如何使用父类中的字段实例化对象？

Question

I am trying to get an understanding of object oriented programming in Java and I have this problem.

Say for example, I have aa parent class like this:

public class Shape {
    private int location;
    private Color color;
    // methods such as getLocation() and getColor()

    public Shape(int initialLocation, Color initialColor) {
        location = initialLocation;
        color = initialColor;
    }


}

How do I make my child class so that I can construct, say, a rectangle with an initial location and an initial color in a main method? Do I create a constructor in the Rectangle class? I can't because location and color are private fields. Do I create accessor methods for location and color and just set the location and color after instantiation? I guess, but is there a way to do this without accessors?

public class Rectangle extends Shape {
    public Rectangle(int initialLocation, Color initialColor) {
        super(initialLocation, initialColor);
    }

}

I just can't wrap my head around this fundamental concept. Any help?

Answer 1

Reuse constructors

public class Shape {
    private int location;
    private Color color;

    public Shape(int location, Color color) {
        this.location = location;
        this.color = color;
    }
    // methods such as getLocation() and getColor()
}

and

public class Rectangle extends Shape {
    public Rectangle(int location, Color color /*, more */) { 
        super(location, color);
        // more
    }
}

This official tutorial explains its use.

Answer 2

如果要扩展变量，可以将其修饰符更改为protected ，以便可以继承，否则private就像它们对于子类不存在一样。

Answer 3

Although you can define the instance variables as protected , however, this goes against the Object Oriented principle of encapsulation. I would use the getters and setters for each instance variable of the class Shape. Also, if you create a Constructor within Shape you could call the super constructor in Rectangle to set the location and colour in Rectangle.

public class Rectangle extends Shape {
    public Rectangle(int location, Color color) { 
        super(location, color);
    }
}

as long as you have the following constructor in Shape:

public class Shape {
    // location and color define.

    public Shape(int location, Color color) {
        this.location = location;
        this.color = color;
    }
    // getters and setters which are public for location and color
}

Answer 4

Private members in a base class that are only accessed by subclasses are pointless! If you want to read them, you'll need at least a public or protected getter. If you want to write them, you'll need at least a public or protected setter and/or a constructor that initializes them.