Scala编译器如何从超类调用抽象方法？

Question

I am new to Scala world and bit confused with OOPS concept . Here is my code snippet.

abstract class Element {
  def contents: Array[String]
  val lenth = contents.size
  val maxLength = contents.map(_.size).max
}
class ArrayElement(var contents: Array[String]) extends Element

From my understanding Scala compiler will generate contents , contents_= method for us. hence we can avoid to define def contents abstract method in base class. is it my understanding correct ? if it yes

val names = Array("welcome", "apple", "Test")
    val names1 = Array("apple", "Test")
    var t = new ArrayElement(names);
    println(t.contents.mkString(",")) //Op : welcome,apple,Test
    t.contents = names1 // because of var

#1 println(t.contents.mkString(",")) // op : apple,Test #2 println(t.lenth) // here i am getting 3 (length of names) . but i am expecting 2 . Why ? #3 println(t.maxLength) // same names reference here . Why ?

'#1 is giving updated names list. But #2 and #3 is giving old name references. Why ?

Answer 1

If you decompile Element class you will find something similar to this:

public abstract class Element {
  private final int lenth =  // call contents()
  private final int maxLength = // call contents()

  public abstract String[] contents();

  public int lenth() {
    return this.lenth;
  }

  public int maxLength() {
    return this.maxLength;
  }
}

So lenth and maxLength fields are initialized once during object creation stage and never change.

And when you decompile ArrayElement class there will be something like this:

public class ArrayElemen extends Element {

  private String[] contents;

  @Override
  public String[] contents() {
      return this.contents;
  }

  //...
}

So when you call t.contents it will be translated to t.contents() returning your changed value.

Answer 2

Both lenth and maxLength are states of the object ArrayElement and it gets updated just once during the initialization of ArrayElement class. After you chnaged contents , both lenth and maxLength are not updated with names1 .

If you are looking for updated lenth and maxLength .

abstract class Element {
  def contents: Array[String]

  //  val lenth = contents.size
  //  val maxLength = contents.map(_.size).max

  def lenth = contents.size
  def maxLength = contents.map(_.size).max

}

Of course , you can achieve this in different ways.