Java default constructor

Question

What exactly is a default constructor     can you tell me which one of the following is a default constructor and what differentiates it from any other constructor   public Module        this name          this credits   0     this hours   0     public Module String name  int credits  int hours       this name   name     this credits   credits     this hours   hours

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A default constructor is automatically generated by the compiler if you do not explicitly define at least one constructor in your class. You've defined two, so your class does not have a default constructor.

Per The Java Language Specification Third Edition:

8.8.9 Default Constructor

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

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When you don   t define any constructor in your class  compiler defines default one for you  however when you declare any constructor  in your example you have already defined a parameterized constructor   compiler doesn   t do it for you   Since you have defined a constructor in class code  compiler didn   t create default one  While creating object you are invoking default one  which doesn   t exist in class code  Then the code gives an compilation error

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If a class doesn t have any constructor provided by programmer  then java compiler will add a default constructor with out parameters which will call super class constructor internally with super   call  This is called as default constructor   In your case  there is no default constructor as you are adding them programmatically  If there are no constructors added by you  then compiler generated default constructor will look like this   public Module        super        Note  In side default constructor  it will add super   call also  to call super class constructor   Purpose of adding default constructor   Constructor s duty is to initialize instance variables  if there are no instance variables you could choose to remove constructor from your class  But when you are inheriting some class it is your class responsibility to call super class constructor to make sure that super class initializes all its instance variables properly    That s why if there are no constructors  java compiler will add a default constructor and calls super class constructor

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When we do not explicitly define a constructor for a class  then java creates a default constructor for the class  It is essentially a non-parameterized constructor  i e  it doesn t accept any arguments   The default constructor s job is to call the super class constructor and  initialize all instance variables  If the super class constructor is not present then it automatically initializes the instance variables to zero  So  that serves the purpose of using constructor  which is to initialize the internal state of an object so that the code creating an instance will have a fully initialized  usable object    Once we define our own constructor for the class  the default constructor is no longer used  So  neither of them is actually a default constructor

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A default constructor is created if you don t define any constructors in your class  It simply is a no argument constructor which does nothing  Edit  Except call super    public Module

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I hope you got your answer regarding which is default constructor  But I am giving below statements to correct the comments given         Java does not initialize any local variable to any default value  So   if you are creating an Object of a class it will call default   constructor and provide default values to Object    Default constructor provides the default values to the object like 0    null etc  depending on the type       Please refer below link for more details   https   www javatpoint com constructor

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Java provides a default constructor which takes no arguments and performs no special actions or initializations  when no explicit constructors are provided   The only action taken by the implicit default constructor is to call the superclass constructor using the super   call  Constructor arguments provide you with a way to provide parameters for the initialization of an object   Below is an example of a cube class containing 2 constructors   one default and one parameterized constructor    public class Cube1       int length      int breadth      int height      public int getVolume             return  length   breadth   height              Cube1             length   10          breadth   10          height   10             Cube1 int l  int b  int h            length   l          breadth   b          height   h             public static void main String   args            Cube1 cubeObj1  cubeObj2          cubeObj1   new Cube1            cubeObj2   new Cube1 10  20  30           System out println  Volume of Cube1 is       cubeObj1 getVolume             System out println  Volume of Cube1 is       cubeObj2 getVolume

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General terminology is that if you don t provide any constructor in your object a no argument constructor is automatically placed which is called default constructor   If you do define a constructor same as the one which would be placed if you don t provide any it is generally termed as no arguments constructor Just a convention though as some programmer prefer to call this explicitly defined no arguments constructor as default constructor  But if we go by naming if we are explicitly defining one than it does not make it default   As per the docs  If a class contains no constructor declarations  then a default constructor with no formal parameters and no throws clause is implicitly declared   Example  public class Dog       will automatically be modified by adding default constructor  as follows  public class Dog      public Dog                 and when you create it s object   Dog myDog   new Dog      this default constructor is invoked

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default constructor refers to a constructor that is automatically generated by the compiler in the absence of any programmer-defined constructors   If there s no constructor provided by programmer  the compiler implicitly declares a default constructor which calls super    has no throws clause as well no formal parameters   E g   class Klass            Default Constructor gets generated     new Klass        Correct -------------------------------------  class KlassParameterized        KlassParameterized   String str            Parameterized Constructor            do Something           new KlassParameterized         Wrong  - you need to explicitly provide no-arg constructor  The compiler now never declares default one    --------------------------------  class KlassCorrected        KlassCorrected           No-arg Constructor            Safe to Invoke           KlassCorrected   String str            Parameterized Constructor            do Something           new KlassCorrected           RIGHT  -- you can instantiate

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A default constructor does not take any arguments   public class Student           default constructor     public Student

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When you create a new Module object  java compiler add a default constructor for you because there is no constructor at all  class Module      you will never see the default constructor  If you add any kind of constructor even and non-arg one  than java thing you have your own and don t add a default constructor anymore  This is an non-arg constructor that internelly call the super   constructor from his parent class even you don t have one   if your class don t have a parent class than Object Class constructor will be call      class Module           Module         this look like a default constructor but in not

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Neither of them  If you define it  it s not the default   The default constructor is the no-argument constructor automatically generated unless you define another constructor  Any uninitialised fields will be set to their default values  For your example  it would look like this assuming that the types are String  int and int  and that the class itself is public   public Module       super      this name   null    this credits   0    this hours   0      This is exactly the same as  public Module        And exactly the same as having no constructors at all  However  if you define at least one constructor  the default constructor is not generated   Reference  Java Language Specification     If a class contains no constructor declarations  then a default constructor with no formal parameters and no throws clause is implicitly declared     Clarification  Technically it is not the constructor  default or otherwise  that default-initialises the fields  However  I am leaving it the answer because    the question got the defaults wrong  and  the constructor has exactly the same effect whether they are included or not

User · Answer

Hi  As per my knowledge let me clear the concept of default constructor      The compiler automatically provides a no-argument  default constructor   for any class without constructors  This default constructor will call   the no-argument constructor of the superclass  In this situation  the   compiler will complain if the superclass doesn t have a no-argument   constructor so you must verify that it does  If your class has no   explicit superclass  then it has an implicit superclass of Object    which does have a no-argument constructor    I read this information from the Java Tutorials

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