Implementing Java interface in Scala results in incompatible type map-CodePudding

I have a Java interface that I want to implement in Scala. The interface contains the following method-declaration:

 List<Map<String, Object>> xyz(Map<String, Object> var1, Map<String, Object> var2);

My Scala implementation is:

override def testComponent( input: java.util.Map[String, Object], metadata: java.util.Map[String, Object] )
  {

    val list: util.List[Map[String, AnyRef]] = new util.ArrayList[Map[String, AnyRef]]

     return list
  }

ERROR : overriding method xyz in trait ITester of type (x$1: java.util.Map[String,Object], x$2: java.util.Map[String,Object])java.util.List[java.util.Map[String,Object]]; method testComponent has incompatible type override def testComponent( input: java.util.Map[String, Object], metadata: java.util.Map[String, Object] )

I don't know what exactly the issue is.

CodePudding user response：

Your definition of testComponent has type Unit because it's defined using the (deprecated in recent versions of Scala) procedure syntax of def name(args) { } (i.e. no = {). It first passes compilation because Scala allows a value to be discarded when Unit is expected (that behavior can be made a warning with the compiler option -Ywarn-value-discard). You can solve this by being explicit that you want this method to be called for value by including the =.

override def testComponent(input: java.util.Map[String, Object], metadata: java.util.Map[String, Object]) = {
  val list: util.List[Map[String, AnyRef]] = new util.ArrayList[Map[String, AnyRef]]
  list  // return should be avoided in Scala: it's not necessary and can introduce some really subtle bugs
}

It's generally a good idea to be explicit about the result type of a public method. Alternatively, this should also work:

override def testComponent(input: java.util.Map[String, Object], metadata: java.util.Map[String, Object]) =
  new ArrayList[Map[String, AnyRef]]