Why might one also use a blank constructor?

Question

I was reading some Java recently and came across something (an idiom?) new to me: in the program, classes with multiple constructors would also always include a blank constructor. For example:

public class Genotype {
  private boolean bits[];
  private int rating;
  private int length;
  private Random random;

  public Genotype() {              //  <= THIS is the bandit, this one right here
    random = new Random();
  }

  /* creates a Random genetoype */
  public Genotype(int length, Random r) {
    random = r;
    this.length = length;
    bits = new boolean[length];

    for(int i=0;i<length;i++) {
        bits[i] =random.nextBoolean();
    }
  }

  /* copy constructor */
  public Genotype(Genotype g,Random r) {
    random = r;
    bits = new boolean[g.length];
    rating = g.rating;
    length = g.length;

    for(int i=0;i<length;i++) {
        bits[i] = g.bits[i];
    }

  }
}

The first constructor doesn't seem to be a "real" constructor, it seems as though in every case one of the other constructors will be used. So why is that constructor defined at all?

Answer 1

Is the object Serializable?

To allow subtypes of non-serializable classes to be serialized, the subtype may assume responsibility for saving and restoring the state of the supertype's public, protected, and (if accessible) package fields. The subtype may assume this responsibility only if the class it extends has an accessible no-arg constructor to initialize the class's state. It is an error to declare a class Serializable if this is not the case. The error will be detected at runtime.

During deserialization, the fields of non-serializable classes will be initialized using the public or protected no-arg constructor of the class. A no-arg constructor must be accessible to the subclass that is serializable. The fields of serializable subclasses will be restored from the stream

Answer 2

That is NOT a copy constructor. Basically you want empty constructors when working with some framework. Shall there always be an empty constructor, of course, public or private, but at least it allows you to keep control of how the class is being (or not) instantiated.

Answer 3

The default constructor is'nt a good pratice for the functional view. The default constructor is used if the object have a global visibility into a method: for example, you want log the actual state of a object in a try/catch you can code

MyObejct myObject=null
try{...
}catch(Exception e){
    log.error(myObject);//maybe print null. information?
}

or do you prefer

MyObejct myObject=new Object();
try{...
}catch(Exception e){
log.error(myObject);//sure print  myobject.toString, never null. More information
}

?

Anotherway the create a EMPTY object have'nt a lot of logic, but instatiate a NULL object is harmuful in my opinion. You can read this post

Answer 4

Default constructor is NOT mandatory.

If no constructors defined in the class then default (empty) constructor will be created automatically. If you've provided any parametrized constructor(s) then default constructor will not be created automatically and it's better to create it by yourself. Frameworks that use dependency injection and dynamic proxy creation at runtime usually require default constructor. So, it depends on use cases of class that you write.