问题
I was wondering what it means to say a field is hidden between 2 java classes and what it means when running code in terms of resulting output?
I have an abstract class with a protected static boolean field = false
and a sub class
which has a boolean field with the same name but is not static and set to true
.
If I had this code:
Superclass d = new subclass();
what would be the value of the boolean field in the superclass and the boolean field
in the subclass? Does subclass field stay as false
after the assignment above?
回答1:
static
members are never overridden (and certainly not by non-static members). And since you should access them like this: ClassName.member
there is also no need to worry about hiding them.
In your case, you would access the Superclass
field like this: Superclass.field
. And the field of a Subclass
instance like this: subclass.field
. If you have, however a Subclass
instance in a Superclass
variable like above, this code: d.field
will access the static field defined in Superclass
, which will be false
in your case.
But this does not change the value of the Subclass
instance, it just accesses the "wrong" member! You can verify this by putting the instance in d
back into a Subclass
variable and reading field
again.
回答2:
Java Language Specification
If the class declares a field with a certain name, then the declaration of that field is said to hide any and all accessible declarations of fields with the same name in superclasses, and superinterfaces of the class.
A hidden field can be accessed by using a qualified name if it is
static
, or by using a field access expression that contains the keywordsuper
or a cast to a superclass type.See more in http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html
Example Code
class A { static int field; } class B extends A { int field; void doSomething() { System.out.println(super.field); // From A System.out.println(field); // From B } } class Main { public static void main(String[] args) { B b = new B(); System.out.println(b.field); // From B System.out.println(((A) b).field); // From A System.out.println(A.field); // From A } }
回答3:
what would be the value of the boolean field in the superclass and the boolean field in the subclass?
The value of field
variable in superclass will remain false and the value of field
in the subclass will remain true.
Does subclass field stay as FALSE after the assignment above?
No. You cannot override static variables in Java. What essentially happens is the definition in the sub class hides the variable declared in the super class.
For a nice example and explanation, see SO Question
I also suggest you try it out yourself to see what happens.
来源:https://stackoverflow.com/questions/10501503/java-field-hiding