如何在父类构造函数所需的子类中初始化参数对象？

Question

So, this might sound a bit weird but let me explain. I have a super class that requires multiple parameters. One such parameter is a BufferedImage object. Now obviously, to initialize this BufferedImage in the child class to use as a parameter, I need to use try and catch blocks. The only way I can do that is in a method of the child class called in the constructor. The problem is, the super() constructor must be the first thing in the child class constructor. So I can't call the method to initialize my BufferedImage before calling super() . How can I initialize my BufferedImage object correctly before using it as a parameter when calling super() in my child class constructor?

Example: Super/Parent Class

public class CombatEntity {
    BufferedImage sprite; 
    public CombatEntity(String name, BufferedImage sprite) {
        //do something
    }
}

Example: Child Class

public class Batman {
     BufferedImage sprite;
     Batman() {
         super("Bruce Wayne", sprite); //sprite hasn't been properly initalized

     }

     void getSprite() { //I need to call this method before super in order to initalize my image
          try {
              File f = new File("Batman.png");
              sprite = ImageIO.read(f);
          }

          catch(Exception e) {
              //whatever
          }

     }

}

Answer 1

Do something like this:

At the parent class create normal constructor, which takes name and sprite parameters. Generate getters and setters method following JavaBeans specification.

CombatEntity:

   public class CombatEntity {

    protected String name;

    protected BufferedImage sprite; 

    public CombatEntity(String name, BufferedImage sprite) {
        this.name = name;
        this.sprite = sprite;
    }

    /*
     * Getters and Setters 
     */

    public String getName() {
        return name;
    }

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

    public BufferedImage getSprite() {
        return sprite;
    }

    public void setSprite(BufferedImage sprite) {
        this.sprite = sprite;
    }   
}

In Batman (child) class create two constructors - one with no parameters, which you can use to create Batman object before you initialize sprite image, and second similiar to parent constructor. When you invoke constructor with no parameters, it invoke parent constructor and set it's parameters to default. Then you can execute generateSpriteImage(String spriteImagePath) to create sprite image from injected path.

Batman:

public class Batman extends CombatEntity{

    //Default constructor with no parameters 
    public Batman(){
        super("", null);
    }

    public Batman(String name, BufferedImage sprite){
        super(name, sprite);
    }

    public void generateSpriteImage(String spriteImagePath) {
        try {
            File file = new File(spriteImagePath);
            this.sprite = ImageIO.read(file);
        }

        catch(Exception e) {
            //whatever
        }
   }
}

Hope this will help you.

Answer 2

The only way to fix this is to demand an Image parameter in your Batman constructor. This issue is not uncommon, and is one of the reasons to comply with the Javabeans pattern, where every class has a null constructor as well as getters and setters.

Answer 3

Your CombatEntity could define a abstract method getSprite() and call this one in the constructor. The child class (Batman) will have to implement this method.

The advantage is that you don't need to call an extra method after constructing the object (as suggested in the other answer).