通过对象在静态方法上使用非静态变量？ Java的

Question

Since we can't use this inside a static method, and we also cannot use non-static variables, why is it that we can use objects, which use nonstatic variables inside static methods?

Here is what I mean:

public int x;
public int y;

public Account(int a, int b) {
    this.x = a;
    this.y = b;
}

public static void Swap(Account acc) {
    int holder;
    holder = acc.x;
    acc.x = acc.y;
    acc.y = holder;
}

So Swap() will work, even though the variables inside of the object are not static. I don't understand this part. Would appreciate some help. TIA!

Answer 1

static methods cannot access instance variable of the current ( this ) instance, since no such instance exists in their context.

However, if you pass to them a reference to an instance, they can access any instance variables and methods visible to them.

In case of your swap example, if that method wasn't static , you could have removed the acc argument and operate on the instance variables of this :

public void swap() {
    int holder;
    holder = this.x;
    this.x = this.y;
    this.y = holder;
}

Answer 2

You cannot use this in a static method because Java does not know which instance (which this) you refer to.

You can pass a reference to an object as a parameter acc to a static method because the caller specifies which instance to pass.

Java knows which instance you mean when your static method refers to acc . So you can use any accessible fields or methods of acc .