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31 Combining Scala and Java

31.1 Using Scala from Java

// In file java/App.scala

  object App {
    def main(args: Array[String]) {
      println("Hello, world!")
    }
  }


  $ javap App$
  public final class App$ extends java.lang.Object
  implements scala.ScalaObject{
      public static final App$ MODULE$;
      public static {};
      public App$();
      public void main(java.lang.String[]);
      public int $tag();
  }


  $ javap App
  Compiled from "App.scala"
  public final class App extends java.lang.Object{
      public static final int $tag();
      public static final void main(java.lang.String[]);
  }


  

31.2 Annotations

  // Java serial version marker
  private final static long SerialVersionUID = 1234L


// In file java/Reader.scala

  import java.io._
  class Reader(fname: String) {
    private val in =
      new BufferedReader(new FileReader(fname))
 
    @throws(classOf[IOException])
    def read() = in.read()
  }


  $ javap Reader
  Compiled from "Reader.scala"
  public class Reader extends java.lang.Object implements
  scala.ScalaObject{
      public Reader(java.lang.String);
      public int read()       throws java.io.IOException;
      public int $tag();
  }
  $


// In file java/SetTest.scala

  import org.junit.Test
  import org.junit.Assert.assertEquals

  class SetTest {

    @Test
    def testMultiAdd {
      val set = Set() + 1 + 2 + 3 + 1 + 2 + 3
      assertEquals(3, set.size)
    }
  }


  $ scala -cp junit-4.3.1.jar:. org.junit.runner.JUnitCore SetTest
  JUnit version 4.3.1
  .
  Time: 0.023
 
  OK (1 test)


// In file java/Ignore.java

  import java.lang.annotation.*; // This is Java
  @Retention(RetentionPolicy.RUNTIME)
  @Target(ElementType.METHOD)
  public @interface Ignore { }


// In file java/Tests.scala

  object Tests {
    @Ignore
    def testData = List(0, 1, -1, 5, -5)

    def test1 {
      assert(testData == (testData.head :: testData.tail))
    }

    def test2 {
      assert(testData.contains(testData.head))
    }
  }


// In file java/FindTests.scala

  for {
    method <- Tests.getClass.getMethods
    if method.getName.startsWith("test")
    if method.getAnnotation(classOf[Ignore]) == null
  } {
    println("found a test method: " + method)
  }


  $ javac Ignore.java
  $ scalac Tests.scala
  $ scalac FindTests.scala
  $ scala FindTests
  found a test method: public void Tests$.test2()
  found a test method: public void Tests$.test1()


  

31.3 Existential types

  type forSome { declarations }


  Iterator[T] forSome { type T }


  Iterator[T] forSome { type T <: Component }


  Iterator[T] forSome { type T <: Component }


// In file java/Wild.java

  // This is a Java class with wildcards
  public class Wild {
    Collection<?> contents() {
      Collection<String> stuff = new Vector<String>();
      stuff.add("a");
      stuff.add("b");
      stuff.add("see");
      return stuff;
    }
  }


  scala> val contents = (new Wild).contents
  contents: java.util.Collection[?0] forSome { type ?0 } =
     [a, b, see]


  scala> contents.size()
  res0: Int = 3


  import scala.collection.mutable.Set
  val iter = (new Wild).contents.iterator
  val set = Set.empty[???]     // what type goes here?
  while (iter.hasMore)
    set += iter.next()


// In file java/UseWild.scala

  import scala.collection.mutable.Set
  import java.util.Collection

  abstract class SetAndType {
    type Elem
    val set: Set[Elem]
  }

  def javaSet2ScalaSet[T](jset: Collection[T]): SetAndType = {
    val sset = Set.empty[T]  // now T can be named!

    val iter = jset.iterator
    while (iter.hasNext)
      sset += iter.next()

    return new SetAndType {
      type Elem = T
      val set = sset
    }
  }


  

31.4 Using synchronized

// In file concurrency/Synchronized.scala

  var counter = 0
  synchronized {
    // One thread in here at a time
    counter = counter + 1
  }


  

31.5 Compiling Scala and Java together

  $ scalac -d bin InventoryAnalysis.scala InventoryItem.java \
         Inventory.java
  $ javac -cp bin -d bin Inventory.java InventoryItem.java \
         InventoryManagement.java 
  $ scala -cp bin InventoryManagement
  Most expensive item = sprocket($4.99)


  

31.6 Conclusion