Question

As I’m new to java, I would like to know,
1. How many Default constructors are defined when i Make an Empty Class?
2. Does i need to define copy constructor or it is auto defined as in C++?

Answer 1

There is only one default constructor, which is only defined when you declare no constructors for a class. Otherwise, the declared constructors will be the only constructors. If you do not include a constructor, the compiler inserts code equivalent to

public ClassName() {
  super();
}

In addition, if you do declare constructors for a class, and you don’t explicitly define the constructor call of the super class, the compiler will insert a parameter matching call to the super class.

public ClassName extends SuperClassName {

  public ClassName(String item, List stuff) {
    // no explicit super class constructor called
    ...
  }

}

gets transformed in the compiler to something like

public ClassName extends SuperClassName {

  public ClassName(String item, List stuff) {
    // explicit super class constructor call to SuperClassName(item, stuff);
    super(item, stuff); 
    ...
  }

}

To define a copy constructor, there is no special syntax, you basically define a constructor that takes another instance of your class as an argument.

public ClassName(ClassName other) {
  field = other.field;
  field2 = other.field2;
  field3 = new ArrayList(other.field3);
  ...
}

There is no such thing as a default copy constructor, actually there is no such thing as a copy constructor, there are just a constructors. If you decide to create one or more that seem to copy an object, so be it.

The reason that Java needs less types of items is somewhat tied to not overloading basic operators. If you don’t put more meanings on = then you don’t need as many types of constructors to support different methods of object allocation.

One of the reasons that = can be used so simply is due to Java only passing references, which are sort of like pointers; but, they are strongly typed, you cannot do any pointer math, nor access offsets. As such, they are only good for assignment and passing by reference, which prevents most pointer issues.