Methods vs Constructors in Java

Question

I have just started programming with Java  The text we use is lacking when talking about methods and constructors  I m not sure what a method or a constructor is exactly and what makes each unique  Can someone please help me define them and differentiate between the two

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The main difference is  1 Constructor are used to initialize the state of object where as method is expose the behaviour of object             2 Constructor must not have return type where as method must have return type       3 Constructor name same as the class name where as method may or may not the same class name    4 Constructor invoke implicitly where as method invoke explicitly      5 Constructor compiler provide default constructor where as method compiler does t provide

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A constructor is a special kind of method that allows you to create a new instance of a class  It concerns itself with initialization logic

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Here are some main key differences between constructor and method in java   Constructors are called at the time of object creation automatically  But methods are not called during the time of object creation automatically  Constructor name must be same as the class name  Method has no such protocol  The constructors can   t have any return type  Not even void  But methods can have a return type and also void  Click to know details - Difference between constructor and method in Java

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the difference r    Constructor must have the name same as class but method can be made by any name  Constructor are not inherited automatically by child classes while child inherit method from their parent class unless they r protected by private keyword  Constructor r called explicitly while methods implicitaly  Constructor doesnot have any return type while method have

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Constructor typically is Method   When we create object of a class new operator use then we invoked a special kind of method called constructor   Constructor used to perform initialization  of  instance variable   Code   public class Diff   public Diff       same as class name so constructor           String A    Local variable A in Constructor            System out println A   Contructor Print me             public void Print            String B    Local variable B in Method           System out println B   Method print me                 public static void main String args             Diff ob   new Diff                       Output   Local variable A in Constructor Contructor Print me   So only show here Constructor  method Diff   statement because we create Diff class object  In that case constructor always come first to instantiate Class here  class Diff     typically   Constructor is set up feature   Everything start with here  when we call ob object in the main method constructor takes this class and create copy and it s load into the   Java Virtual Machine Class loader      This class loader takes this copy and load into memory so we can now use it by referencing   Constructor done its work then Method are come and done its real implementation   In this program when we call  ob print      then method will coming   Thanks   Arindam

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The important difference between constructors and methods is that constructors  initialize objects that are being created with the new operator  while methods perform operations on objects that already exist   Constructors can t be called directly  they are called implicitly when the new keyword creates an object   Methods can be called directly on an object that has already been created with new   The definitions of constructors and methods look similar in code   They can take parameters  they can have modifiers  e g  public   and they have method bodies in braces   Constructors must be named with the same name as the class name   They can t return anything  even void  the object itself is the implicit return    Methods must be declared to return something  although it can be void

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Constructor is special function used to initialise the data member  where the methods are functions to perform specific task   Constructor name is the same name as the class name  where the method name may or may not or be class name   Constructor does not allow any return type  where methods allow return type

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In Java  classes you write are Objects  Constructors construct those objects  For example if I have an Apple class like so   public class Apple         instance variables     String type     macintosh  green  red                      This is the default constructor that gets called when you use        Apple a   new Apple    which creates an Apple object named a               public Apple                in here you initialize instance variables  and sometimes but rarely            do other functionality  at least with basic objects          this type    macintosh      the  this  keyword refers to  this  object  so this type refers to Apple s  type  instance variable                        this is another constructor with a parameter  You can have more than one        constructor as long as they have different parameters  It creates an Apple        object when called using Apple a   new Apple  someAppleType                public Apple String t               when the constructor is called  i e new Apple     this code is executed         this type   t                        methods in a class are functions  They are whatever functionality needed        for the object             public String someAppleRelatedMethod            return  hello  Apple class               public static void main String   args               construct an apple         Apple a   new Apple  green                a  is now an Apple object and has all the methods and            variables of the Apple class             To use a method from  a           String temp   a someAppleRelatedMethod            System out println temp           System out println  a s type is     a type             Hopefully I explained everything in the comments of the code  but here is a summary  Constructors  construct  an object of type of the class  The constructor must be named the same thing as the class  They are mostly used for initializing instance varibales Methods are functionality of the objects

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A  method  is a  subroutine  is a  procedure  is a  function  is a  subprogram  is a      The same concept goes under many different names  but basically is a named segment of code that you can  call  from some other code   Generally the code is neatly packaged somehow  with a  header  of some sort which gives its name and parameters and a  body  set off by BEGIN  amp  END or    amp    or some such   A  consrtructor  is a special form of method whose purpose is to initialize an instance of a class or structure   In Java a method s header is  lt qualifiers gt   lt return type gt   lt method name gt     lt parameter type 1 gt   lt parameter name 1 gt    lt parameter type 2 gt   lt parameter name 2 gt         lt exceptions gt  and a method body is bracketed by        And you can tell a constructor from other methods because the constructor has the class name for its  lt method name gt  and has no declared  lt return type gt     In Java  of course  you create a new class instance with the new operator -- new  lt class name gt     lt parameter list gt

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The Major difference is Given Below -   1  Constructor must have same name as the class name while this is not the case of methods  class Calendar      int year   0      int month  0         constructor     public Calendar int year  int month           this year   year          this month   month          System out println  Demo Constructor                 Method     public void Display             System out println  Demo method               2  Constructor initializes objects of a class whereas method does not  Methods performs operations on objects that already exist  In other words  to call a method we need an object of the class   public class Program        public static void main String   args               constructor will be called on object creation         Calendar ins    new Calendar 25  5              Methods will be called on object created         ins Display                3  Constructor does not have return type but a method must have a return type  class Calendar         constructor     no return type     public Calendar int year  int month                 Method have void return type     public void Display             System out println  Demo method

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Other instructors and teaching assistants occasionally tell me that constructors are specialized methods   I always argue that in Java constructors are NOT specialized methods   If constructors were methods at all  I would expect them to have the same abilities as methods    That they would at least be similar in more ways than they are different   How are constructors different than methods   Let me count the ways      Constructors must be invoked with the new operator while methods may not be invoked with the new operator   Related  Constructors may not be called by name while methods must be called by name  Constructors may not have a return type while methods must have a return type  If a method has the same name as the class  it must have a return type   Otherwise  it is a constructor   The fact that you can have two MyClass   signatures in the same class definition which are treated differently should convince all that constructors and methods are different entities   public class MyClass      public MyClass                                            constructor    public String MyClass     return  MyClass   method        method    Constructors may initialize instance constants while methods may not  Public and protected constructors are not inherited while public and protected methods are inherited  Constructors may call the constructors of the super class or same class while methods may not call either super   or this      So  what is similar about methods and constructors    They both have parameter lists  They both have blocks of code that will be executed when that block is either called directly  methods  or invoked via new  constructors     As for constructors and methods having the same visibility modifiers    fields and methods have more visibility modifiers in common    Constructors may be  private  protected  public  Methods may be  private  protected  public  abstract  static  final  synchronized  native   strictfp  Data fields may be  private  protected  public  static  final  transient  volatile    In Conclusion  In Java  the form and function of constructors is significantly different than for methods   Thus  calling them specialized methods actually makes it harder for new programmers to learn the differences   They are much more different than similar and learning them as different entities is critical in Java     I do recognize that Java is different than other languages in this regard  namely C    where the concept of specialized methods originates and is supported by the language rules   But  in Java  constructors are not methods at all  much less specialized methods   Even javadoc recognizes the differences between constructors and methods outweigh the similarities  and provides a separate section for constructors

[java] Methods vs Constructors in Java

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