Question

The following code gives a compilation error:

class parent {
  parent(int a){}
}

class child extends parent{}

Error:

Main.java:6: cannot find symbol
symbol  : constructor parent()
location: class parent
class child extends parent{}
^
1 error

I was trying to do different things and found that adding a return type to the parent constructor got rid of the error!!!

class parent {
  int parent(int a){}
}

class child extends parent{}

I’ve read that constructors should not have return type, which clearly is not correct all the times. So my question is when should we have return type for constructor?

Answer 1

On constructor not having return type

Constructor must not have a return type. By definition, if a method has a return type, it’s not a constructor.

JLS 8.8 Constructor Declarations

A constructor is used in the creation of an object that is an instance of a class. [The name must match the class name, but], in all other respects, the constructor declaration looks just like a method declaration that has no result type.

---

On default constructors

The following snippet does give a compilation error:

class Parent {
   Parent(int a){}
}

class Child extends Parent{

   // DOES NOT COMPILE!!
   // Implicit super constructor parent() is undefined for default constructor.
   // Must define an explicit constructor

}

The reason is not because of a return type in a constructor, but because since you did not provide ANY constructor for Child, a default constructor is automatically created for you by the compiler. However, this default constructor tries to invoke the default constructor of the superclass Parent, which does NOT have a default constructor. THAT’S the source fo the compilation error.

Here’s the specification for the default constructor:

JLS 8.8.9 Default Constructor

If a class contains no constructor declarations, then a default constructor that takes no parameters is automatically provided:

• If the class being declared is the primordial class Object, then the default constructor has an empty body.
• Otherwise, the default constructor takes no parameters and simply invokes the superclass constructor with no arguments.

The following is a simple fix:

class Parent {
   Parent(int a){}
}

class Child extends Parent{

   // compiles fine!
   Child() {
      super(42);
   }

}

---

On methods having the same name as the constructor

The following snippet DOES compile:

// COMPILES FINE!!

class Parent  {

   // method has same name as class, but not a constructor
   int Parent(int a) {
      System.out.println("Yipppee!!!");
      return 42;
   }

   // no explicit constructor, so default constructor is provided
}

class Child extends Parent {

   // no explicit constructor, so default constructor is provided

}

There is in fact no explicit constructor in the above snippet. What you have is a regular method that has the same name as the class. This is allowed, but discouraged:

JLS 8.4 Method Declarations

A class can declare a method with the same name as the class or a field, member class or member interface of the class, but this is discouraged as a matter of style.

You will find that if you create a new Parent() or a new Child(), "Yipppee!!!" will NOT be printed to standard output. The method is not invoked upon creation since it’s not a constructor.