类中的this关键字

Question

I am learning Java, and going through the documentation.

There is a line on this page which I am not able to understand -

... Also, class methods cannot use the this keyword as there is no instance for this to refer to. ...

I thought that it was only static class methods which could not use the this keyword.

To test this, I wrote the following, which compiles.

import java.math.*;

class Point {

    public int x, y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public double getDistanceFromOrigin() {
        return Math.sqrt(this.x*this.x + this.y*this.y);
    }

}

I have a class all right, in which a method refers to this .

Am I misinterpreting things in some way?

Answer 1

Class methods are static methods. A "class method" is a method that is bound to the class definition (using the static keyword), as opposed to object/instance methods, which you write so that you can call them on objects you build based on that class.

The code you've written has two object/instance methods, and no class methods. If you want a class method in Java, you make it static, and then you cannot use this .

Answer 2

I thought that it was only static class methods which could not use the this keyword.

You're right. The static methods in a class belong to the class, not to the object reference. So to prove your sentence, just add a static method and use this keyword in it. For example:

class Point {

    public int x, y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public double getDistanceFromOrigin() {
        return Math.sqrt(this.x*this.x + this.y*this.y);
    }

    public static double getDistanceBetweenPoints(Point point1, Point point2) {
        //uncomment and it won't compile
        //double result = this.x;
        //fancy implementation...
        return 0;
    }

}

Answer 3

您是正确的，只有static类方法不能使用this关键字，但是您的代码示例是非静态的，因此this是完全有效的。

Answer 4

You are using this inside an instance method which will refer to the current instance.

public double getDistanceFromOrigin() {
    return Math.sqrt(this.x*this.x + this.y*this.y);
}

If you change the method to a static method , this will not be available , because static methods are tied to the class and not to a particular instance of the class whereas this refers to the current instance of a class if used inside a method.

public static double getDistanceFromOrigin() {
    return Math.sqrt(this.x*this.x + this.y*this.y); // compilation error here
}

Answer 5

After reading the content at the link you've posted, It seems to use the verbiage Class methods to refer to static methods

---

Class Methods

The Java programming language supports static methods as well as static variables. Static methods, which have the static modifier in their declarations, should be invoked with the class name, without the need for creating an instance of the class, as in

You can't use this in static methods because there is no instance(no this ) to reference.