Why can't we use 'this' keyword in a static method

问题

class Sub {
    static int y;
    public static void foo() {
         this.y = 10;
    }
}

I understand that this represents the object invoking the method and that static methods are not bound to any object. But in the above mentioned case, the variable y is also static.

If we can invoke static method on class object, why can't we allow static methods to set the static variables of the class.

What is the purpose of this additional constraint?

回答1:

Because this refers to the object instance. There is no object instance in a call of a static method. But of course you can access your static field (only the static ones!). Just use

class Sub {
    static int y;
    public static void foo() {
         y = 10;
    }
}

If you want to make sure you get the static field y and not some local variable with the same name, use the class name to specify:

class Sub {
    static int y;
    public static void foo(int y) {
         Sub.y = y;
    }
}

回答2:

The main reason why we can not use "this" in static method context:-

this :- "this" means current class OBJECT , so its clear that "this" only come in the picture once we intended to create an Object of that class.

static method:- there is no need to create an object in order to use static method. means "instance" or object creation doesn't any sense with "static" as per Java rule.

So There would be contradiction,if we use both together(static and this) . That is the reason we can not use "this" in static method.

回答3:

this is referring to this instance of the object Sub. As the method is static, there is not an instance of Sub.

回答4:

To make your code work write it like this:

class Sub {
    static int y;
    public static void foo() {
         Sub.y = 10;
    }
}

You can set static fields in static methods, but you don't have access to this in static method because this represents the current instance of the object, and in a static method you have no instance.

回答5:

This means "this" object but there isn't one. In your case you can use the class name as @tibtof suggests.

回答6:

Keyword "this" refers to the object that you are operation with. In your case this inside any non-static methods or constructor (if you have one and and if you use "this" inside that), then "this" refers to that particular instance of the class Sub.So it is applicable only when the object is created. But anything in the static context of a class, you can use without even creating object for that as it is resolved during the class loading. "this" resolved only when object is created ( you can even say dynamically for which object). So "this" make sense in static context. Hope it helps. God bless.

回答7:

"this" keyword is only applicable where an instance of an object is created. And in static method no instance is created because static method belongs to class area.

回答8:

I agree with all other people who replied before me. Let me try it in different way to answer this:

I guess, instance method/non-static method belongs to instance of a class (meaning sooner or later we need object ref to access it) so this keyword make sense inside instance block or method. But static keyword to any member of a class is interpreted as direct asset to class which a object if existed then has access to it. So in static context it's not sure that object is existing somewhere. That's why using this inside static area is not allowed in java.

回答9:

when we declare variable and method is static then this is share by among object where this keyword only pointing to current object. suppose you have created five object of class foo then only one copy of made of (int y) shred by all object.so if you access int y using this keyword then compiler get a ambiguity which object have to point because static int y is shared by all object . you have access static variable using class name.

来源：https://stackoverflow.com/questions/11664522/why-cant-we-use-this-keyword-in-a-static-method