public class member in scala class not accessible in java class

Question

I'm learning scala, and to practice, I have started to convert some of my existing java classes into scala, so I can also learn about scala-java inter-op. Following is my project setup:

Scala class :

@Entity
class DemoBeanScala{
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @BeanProperty
    var Id: Long= _

    @BeanProperty
    var str: String= _

    @BeanProperty
    var num: Int = _

    @BeanProperty
    var d:Double = _

    def testFunc(printText: () => Unit){
      printText()
    }

    val pr = () => {
      println("functional programming test")
    }
}

Java class(extract):

@RequestMapping("/demo")
public DemoBeanScala demo(@RequestParam(value="id") Long Id, @RequestParam(value="str") String str, @RequestParam(value="num") int num, @RequestParam(value="d") double d)
{
    DemoBeanScala dbs = new DemoBeanScala();
    dbs.setId(123456);
    dbs.setStr("sample text");
    dbs.setNum(1);
    dbs.setD(2.1);
    dbs.testFunc(dbs.pr);
    return dbs;
}

From what I have learnt, pr in DemoBeanScala class should be accessible in my java class, since no modifier is declared before it. But, after maven compilation(using scala:compile) and running java code, I'm getting an error saying that pr is private in DemoBeanScala class. What am I doing wrong?

Cyäegha · Accepted Answer · 2016-05-03 07:45:04Z

1

If you look at your compiled class in javap, you will see somethign like this :

public class DemoBeanScala {
  private final scala.Function0<scala.runtime.BoxedUnit> pr;
  public scala.Function0<scala.runtime.BoxedUnit> pr();
  // And some other stuff...
}

You will notice two important difference with Java :

Scala respects the uniform access principle, so there is no difference between calling a method without a parameter list or accessing a property. To make that work, the scala compiler will generate a public accessor method and a private field to represent a public property.
Java doesn't have methods without a parameter list: they always have at least an empty parameter list.

So, from Java, you need to access the pr property with dbs.pr(), not dbs.pr (or generate a Java-style getter with @BeanProperty, as you did for your other properties, and access it as dbs.getPr()).

answered May 3, 2016 at 7:45

Cyäegha

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Manas Over a year ago

Thanks for the detailed explanation. using dbs.pr() worked. I'm even more intrigued about java-scala inter-op than I was before :)

Community · Accepted Answer · 2017-05-23 11:52:43Z

1

When you add @BeanProperty that particular property is not visible in java class, only the generated public setters and getters are available or you may access scala like setter (pr_$eq()) and getter (pr()).

I could not find a proper reference, but the following links may helpful for you, they are not directly related to this question, but they will help you understanding.

Scala: Can I declare a public field that will not generate getters and setters when compiled? https://issues.scala-lang.org/browse/SI-4481

edited May 23, 2017 at 11:52

CommunityBot

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answered May 3, 2016 at 7:45

RP-

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1 Comment

Manas Over a year ago

Thanks for the explanation, and the reference :)

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