了解Scala中的Case类和Traits

Question

I have a simple trait as defined below:

trait MyTrait {

  def myStringVal: String

}

My case class which implements this trait is as below:

case class MyCaseClass(myStringVal: String) extends MyTrait {
  ...
  ...
}

Coming from a Java world, I find it a bit difficult to fathom the fact that MyCaseClass actually implements this just by defining a parameter to MyCaseClass. I understand that thy byte code would actually write the getter and setter. But how is this possible without any var or val?

My understanding is that if there is no var or val, then there is no getter or setter method generated. In that case how is the above case class MyCaseClass implementing myStringVal method?

Sometime too much of this Scala magic is difficult to understand especially with legacy code.

Answer 1

You might want to check out this blog article covering what case classes exactly are and why they are so useful.

In your example, the trait MyTrait has no use, except being able to function like a java interface. Note, that the default visibility in scala is public. By default case class parameters are immutable so in your example val is automatically inferred by the compiler for the myStringVal argument.

What magic do case classes do?!

• Convert all constructor parameters to public readonly ( val ) by default fields
• Generate the toString() , equals() and hashcode() methods using all constructor params for each method
• Generate companion object with the same name containing an appropriate apply() and unapply() method, which are basically just a convenience constructor allowing to instantiate without using the new keyword and an extractor which by default generates an option-wrapped tuple of the case class parameters.

EDIT: Sample compiler output for (case) classes (copied from scalatutorial.de )

A simple scala class definition like

class A1(v1: Int, v2: Double)

gets compiled to the java code

public class A1 extends java.lang.Object implements scala.ScalaObject {
  public A1(int, double);
}    

the analogous case class

case class A2(v1: Int, v2: Double)

gets compiled to the following java classes

public class A2 extends java.lang.Object implements 
scala.ScalaObject,scala.Product,java.io.Serializable {
  public static final scala.Function1 tupled();
  public static final scala.Function1 curry();
  public static final scala.Function1 curried();
  public scala.collection.Iterator productIterator();
  public scala.collection.Iterator productElements();
  public double copy$default$2();
  public int copy$default$1();
  public int v1();
  public double v2();
  public A2 copy(int, double);
  public int hashCode();
  public java.lang.String toString();
  public boolean equals(java.lang.Object);
  public java.lang.String productPrefix();
  public int productArity();
  public java.lang.Object productElement(int);
  public boolean canEqual(java.lang.Object);
  public A2(int, double);
}


public final class A2$ extends scala.runtime.AbstractFunction2 
implements scala.ScalaObject {
  public static final A2$ MODULE$;
  public static {};
  public scala.Option unapply(A2);
  public A2 apply(int, double);
  public java.lang.Object apply(java.lang.Object, java.lang.Object);
}   

Answer 2

Scala case classes have a plenty of boilerplate implemented for you, and having all the constructor parameters automatically exposed as val s is one of these things.

If you try avoiding val s in a regular class, like that:

trait MyTrait {
  def myVal: String
}

class MyClass(myVal: String) extends MyTrait

Compiler will show you the error message, that MyClass has to be abstract, as it does't override myVal method, but adding val or var to the class constructor parameter will solve the issue.

Answer 3

Case classes are different -- some default methods are generated for them. This includes val getters for the parameters. Think of case classes as POJOs -- this is a useful bit of syntactic sugar, since they don't need private members.

Some other useful methods are generated too, for example copy , toString , apply and unapply .