How is val in scala different from const in java?

问题

Anyone care to elaborate on how val in scala is different from const in java?
What are the technical differences? I believe I understand what "const" is in c++ and java. I get the feeling that "val" is somehow different and better in some sense but I just can't put my finger on it. Thanks

回答1:

const in Java has no function—it's reserved but you can't actually use it for anything. Declaring a Java variable as final is roughly equivalent.

Declaring a variable as a val in Scala has similar guarantees to Java final—but Scala vals are actually methods unless they're declared as private[this]. Here's an example:

class Test(val x: Int, private[this] val y: Int) {
  def z = y
}

Here's what the compiled classfile looks like:

$ javap -p Test
Compiled from "Test.scala"
public class Test {
  private final int x;
  private final int y;
  public int x();
  public int z();
  public Test(int, int);
}

So it's clear from this example that private[this] val is actually Scala's equivalent of Java's final in that it just creates a field (no getter method). However, it's a private field, so even that's not quite the same.

Another fun fact: Scala also has a final keyword! Scala's final behaves similarly to how final works for classes in Java—i.e. it prevents overriding. Here's another example:

final class Test(final val x: Int, final var y: Int) { }

And the resulting class:

$ javap -p Test
Compiled from "Test.scala"
public final class Test {
  private final int x;
  private int y;
  public final int x();
  public final int y();
  public final void y_$eq(int);
  public Test(int, int);
}

Notice that the final var definition makes the getter and setter methods final (i.e. you can't override them), but not the backing variable itself.

回答2:

A Scala val is equivalent to a final variable or field in Java. A Scala var is equivalent to a non-final variable or field in Java. (By the way, neither "var" nor "const" are Java terms.)

The aspect that's "better" about Scala's syntax choice to use val and var is that code using non-modifiable values is generally easier to understand. In Java, final is "syntactic vinegar", and style guides tend to argue over whether code should use final to encourage better coding or omit final to avoid the clutter. Scala doesn't have this conundrum because the var and val are exactly the same length, so you're a bit more free to just choose the one that makes the most sense.

来源：https://stackoverflow.com/questions/25417450/how-is-val-in-scala-different-from-const-in-java