_简写
  elements.foreach(println(_))
  // 当方法入参为单个参数时且正好是匿名函数的元素时, 进一步简写
  elements.foreach(println)
  var sum = 0;
  for (elem <- elements) {
    sum += elem
  }
  sum
}

def fun: (Int, Int) => Int = (a: Int, b: Int) => {
  a + b
}

def main(args: Array[String]): Unit = {
  
  val value1: (Int) => Unit = (a: Int) => {
    println(a)
  }
  value1(1)
  
  val value2 = () => {
    println("我爱学习")
  }
  value2()
  
  val value3 = (a: Int, b: Int) => {
    a + b
  }
  println(value3(4, 4))
}

def fun5(num: Int) = {
  def fun6(a: Int, b: Int): Int = {
    if (a == 1) {
      b
    } else {
      fun6(a - 1, a * b)
    }
  }
  fun6(num, 1)
}

def showLog(date :Date, log :String)= {
  println(s"date is $date, log is $log")
}
def main(args: Array[String]): Unit = {
  val date = DateUtil.date(new Date())
  showLog(date,"当前时间")
  // 想要调用log, 以上变化的是第二个参数, 可以用偏应用函数处理
  // 把showLog()方法定义为偏应用函数
  val logWithDate = showLog(date,_:String)
  // 第二种函数写法
  def logWithDate2 = showLog(date,_:String)
  logWithDate("偏应用函数-log11")
  logWithDate("偏应用函数-log22")
  logWithDate("偏应用函数-log33")
}

  
  def fun(a: Int, b: Int): Int = {
    a + b
  }
  
  private val function: (Int, Int) => Int = fun _
  
  def fun1(f: (Int, Int) => Int, a: Int): Int = {
    f(a, 100)
  }
  
  def fun2(a: Int, b: Int): (Int, Int) => Int = {
    // 在内部定义了一个方法
    def f2(v1: Int, v2: Int): Int = {
      v1 + v2 + a + b
    }
    f2
  }
  
  def fum3(f: (Int, Int) => Int): (String, String) => String = {
    val i = f(1, 2)
    def fun1(s1: String, s2: String): String = {
      s1 + "@" + s2 + "$" + i
    }
    fun1
  }
  
  def main(args: Array[String]): Unit = {
    // 显示声明过的函数作为参数
    println("显示声明过的函数作为参数: " + fun1(fun, 100))
    // 匿名函数作为参数: 匿名函数入参的类型可以省略不写
    println("匿名函数作为参数: " + fun1((a: Int, b: Int) => {
      a * b
    }, 100))
    // 返回值是一个函数
    println("返回值是一个函数: " + fun2(1, 2)(3, 4))
    // 入参和出参都是函数
    println("入参和出参都是函数: " + fum3(fun)("hello", "Scala!"))
    println("入参和出参都是函数: " + fum3((a, b) => a * b)("hello", "scala!"))
  }

def fun7(a: Int, b: Int)(c: Int, d: Int) = {
  a + b + c + d
}

def main(args: Array[String]): Unit = {
  println(fun7(1, 2)(3, 4))
}