Java Static confusion | 易学教程

问题

I'm working with Java; I have worked with C++ before. I am thinking about static usage in Java. If I create static methods and variables in the class, Why can I access them through the object also?

Example:

class Test{
  static int count=0;
  int id;
  static void updatec(){
    count++
   }
}

class TestMain
{
   public static void main(String args[])
   {
         Test.count=1;
         Test t = new Test();
         t.count=5; // Valid WHY ?????
   }
}

Why this is allowed? Java's site says we should not use obj.static method/variable.
Why it is allowed?

回答1:

Static doesn't mean that the field is only for the class. It means it for the class and all its instances.

In this example, the class variable origin of the class Point is referenced both using the class name as a qualifier, in Point.origin, and using variables of the class type in field access expressions (§15.11), as in p.origin and q.origin. These two ways of accessing the origin class variable access the same object, evidenced by the fact that the value of the reference equality expression (§15.21.3):
q.origin==Point.origin is true

But you're right it's usually a bad idea to refer to a static field/method/class from a non-static context, it can confuse the developer.

Resources :

JLS - static Fields

回答2:

It's legal to access static fields through an instance, although you'll generally get a warning about it. The field is still static, however, so there's only one per class:

     Test t = new Test();
     Test u = new Test();
     t.count = 5;
     System.out.println(u.count); // Outputs 5

回答3:

There is no good reason why this is allowed. It's confusing and it serves no purpose. Good tools will warn you about this (or maybe even give you the option to fail compilations over it). But it is part of the language and so it will never be taken out because that would break existing code.

回答4:

It really should not have been supported. This support leads to this dubious code in org.apache.commons.cli command-line parser.

class OptionBuilder

public static Option create();
public static OptionBuilder withLongOpt(String newLongopt);
public static OptionBuilder withDescription(String newDescription);

This leads to this:

Option opt =
  OptionBuilder
    .withLongOpt( "opt" )
    .withDescription( "opt description" )
    .create( );

It would be much better and cleaner and thread-safe, if there were a default OptionBuilder constructor and all static methods were instance methods. That's the model used by StringBuilder/StringBuffer

回答5:

Your snippet is perfectly legit, either in Java or C++ equivalent.

It seems you're confusing access restriction (private, protected, public) and instance/class distinction for members (static keyword).

Since you have an instance of class Test (named t) in your static main method, you can use instance methods/members of class Test, that's conforming with the documentation you quoted.

If the count field was private or protected, you wouldn't be able to access it, but that would have nothing to do with static.

回答6:

You shouldn't doesn't mean you can't.

Anyway, this is allowed in C++ also.

回答7:

static in this case means there's one shared instance per class, not per instance. So when you change t.count, you're changing the 'count' member for the whole class and ALL instances.

That is, these will print the same value:

System.out.println(t.count);
System.out.println(Test.count);

There's nothing illegal about it, although it's usually not a great idea because it looks like instance data.

来源：https://stackoverflow.com/questions/3610309/java-static-confusion

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