Beginning Scala study note Scala and Java Interoperability

Posted 2020-08-26 mengrennwpu

1. Translating Java Classes to Scala Classes

　　Example 1:

# a class declaration in Java
public class Book{}

# Scala equivalent of a class declaration
class Book

 　　Example 2:

# a Java class with a Construtor
public class Book{
    private final int isbn;
    private final String title;

    public Book(int isbn, String title){
        this.isbn = isbn;
        this.title = title;
    }   

    public int getIsbn(){
        return isbn;
    }

    public String getTitle(){
        return title;
    }   
}

# Scala equivalent 
class Book(val isbn: Int, val title: String)

 　　Example 3:

# constructor calling superclass in Java
public class NonFiction extends Book{
    public NOnFiction(String title){
        super(title);
    }
}

# Scala equivalent
class NonFiction(title: String) extends Book(title)

 　　Example 4:

# mutable instance variable in Java
public class Book{
    private String title = "Beginning Scala";

    public String getTitle(){
        return title;
    }

    public void setTitle(String t){
        title = t;
    }
}

# Scala equialent
class Book{
    var title = "Beginning Scala"
}

 　　Example 5:

# immutable instance variable in Java
public class Book{
    private final int isbn = 999;

    public int getIsbn(){
        return isbn;
    }
}

# Scala equivalent
class Book{
    val isbn = 999
}

 　　Translating Java imports to Scala imports:

# import in Java
import com.modA.ClassA;
import com.modB.ClassB1;
import com.modB.ClassB2;
import com.modC.*;

# import in Scala
import com.modA.ClassA
import com.modB.{ClassB1,ClassB2} // you can stack multiple imports from the same package in braces
import com.modC._ // underscore in Scala imports is equivalent of * in Java imports

 　　Example 6:

# Java class with multiple constructors
public class Book {
private Integer isbn;
private String title;
public Book(Integer isbn) {
this.isbn = isbn;
}
public Book(Integer isbn, String title) {
this.isbn = isbn;
this.title = title;
}
public Integer getIsbn() {
return isbn;
}
public void setIsbn(Integer isbn) {
this.isbn = isbn;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}}

# refactoring
class Book(var isbn: Int, var title: String)

  　　If you create the Book instance with a construtor that takes a single "title" parameter, you will get an error.

scala> val book = new Book("test")
<console>:12: error: not enough arguments for constructor Book: (isbn: Int, title: String)Book.
Unspecified value parameter title.
       val book = new Book("test")

 　　We need an extra constructor for this case.

scala> class Book(var isbn: Int, var title: String){
     | def this(title: String) = this(0,title)
     | }
defined class Book
scala> val book = new Book("test")
book: Book = [email protected]
scala> book.isbn
res2: Int = 0
scala> book.title
res3: String = test

 　　You can get and set "isbn" and "title" because of the generated getters and setters that follow the Scala conversion.

2. JavaBeans specification compliant Scala classes

　　To have Java-style getters and setters is to annotate the field with scala.beans.BeanProperty. In this way, you can interact with a Java calss or library that accepts only classes that conform to the JavaBean specification.

scala> import scala.beans.BeanProperty
import scala.beans.BeanProperty

scala> class Book(@BeanProperty var isbn: Int, @BeanProperty var title: String)
defined class Book

 　　After compiling Book.scala with scalac command and disassembling it with javap command:

public class Book {
  public int isbn();
  public void isbn_$eq(int);
  public void setIsbn(int);
  public java.lang.String title();
  public void title_$eq(java.lang.String);
  public void setTitle(java.lang.String);
  public int getIsbn();
  public java.lang.String getTitle();
  public Book(int, java.lang.String);
}

  　　The methods getTitle,setTitle,getIsbn,setIsbn have all been generated because of the @BeanProperty annotation. Note that use the @BeanProperty annotation on your fields, also making sure you declare each field as a var. If you declare your fields a type val, the setter methods won‘t be generated.

　　You can use @BeanProperty annotation on class constructor parameters, even on the fields in a Scala class.

3. Java interfaces and Scala traits

　　A java class can‘t extend a Scala trait that has implemented methods.

# A regular Java interface declaration

public interface Book{
    public abstract boolean isBestSeller();      
}

# Scala equivalent
trait Book{ def isBestSeller: Boolean}

 　　Note that in scala, if there is no  = assignment, then the methods denoted with a def keyword or the functions denoted with a val keyword are abstract. That means if there‘s no definition provided with =, then it‘s automatically abstract.

# a concrete Java method
public String someMethod(int arg1, boolean arg2){return "voila"}

# Scala equivalent
def someMethod(arg1: Int, arg2: Boolean) :String = "volia"

# an abstract Java method
abstract int doTheMath(int i)

# Scala equivalent
def doTheMath(i: Int): Int

 　　Example: you need to be able to user an add method from a Java application:

# a Scala trait
trait Computation{def add(a: Int, b:Int) = a + b}

# a Java application
public class DoTheMath{
    public static void main(String[] args){
        DoTheMath d = new DoTheMath();   
    }
}

 　　Java class DoTheMath cannot implement the trait Computation because Computation is not like a regular Java interface. To be able to use the implemented method add of a Scala trait Computation from Java class DoTheMath, you must wrap the trait Computation in a Scala class.

# Scala class that wraps the trait Computation
class JavaInteroperableComputation extends Computation

# accessing the add method of the Scala trait from Java class
public class DoTheMath extends JavaInteroperableComputation{
    public static void main(String[] args){
        DoTheMath d = new DoTheMath();
       d.add(3,1);
    }
}

　　Note that wrap you scala traits with implemented behavior in the Scala class for its Java callers.

4. Java static members and Scala objects

# a singleton in Scala
public class Book{
    private static Book book;
    private Book(){}
    public static synchronized Book getInstance(){
        if(book == null){
            book = new Book();
        }
        return book;
    }
}

# Scala equivalent, object can extend interfaces and traits
object Book{}

　　The companion object enables storing of static methods and from this, you have full access to the class‘s members, including private ones. Scala alows you to declare both an object and a class of the same name, placing the static members in the object and the instance members in the class.

# Java class with instance and static methods
public class Book{
    public String getCategory(){
        return "Non-Fiction";
    }

   public static Book createBook(){
        return new Book();
   }   
}

# Scala equivalent
class Book{
    def getCategory() = "Non-Fiction"
}

object Book{
    def createBook() = new Book()
}

5.  Handling Exceptions

# a Scala method that throws an Exceotion
class SomeClass{
    def aScalaMethod{ throw new Exception("Exception")}
}

# calling a Scala method from a Java class
publc static void main(String[] args){
    SomeClass s = new SomeClass();
    s.aScalaMethod();
}

# the uncaught exception causes the Java method to fail
[error] (run-main) java.lang.Exception: Exception!
java.lang.Exception: Exception!
at SomeClass.aScalaMethod

　　For the Java callers of you Scala methods, add the @throws annotation to your Scala methods so they will know which methods can throw exception and what exception they throw.

# annotating Scala method with @throws
class SomeClass{
    @throws(classOf[Excepion])
    def aScalaMethod{ throw new Exception("Exception")}
}

　　If you attempt to call aScalaMethod from a Java class without wrapping it in a try/catch block, or declaring that your Java method throws an exception, the compiler will throw an eeor.

#calling annotated aScalaMethod from Java
SomeClass s = new SomeClass();
try{
    s.aScalaMethod();
}catch(Exception e){
    System.err.println("Caught the exception");
    e.printStackTrace();
}