Beginning Scala Study Note (9) Scala and Java interoperability

Source: Internet
Author: User
Tags throw exception traits

1. Translating Java Classes to Scala Classes

Example 1:

inch Javapublic class book{}# Scala equivalent of a class Declarationclass book

Example 2:

# A Java class with a Construtorpublic class book{    int  ISBN;    Private final String title;    Public book (int  ISBN, String title) {        = ISBN;         = title;    }        int GETISBN () {        return ISBN;    }    Public String GetTitle () {        return title;    }   } # Scala equivalent class book (Val Isbn:int, Val title:string)

Example 3:

inch Javapublic class Nonfiction extends book{public    nonfiction (String title) {        Super (title);    }} # Scala Equivalentclass Nonfiction (title:string) extends book (title)

Example 4:

inch Javapublic class book{    "Beginning Scala";    Public String GetTitle () {        return title;    }    public void Settitle (String t) {        = t;    }} # Scala Equialentclass book{    "Beginning Scala"}

Example 5:

inch Javapublic class book{    int999;     int GETISBN () {        return ISBN;    }} # Scala Equivalentclass book{    999}

Translating Java Imports to Scala imports:

inch javaimport com.moda.classa;import com.modb.classb1;import com.modb.classb2;import Com.modC. *in// your can stack multiple importsfrom the same package in braces//
     underscore in Scala imports are equivalent of * in Java imports

Example 6:

= = = = = title;}} # Refactoringclass Book (var isbn:int, var title:string)

If You create the book instance with a Construtor this takes a single "title" parameter, you'll get an error.

scala> val book = new book ("test")<console>: Constructor Book: (Isbn:int, title:string) book.unspecified value parameter title.        = 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 Bookscala> Val book = new book ("test"= [email Protected]scala>0Scala>= 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 has Java-style getters and settersare to annotate the field with Scala.beans.BeanProperty. In this is the can interact with a Java CALSS or library that accepts only classes that conform to the JavaBean specific ation.

scala> Import scala.beans.BeanPropertyimport Scala.beans.BeanPropertyscala> 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   int   int   GETISBN ();  Public java.lang.String getTitle (); Public book ( int   

The methods GETTITLE,SETTITLE,GETISBN,SETISBN has all been generated because of the @BeanProperty annotation. Note that use the @BeanProperty annotation in your fields, also making sure you declare each field as a var. If you declare your fields a type Val, the setter is 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 Declarationpublic interface book{public    abstract Boolean isbestseller ();      }

# Scala equivalent
Trait book{def Isbestseller:boolean}

  Note in Scala, if there are no  = assignment, then the methods denoted with a def keyword or the funct Ions denoted with a val keyword is abstract. That means if there's no definition provided with = and then it's automatically abstract.

# A concrete Java methodpublic String somemethod (int"voila"  " Volia "  int dothemath (int  i) # Scala equivalentdef Dothemath (i:int): int

Example:you need to being able to the user an Add method from a Java application:

= A + b}# a Java applicationpublic class dothemath{public    static void Main (string[] args) {        = new DoT Hemath ();       }}

Java class Dothemath cannot implement the trait computation because computation are not like a regular Java interface. To is able to use the implemented method add of a Scala trait computation from Java class Dothemath, you must wrap the TR AIT computation in a Scala class.

# Scala class that wraps the trait Computationclass Javainteroperablecomputation extends computation# accessing the ad D method of the Scala trait from Java classpublic class Dothemath extends javainteroperablecomputation{public    static void Main (string[] args) {        = new Dothemath ();       D.add (3,1);}    }

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

4. Java static members and Scala objects

inch Scalapublic class book{    private static book book;    Private book () {} public    static synchronized book getinstance () {        ifnull) {            = new book ();        }        return book;    }} # Scala equivalent, object can extend interfaces and traitsobject book{}

  the companion object enables storing of static methods and from this, you has full access to the class's members, in cluding private ones. Scala Alows declare both an object and a class of the same name, placing the static members in the object and the I Nstancemembers in the class.

# Java class with instance and static Methodspublic class book{public    String getcategory () {        " C4>non-fiction";    }   public static book Createbook () {        return to new book ();   }   } # Scala Equivalentclass book{    "non-fiction"}Object  book{    = new book ()}

5. Handling Exceptions

# A Scala method that throws an Exceotionclass someclass{    def ascalamethod{throw new Exception ("
   
    exception
    "
    )}}# calling a Scala method from a Java classpublc static void Main (string[] args) {     =
     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 your Scala methods, add the @throws annotation to your Scala methods so they would know which method s can throw exception and what exception they throw.

# annotating Scala method with @throwsclass someclass{    @throws (Classof[excepion])    def ascalamethod{throw New Exception ("Exception")}}

  If you attempt the Ascalamethod from a Java class without wrapping it in a try/catch block, or declaring that your Jav A method throws an exception, the compiler would throw an eeor.

= new SomeClass (); try{    S.ascalamethod ();} catch (Exception e) {    System.err.println ("caught the Exception");    E.printstacktrace ();}

Beginning Scala Study Note (9) Scala and Java interoperability

Contact Us

The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion; products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email.

If you find any instances of plagiarism from the community, please send an email to: info-contact@alibabacloud.com and provide relevant evidence. A staff member will contact you within 5 working days.

A Free Trial That Lets You Build Big!

Start building with 50+ products and up to 12 months usage for Elastic Compute Service

  • Sales Support

    1 on 1 presale consultation

  • After-Sales Support

    24/7 Technical Support 6 Free Tickets per Quarter Faster Response

  • Alibaba Cloud offers highly flexible support services tailored to meet your exact needs.