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31 Combining Scala and Java

Sample run of chapter's interpreter examples

• 31.1 Using Scala from Java
• 31.2 Annotations
• 31.3 Wildcard types
• 31.4 Compiling Scala and Java together
• 31.5 Java 8 integration
• 31.6 Conclusion

31.1 Using Scala from Java

// In file combining-scala-and-java/App.scala

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


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


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


  

31.2 Annotations

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


// In file combining-scala-and-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 {
    public int read() throws java.io.IOException;
    public Reader(java.lang.String);
  }


// In file combining-scala-and-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 combining-scala-and-java/Ignore.java

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


// In file combining-scala-and-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 combining-scala-and-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 Wildcard types

// In file combining-scala-and-java/Wild.java

  // This is a Java class with wildcards
  import java.util.*;
  public class Wild {
    public 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[_] = [a, b, see]


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


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


// In file combining-scala-and-java/UseWild.scala

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

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

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

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

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


  

31.4 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)


  

31.5 Java 8 integration

  JButton button = new JButton(); // This is Java
  button.addActionListener(
    new ActionListener() {
      public void actionPerformed(ActionEvent event) {
        System.out.println("pressed!");
      }
    }
  );


  JButton button = new JButton(); // This is Java 8
  button.addActionListener(
    event -> System.out.println("pressed!")
  );


  val button = new JButton
  button.addActionListener( 
    _ => println("pressed!")
  )


  scala> trait Increaser {
           def increase(i: Int): Int
         }
  defined trait Increaser


  scala> def increaseOne(increaser: Increaser): Int =
           increaser.increase(1)
  increaseOne: (increaser: Increaser)Int


  scala> increaseOne(
           new Increaser {
             def increase(i: Int): Int = i + 7
           }
         )
  res0: Int = 8


  scala> increaseOne(i => i + 7) // Scala 2.12
  res1: Int = 8


  scala> import java.util.function.IntUnaryOperator
  import java.util.function.IntUnaryOperator

  scala> import java.util.Arrays
  import java.util.Arrays

  scala> val stream = Arrays.stream(Array(1, 2, 3))
  stream: java.util.stream.IntStream = ...

  scala> stream.map(
           new IntUnaryOperator {
             def applyAsInt(i: Int): Int = i + 1
           }
         ).toArray
  res3: Array[Int] = Array(2, 3, 4)


  scala> val stream = Arrays.stream(Array(1, 2, 3))
  stream: java.util.stream.IntStream = ...

  scala> stream.map(i => i + 1).toArray // Scala 2.12
  res4: Array[Int] = Array(2, 3, 4)


  scala> val f = (i: Int) => i + 1
  f: Int => Int = ...


  scala> val stream = Arrays.stream(Array(1, 2, 3))
  stream: java.util.stream.IntStream = ...

  scala> stream.map(f).toArray
  <console>:16: error: type mismatch;
   found   : Int => Int
   required: java.util.function.IntUnaryOperator
         stream.map(f).toArray
                    ^


  scala> stream.map(i => f(i)).toArray
  res5: Array[Int] = Array(2, 3, 4)


  scala> val f: IntUnaryOperator = i => i + 1
  f: java.util.function.IntUnaryOperator = ...

  scala> val stream = Arrays.stream(Array(1, 2, 3))
  stream: java.util.stream.IntStream = ...

  scala> stream.map(f).toArray
  res6: Array[Int] = Array(2, 3, 4)


  

31.6 Conclusion