Purpose of a constructor in Java

Question

What is the purpose of a constructor  I ve been learning Java in school and it seems to me like a constructor is largely redundant in things we ve done thus far  It remains to be seen if a purpose comes about  but so far it seems meaningless to me  For example  what is the difference between the following two snippets of code   public class Program           public constructor              function                      private void function                do stuff               public static void main String   args             constructor a   new constructor               This is how we were taught do to things for assignments  but wouldn t the below do the same deal   public class Program           public static void main String   args            function                      private void function               do stuff           The purpose of a constructor escapes me  but then again everything we ve done thus far has been extremely rudimentary

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I thought a constructor might work like a database in which only it defines variables and methods to control them

Then I saw objects that use variables and methods not defined in its constructor. So the discussion that makes the most sense to me is the one that tests variables values for validity, but the class can create variables and methods that are not defined in the constructor - less like a database and more like an over pressure valve on a steam locomotive. It doesn't control the wheels,etc.

Far less of a definition than I thought.

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Constructor is basicaly used for initialising the variables at the time of creation of object

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Well  first I will tell the errors in two code snippets   First code snippet  public class Program       public constructor      Error - Return type for the method is missing               function                      private void function                   do stuff               public static void main String   args                constructor a   new constructor       Error - constructor cannot be resolved to a type           As you can see the code above  constructor name is not same as class name  In the main   method you are creating a object from a method which has no return type   Second code snippet  public class Program       public static void main String   args                function       Error - Cannot make a static reference to the non-static method function   from the type Program            private void function                   do stuff            Now in this code snippet you re trying to create a static reference to the non-static method function   from the type Program  which is not possible   So the possible solution is this   First code snippet  public class Program       public Program                 function                      private void function                   do stuff               public static void main String   args                Program a   new Program            a function              Second code snippet  public class Program       public static void main String   args                Program a   new Program            a function               private void function                   do stuff            Finally the difference between the code snippets is that  in first code snippet class name is not same as the class name  While in the second code snippet there is no constructor defined   Meanwhile to understand the purpose of constructor refer resources below   https   docs oracle com javase 7 docs api java lang reflect Constructor html  http   www flowerbrackets com learn-constructor-in-java   http   www flowerbrackets com java-constructor-example

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As mentioned in LotusUNSW answer Constructors are used to initialize the instances of a class    Example   Say you have an Animal class something like  class Animal     private String name     private String type      Lets see what happens when you try to create an instance of Animal class  say a Dog named Puppy  Now you have have to initialize name   Puppy and type   Dog  So  how can you do that  A way of doing it is having a constructor like      Animal String nameProvided  String typeProvided            this name   nameProvided           this type   typeProvided           Now when you create an object of class Animal  something like Animal dog   new Animal  Puppy    Dog    your constructor is called and initializes name and type to the values you provided i e  Puppy and Dog respectively   Now you might ask what if I didn t provide an argument to my constructor something like   Animal xyz   new Animal      This is a default Constructor which initializes the object with default values i e  in our Animal class name and type values corresponding to xyz object would be name   null and type   null

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The class definition defines the API for your class  In other words  it is a blueprint that defines the contract that exists between the class and its clients--all the other code that uses this class  The contract indicates which methods are available  how to call them  and what to expect in return   But the class definition is a spec  Until you have an actual object of this class  the contract is just  a piece of paper   This is where the constructor comes in   A constructor is the means of creating an instance of your class by creating an object in memory and returning a reference to it  Something that should happen in the constructor is that the object is in a proper initial state for the subsequent operations on the object to make sense    This object returned from the constructor will now honor the contract specified in the class definition  and you can use this object to do real work   Think of it this way  If you ever look at the Porsche website  you will see what it can do--the horsepower  the torque  etc  But it isn t fun until you have an actual Porsche to drive   Hope that helps

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It s used to set up the contents and state of your class  Whilst it s true you can make the simpler example with the main method you only have 1 main method per app so it does not remain a sensible approach   Consider the main method to simply start your program and should know no more than how to do that  Also note that main   is static so cannot call functions that require a class instance and the state associated  The main method should call new Program   function   and the Program constructor should not call function   unless it is required for the setup of the class

