什么是Scala对象的Java等价物？ [英] What is the Java equivalent of a Scala object?

问题描述

在Scala中，我们可以写

In Scala, we can write

object Foo { def bar = {} }

编译器如何实现？我可以从Java
调用 Foo.bar（）; 但是新的Foo（）; 来自Java给出错误找不到符号符号：构造函数Foo（）

How is this implemented by the compiler? I am able to call Foo.bar(); from Java but new Foo(); from Java gives the error cannot find symbol symbol: constructor Foo()

• 是否JVM本身支持单例吗？


• 是否可以在Java中使用没有构造函数的类？

注意：这是代码输出 scalac -print

package <empty> {
  final class Foo extends java.lang.Object with ScalaObject {
    def bar(): Unit = ();
    def this(): object Foo = {
      Foo.super.this();
      ()
    }
  }
}

推荐答案

对单身人士的支持不在语言层面，但该语言提供了足够的设施来创建它们。

Support for singletons is not on a language level, but the language provides enough facilities to create them without any trouble.

考虑以下代码：

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;
    }
}

这是维基百科的一个例子，它解释了如何可以制作单身人士。实例保存在私有字段中，构造函数在类外部不可访问，该方法返回此单个实例。

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.

至于构造函数：每个默认情况下，class有一个所谓的默认构造函数，它不带参数，只调用超类的no-args构造函数。如果超类没有任何没有参数的可访问构造函数，则必须编写一个显式构造函数。

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.

所以一个类必须有一个构造函数，但是你没有如果超类有一个no-args构造函数，则必须编写它。

So a class must have a constructor, but you don't have to write it if the superclass has a no-args constructor.

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