scala this.type

 class A {def method1: A = this }
 class B extends A (def method2: B = this}
 val b = new B

如果调用b.method2.method1是可以的，但是如果想调用b.method1.method2就不行了。因为method1返回的是A类型的。
当然你可以在B中覆盖method1，以返回正确类型。但是scala中解决这个问题的办法就是this.type

class A { def method1: this.type = this }
class B extends A { def method2: this.type = this }
val b = new B

如果调用b.method1则编译器会知道method1返回的是B类型的。
spark 源码中大量使用了这种方式，比如spark mllib kmeans

  @Since("0.8.0")
  def setMaxIterations(maxIterations: Int): this.type = {
    require(maxIterations >= 0,
      s"Maximum of iterations must be nonnegative but got ${maxIterations}")
    this.maxIterations = maxIterations
    this
  }

  /**
   * The initialization algorithm. This can be either "random" or "k-means||".
   */
  @Since("1.4.0")
  def getInitializationMode: String = initializationMode

  /**
   * Set the initialization algorithm. This can be either "random" to choose random points as
   * initial cluster centers, or "k-means||" to use a parallel variant of k-means++
   * (Bahmani et al., Scalable K-Means++, VLDB 2012). Default: k-means||.
   */
  @Since("0.8.0")
  def setInitializationMode(initializationMode: String): this.type = {
    KMeans.validateInitMode(initializationMode)
    this.initializationMode = initializationMode
    this
  }