Scala中对象和类之间的区别

Question

I'm just going over some Scala tutorials on the Internet and have noticed in some examples an object is declared at the start of the example.

What is the difference between class and object in Scala?

Answer 1

tl;dr

• class C defines a class, just as in Java or C++.
• object O creates a singleton object O as instance of some anonymous class; it can be used to hold static members that are not associated with instances of some class.
• object O extends T makes the object O an instance of trait T ; you can then pass O anywhere, a T is expected.
• if there is a class C , then object C is the companion object of class C ; note that the companion object is not automatically an instance of C .

Also see Scala documentation for object and class .

object as host of static members

Most often, you need an object to hold methods and values/variables that shall be available without having to first instantiate an instance of some class. This use is closely related to static members in Java.

object A {
  def twice(i: Int): Int = 2*i
}

You can then call above method using A.twice(2) .

If twice were a member of some class A , then you would need to make an instance first:

class A() {
  def twice(i: Int): Int = 2 * i
}

val a = new A()
a.twice(2)

You can see how redundant this is, as twice does not require any instance-specific data.

object as a special named instance

You can also use the object itself as some special instance of a class or trait. When you do this, your object needs to extend some trait in order to become an instance of a subclass of it.

Consider the following code:

object A extends B with C {
  ...
}

This declaration first declares an anonymous (inaccessible) class that extends both B and C , and instantiates a single instance of this class named A .

This means A can be passed to functions expecting objects of type B or C , or B with C .

Additional Features of object

There also exist some special features of objects in Scala. I recommend to read the official documentation .

• def apply(...) enables the usual method name-less syntax of A(...)
• def unapply(...) allows to create custom pattern matching extractors
• if accompanying a class of the same name, the object assumes a special role when resolving implicit parameters

Answer 2

A class is a definition, a description. It defines a type in terms of methods and composition of other types.

An object is a singleton -- an instance of a class which is guaranteed to be unique. For every object in the code, an anonymous class is created, which inherits from whatever classes you declared object to implement. This class cannot be seen from Scala source code -- though you can get at it through reflection.

There is a relationship between object and class . An object is said to be the companion-object of a class if they share the same name. When this happens, each has access to methods of private visibility in the other. These methods are not automatically imported, though. You either have to import them explicitly, or prefix them with the class/object name.

For example:

class X {
  // class X can see private members of object X
  // Prefix to call
  def m(x: Int) = X.f(x)

  // Import and use
  import X._
  def n(x: Int) = f(x)

  private def o = 2
}

object X {
  private def f(x: Int) = x * x

  // object X can see private members of class X
  def g(x: X) = {
    import x._
    x.o * o // fully specified and imported
   }
}

Answer 3

An object has exactly one instance (you can not call new MyObject ). You can have multiple instances of a class.

Object serves the same (and some additional) purposes as the static methods and fields in Java.

Answer 4

Defining an object in Scala is like defining a class in Java that has only static methods. However, in Scala an object can extend another superclass, implement interfaces, and be passed around as though it were an instance of a class. (So it's like the static methods on a class but better).

Answer 5

As has been explained by many, object defines a singleton instance. The one thing in the answers here that I believe is left out is that object serves several purposes.

• It can be the companion object to a class / trait , containing what might be considered static methods or convenience methods.

• It can act much like a module, containing related/subsidiary types and definitions, etc.

• It can implement an interface by extending a class or one or more trait s.

• It can represent a case of a sealed trait that contains no data. In this respect, it's often considered more correct than a case class with no parameters. The special case of a sealed trait with only case object implementors is more or less the Scala version of an enum.

• It can act as evidence for implicit -driven logic.

• It introduces a singleton type.

It's a very powerful and general construct. What can be very confusing to Scala beginners is that the same construct can have vastly different uses. And an object can serve many of these different uses all at once, which can be even more confusing.

Answer 6

The formal difference -

1. you can not provide constructor parameters for Objects
2. Object is not a type - you may not create an instance with new operator. But it can have fields, methods, extend a superclass and mix in traits.

