如何创建子类以使其属性值不被父类覆盖？

Question

*----------------*                 *----------------*         *---
|       Duck     |                 |   RubberDuck   |         |    
|----------------|                 |----------------|         |---
|color: ""       |   extends to    |color: "Yellow" |         |
|numLeg: 2       |   ---------->   |numLeg: 0       |   and   |     ...
|haveWings: True |                 |haveWings: True |         |
|----------------|                 |----------------|         |---
|swim()          |                 |swim()          |         |
*----------------*                 *----------------*         *---

From diagram above, If i want to extend Duck class to another Duck-like class with some different class attributes value. How should i do?

As i already searched some information i knew that attribute could not be overrided. then i think my design was wrong (wrong way in Object-Oriented design). Please give me some recommendation to improve my program (or re-design my program) in the "simple Object-Oriented way".

note that i don't care so much about Duck class (because i just use it as prototype). Duck class will have attributes or doesn't have attributes that's not matter but in my design i just think that mother class should have some default values (that may be my misleading here).

Answer 1

I don't see any prob here :)
Attributes value can be easily change, it's the attributes type which can't be modified.

public class Duck() {
  private String color;
  private int numLeg ;
  private boolean haveWings ;

  public Duck(String aColor, int aNumLeg, boolean hasWings) {
    color=aColor;
    numLeg=aNumLeg;
    haveWings=hasWings;
  }

  public Duck() {
    this("", 2, true); // a little trick for lazy man ;)
  }
}

then, you can easily change the attribute in a subclass, calling the Duck constructor.

public class SubDuck extends Duck {
  public SubDuck() {
    super("Yellow", 0, true);
  }
}

Answer 2

Create in Duck two constructors

1. First one with no arguments, here you define your default values of attributes
2. Second one taking all values -> this one is to be used in subclasses

Answer 3

Use the overloaded constuctor to set parameters.

//This constructor can only be called by inheriting classes 
//Since it is java, other classes in the package can do as well, but you can avoid. 
protected Duck(String color1, int numLeg1, int haveWings1){
    this.color = color1;
    this.numLeg = numLeg1;
    this.haveWings = haveWings1;   
}

//For setting default values and creating duck objects. 
//Not needed if you don't want to generate    plain Ducks
public Duck(){
    this.color = "";
    this.numLeg = 2;
    this.haveWings = true;
}

Then the RubberDuck Constructor where you call the duck-constructor to set the parameters. public RubberDuck(){ super("yellow", 0, true); } And additional classes can use similar way: public PoorDuck(){ super("pink-blue", 1, false); }

Answer 4

class Duck
{
 protected String color;
 protected int numLeg;
 protected boolean haveWings;

 public Duck()
 {
  color = "";
  numLeg = 2;
  haveWings = true;
 }

 public void swim()
 {
   //Do something
 }
}


class RubberDuck extends Duck
{
 public RubberDuck()
 {
  color = "yellow";
  numLeg = 0;
  haveWings = true;
 }

 public void swim()
 {
   //Do something
 }
}

Answer 5

如果您不关心并使用Duck作为原型，那么我建议创建Duck作为interfase或抽象类，然后您可以编写分别实现或扩展Duck的具体类。