class Point{    public double x;    public double y;}

　　对于可变的类来说，应该用包含私有域和公有设值方法的类来代替：

class Point{    private double x;    private double y;    Point(double x, double y) {        this.x = x;        this.y = y;    }    double getX() {        return x;    }    void setX(double x) {        this.x = x;    }    double getY() {        return y;    }    void setY(double y) {        this.y = y;    }}

　　总之，公有类永远都不应该暴露可变的域。