The difference in usage:

• Scala doesn't have static methods or fields. Instead you should use object . You can use it with or without related class. In 1st case it's called a companion object. You have to:
  1. use the same name for both class and object
  2. put them in the same source file.

• To create a program you should use main method in object , not in class .

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

• You also may use it as you use singleton object in java.

  
  
  

Answer 7

The object keyword creates a new singleton type, which is like a class that only has a single named instance. If you're familiar with Java, declaring an object in Scala is a lot like creating a new instance of an anonymous class.

Scala has no equivalent to Java's static keyword, and an object is often used in Scala where you might use a class with static members in Java.

Answer 8

Object is a class but it already has(is) an instance, so you can not call new ObjectName . On the other hand, Class is just type and it can be an instance by calling new ClassName() .

Answer 9

In scala, there is no static concept. So scala creates a singleton object to provide entry point for your program execution. If you don't create singleton object, your code will compile successfully but will not produce any output. Methods declared inside Singleton Object are accessible globally. A singleton object can extend classes and traits.

Scala Singleton Object Example

object Singleton{  
    def main(args:Array[String]){  
        SingletonObject.hello()         // No need to create object.  
    }  
}  


object SingletonObject{  
    def hello(){  
        println("Hello, This is Singleton Object")  
    }  
}  

Output:

Hello, This is Singleton Object

In scala, when you have a class with same name as singleton object, it is called companion class and the singleton object is called companion object.

The companion class and its companion object both must be defined in the same source file.

Scala Companion Object Example

class ComapanionClass{  
    def hello(){  
        println("Hello, this is Companion Class.")  
    }  
}  
object CompanoinObject{  
    def main(args:Array[String]){  
        new ComapanionClass().hello()  
        println("And this is Companion Object.")  
    }  
}  

Output:

Hello, this is Companion Class.
And this is Companion Object.

In scala, a class can contain:

1. Data member

2. Member method

3. Constructor Block

4. Nested class

5. Super class information etc.

You must initialize all instance variables in the class. There is no default scope. If you don't specify access scope, it is public. There must be an object in which main method is defined. It provides starting point for your program. Here, we have created an example of class.

Scala Sample Example of Class

class Student{  
    var id:Int = 0;                         // All fields must be initialized  
    var name:String = null;  
}  
object MainObject{  
    def main(args:Array[String]){  
        var s = new Student()               // Creating an object  
        println(s.id+" "+s.name);  
    }  
} 

I am sorry, I am too late but I hope it will help you.

Answer 10

对象在某种程度上类似于Java中的静态类，静态特性意味着静态类在放入JVM时不需要创建对象，可以直接使用类名和相同的实例（相同的数据状态） ) 在任何使用它的地方共享。

Answer 11

Scala class same as Java Class but scala not gives you any entry method in class, like main method in java. The main method associated with object keyword. You can think of the object keyword as creating a singleton object of a class that is defined implicitly.

more information check this article class and object keyword in scala programming

Answer 12

A class is just like any other class in other languages. You define class just like any other language with some syntax difference.

class Person(val name: String)
val me = new Person("My name")

However, object is a class with single object only. This makes it interesting as it can be used to create static members of a class using companion object . This companion object has access to private members of the class definition and it has the same name as the class you're defining.

class Person(var name: String) {

  import Person._

  def hi(): String = sayHello(name)
}

object Person {
  private def sayHello(name: String): String = "Hello " + name
}

val me = new Person("My name")
me.hi()

Also, noteworthy point is that object class is lazily created which is another important point. So, these are not instantiated unless they are needed in our code.

If you're defining connection creation for JDBC, you can create them inside object to avoid duplication just like we do in Java with singleton objects.

Answer 13

If you are coming from java background the concept of class in scala is kind of similar to Java, but class in scala cant contain static members.

Objects in scala are singleton type you call methods inside it using object name, in scala object is a keyword and in java object is a instance of class

Answer 14

Class & object: a class is a definition which describes all attributes of entity or an object. And object is an instance of a class.