没有抽象类的arg构造函数

Question

I have an abstract class for which I need to provide a constructor that takes 0 parameters. I'm just not sure how to construct it or what would go in it.

enum Engine {STEAM_ENGINE, STEAM_TURBINE, GAS_TURBINE, DIESEL, ELECTRIC, WIND, HUMAN_FORCE};

public abstract class Ship
{   
    protected String name;
    protected int year;
    protected Engine engineType;

    public Ship(String n, int y, Engine t)
    {
       this.name = n;
       this.year = y;
       this.engineType = t;
    }

    public Ship ()
    {

    }

This is the code I have so far. Any input would be appreciated. Especially because I don't really understand the concept of 0-argument constructors in abstract classes.

Thanks for the help!

Answer 1

This would be similar to deb_rider's answer. But I would recommend using this form of code:

public Ship() {
    this("", 0, null);
}

What it does is that it calls another parameterized constructor to initialize the Ship with the default values by making use of the this operator.

---

Why this() instead of this.x = x; this.y = y;... this.x = x; this.y = y;... ?

Simple: Because it's shorter and makes the code more readable :-)

From the link above:

"...From within a constructor, you can also use the this keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation . ..."

Answer 2

Default constructors or non parameterized constructors are used to initialize the attributes to some default values while creating new object of that class when no data is available. You can define your "0-argument" constructor to initialize the values as,

public Ship(){
    this.name="";
    this.year=0;
    ...
}

you can also define some set methods to have the privilege to change the values at the later time. eg,

public void setName(String name){
    this.name=name;
}

Answer 3

First of all, we should understand why we use constructor of abstract class. Here few points which may help you to understand why we use constructor in abstract class.

1. Initialize data member of a class either assigning some value or making constructor chaining like @ambigram_maker's answer. Constructor chaining means calling another constructor from a constructor.

2. We use default constructor(Non-parameter or with 0 parameter) to avoid developer mistake while constructor chaining from child class eg

   public class MyShip extends Ship {
   public MyShip () { super(); } }

Here we'll get error if didn't put default constructor in Ship class.But mostly we use default constructor for default value of data members like

 public Ship()
    {
       this.name = "Default Engine name";
       this.year = 2014;//what you want as a default.I assume current year
       this.engineType = Engine.STEAM_ENGINE;//this is default engine type
    }

I mean default constructor is used to assign default value(change as per your requirement like default value of int is 0 but you may need current year as a default) for data member of a class and you don't need to put all data member inside default constructor.You should put that data members inside default constructor which are required to change like if you want this.year = 0; then you may avoid and this is done by compiler.

Hopefully you get helped.

Thanks