What is the difference between referencing a field

问题:

I have noticed that there are times when coding in Java that I have seen fields called by method:

System.out.println(object.field);

and by class:

System.out.println(Class.field);

I have not seen any clear distinguishing in my textbooks about what the semantics are for these two cases, and I fear that it is going to be, at least for a noob, a subtle point. My intuition is that the class calling will be used for static fields? Thanks guys. So much con'foo'sion.

回答1:

The field Class.field can be accessed without creating an instance of the class. These are static fields which are initialized when the classes are loaded by classloaders.

The other field i.e. object.field can be accessed only when an instance of the class is created. These are instance field initialized when object of the class is created by calling its constructor.

回答2:

object.field should be (see note below) an instance member while Class.field would be a static member.

Note: Like stated by @radai and I think it's worth mentionning, you can also access a static member through an object instance, however that's a very bad practice which is quite misleading.

回答3:

Instance scope versus class scope.

Check this out:

class Foobar {
  public final int x = 5;
  public static final int y = 6;
}

y is a variable that is only created once, at compile time. It is bound to the class, and therefore shared by all of its instances. You reference this with Foobar.y.

System.err.println(Foobar.y);

x on the other hand, is an instance variable, and every Foobar that you create with new will have a copy of it. You would reference it like this:

Foobar foobar = new Foobar();
System.err.println(foobar.x);

But this will not work:

System.err.println(Foobar.x);

回答4:

Refrencing a field by class requires the field to be static.

Refrencing a field by object requires the field can be either static or non-static field .

回答5:

My intuition is that the class calling will be used for static fields

Yes SomeClass.field can be used only if field is static. In this case you can also access it via reference like someClassRef.field but this code will be changed by compiler to ReferenceType.field anyway. Also it can cause some misunderstandings (it may seem that you are trying to use non-static field) so it is better to use static fields by its class.

If field is not static then it must belong to some instance so you will have to call it via reference someClassRef.field