What is the Java equivalent of a Scala object?
When compiling your code, Scala compiler produces an equivalent of the following Java code:
public final class Foo < private Foo() <>// Actually, Foo doesn't have constructor at all // It can be represented in bytecode, // but it cannot be represented in Java language public static final void bar() < Foo$.MODULE$.bar(); >> public final class Foo$ implements ScalaObject < public static final Foo$ MODULE$; static < new Foo$(); >private Foo$() < MODULE$ = this; >public final void bar() < // actual implementation of bar() >> Here Foo$ is an actual implementation of a singleton, whereas Foo provides a static method for interaction with Java.
Is there any way to see the «intermediate» Java code, as you have shown? Did you use a decompiler to generate it?
@Jus12: Perhaps some decompilers are able to show it, but I reconstructed it manually from the output of javap -c -private Foo / javap -c -private Foo$
Hi @axtavt, what is this part called? static < new Foo$(); >other members(methods fields) all have a name. what type of member is this?
Support for singletons is not on a language level, but the language provides enough facilities to create them without any trouble.
Consider the following code:
public class Singleton < private static final Singleton instance = new Singleton(); // Private constructor prevents instantiation from other classes private Singleton() <>public static Singleton getInstance() < return instance; >> This is an example from Wikipedia, which explains how a singleton can be made. An instance is kept in a private field, constructor is inaccessible outside the class, the method returns this single instance.
As for constructors: every class by default has a so-called default constructor which takes no arguments and simply calls the no-args constructor of the superclass. If the superclass doesn’t have any accessible constructor without arguments, you will have to write an explicit constructor.
So a class must have a constructor, but you don’t have to write it if the superclass has a no-args constructor.