Calling Scala from Java

Jaideep Ganguly

I think attempting Scala from java is slightly skewed philosophically. Because both of them differ on paradigms. On top of my head, here are a few differences:

Java	Scala
Three namespaces: packages, methods, fields	Uniform access principle
Has primitives	Single Object model. Everything is an object
Special (irregular) handling of arrays	Arrays are just like any other collection
void, null	Unit, Nothing
...	...

JVM does not have bytecode support for a lot of stuff scala tries. So scala has to resort to many hacks to make those paradigms a reality.

A small example:


x
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trait J {
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  private[J] val x = “Hey”
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}
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class Prac4 extends J
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// Now lets look at the bytecode:
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public class Prac4 implements J {
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  private final java.lang.String J$$x;
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  public java.lang.String J$$x();
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  public void J$_setter_$J$$x_$eq(java.lang.String);
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  public Prac4();
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}

So x is supposed to be final. Well in bytecode it is not. Below is a working java code


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Prac4 prac4 = new Prac4();
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prac4.J$_setter_$prac$J$$x_$eq("jatin");  //sets `x` to `jatin`

x is technically useless. Means its visibility is only intrait J. But here in bytecode we can see that is public and can be accessed.


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System.out.println(prac4.prac$J$$x()); //prints the `x` value

Now look at this:


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public Prac4();
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    descriptor: ()V
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    flags: ACC_PUBLIC
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    Code:
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      stack=1, locals=1, args_size=1
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         0: aload_0
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         1: invokespecial #24                 // Method java/lang/Object."<init>":()V
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         4: aload_0
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         5: invokestatic  #30                 // Method J$class.$init$:(LJ;)V
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         8: return
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}

So it is here at 5, it calls J$class.class which calls InterfaceMethod J.J\$_setter_\$J\$\$x_\$eq:(Ljava/lang/String;) V

Technically the compiler is fooling us. What it portrays is far from what it actually is in reality. All thus due to lack of bytecode support.

Another example:


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def func(arr:Array[_ <: String]) = arr.length
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func(Array[Nothing]())// line -a

Now line-acompiles successfully. But it throws an error at runtime. Why? Because at bycode level, func expects a String[].

At line-(a) we pass Object[]. But why Object[]? Because in scala, Nothing is subtype of every other type. But how do we express this at byte code. We cant >.<. Hence Object[] is used.

Note this issue is only with arrays as they are reified. Works alright with List as it is type-erased.

Bottom-Line: It is futile to attempt calling scala from java. Scala has a rich type system which cannot be directly expressed from Java. Solution: Hava a java wrapper for your scala code.