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Constructors are used to initialize the instances of your classes  You use a constructor to create new objects often with parameters specifying the initial state or other important information about the object  From the official Java tutorial      A class contains constructors that are invoked to create objects from the class blueprint  Constructor declarations look like method declarations   except that they use the name of the class and have no return type  For example  Bicycle has one constructor   public Bicycle int startCadence  int startSpeed  int startGear        gear   startGear      cadence   startCadence      speed   startSpeed          To create a new Bicycle object called myBike  a constructor is called by the new operator       Bicycle myBike   new Bicycle 30  0  8        new Bicycle 30  0  8  creates space in memory for the object and initializes its fields       Although Bicycle only has one constructor  it could have others  including a no-argument constructor       public Bicycle               gear   1            cadence   10            speed   0               Bicycle yourBike   new Bicycle    invokes the no-argument constructor to create a new Bicycle object called yourBike

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A constructor initializes an object when it is created   It has the same name as its class and is syntactically similar to a method   but constructor have no expicit return type Typically   we use constructor to give initial value to the instance variables defined by the class   or to perform any other startup procedures required to make a fully formed object   Here is an example of constructor   class queen       int beauty      queen          beauty   98            class constructor demo      public static void main String   args          queen arth   new queen           queen y   new queen            System out println arth beauty     y beauty               output is   98 98   Here the construcor is    queen    beauty  98      Now the turn of parameterized constructor     class queen       int beauty      queen int x         beauty   x            class constructor demo      public static void main String   args          queen arth   new queen 100          queen y   new queen 98           System out println arth beauty     y beauty               output is   100 98

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A constructor is basically a method that you can use to ensure that objects of your class are born valid  This is the main motivation for a constructor   Let s say you want your class has a single integer field that should be always larger than zero  How do you do that in a way that is reliable   public class C        private int number        public C int number            setNumber number                public void setNumber int number            if  number  lt  1                throws IllegalArgumentException  C cannot store anything smaller than 1                      this number   number             In the code above  it may look like you are doing something redundant  but in fact you are ensuring that the number is always valid no matter what    initialize the instances of a class  is what a constructor does  but not the reason why we have constructors  The question is about the purpose of a constructor  You can also initialize instances of a class externally  using c setNumber 10  in the example above  So a constructor is not the only way to initialize instances   The constructor does that but in a way that is safe  In other words  a class alone solves the whole problem of ensuring their objects are always in valid states  Not using a constructor will leave such validation to the outside world  which is bad design   Here is another example   public class Interval       private long start      private long end       public Interval long start  long end            changeInterval start  end              public void changeInterval long start  long end            if  start  gt   end                throw new IllegalArgumentException  Invalid interval                       this start   start          this end   end            public long duration             return end - start            The Interval class represents a time interval  Time is stored using long  It does not make any sense to have an interval that ends before it starts  By using a constructor like the one above it is impossible to have an instance of Interval at any given moment anywhere in the system that stores an interval that does not make sense

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Let us consider we are storing the student details of 3 students  These 3 students are having different values for sno  sname and sage  but all 3 students belongs to same department CSE  So it is better to  initialize  dept  variable inside a constructor so that this value can be taken by all the 3 student objects   To understand clearly  see the below simple example   class Student       int sno sage      String sname dept      Student                 dept  CSE             public static void main String a                  Student s1 new Student            s1 sno 101          s1 sage 33          s1 sname  John            Student s2 new Student            s2 sno 102          s2 sage 99          s2 sname  Peter             Student s3 new Student            s3 sno 102          s3 sage 99          s3 sname  Peter           System out println  The details of student1 are            System out println  The student no is   s1 sno           System out println  The student age is   s1 sage           System out println  The student name is   s1 sname           System out println  The student dept is   s1 dept             System out println  The details of student2 are    enter code here          System out println  The student no is   s2 sno           System out println  The student age is   s2 sage           System out println  The student name is   s2 sname           System out println  The student dept is   s2 dept            System out println  The details of student2 are            System out println  The student no is   s3 sno           System out println  The student age is   s3 sage           System out println  The student name is   s3 sname           System out println  The student dept is   s3 dept             Output      The details of student1 are   The student no is 101   The student age is 33   The student name is John   The student dept is CSE   The details of student2 are   The student no is 102   The student age is 99   The student name is Peter   The student dept is CSE   The details of student2 are   The student no is 102   The student age is 99   The student name is Peter   The student dept is CSE

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Through a constructor  with parameters   you can  ask  the user of that class for required dependencies  It is used to initialize instance variables and to pass up arguments to the constructor of a super class  super        which basically does the same It can initialize  final  instance variables with code  that may throw Exceptions  as opposed to instance initializer scopes One should not blindly call methods from within the constructor  because initialization may not be finished sufficient in the local or a derived class

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Constructor will be helpful to prevent instances getting unreal values  For an example set a Person class with height   weight  There can t be a Person with 0m and 0kg

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A Java constructor has the same name as the name of the class to   which it belongs   Constructor   s syntax does not include a return type  since constructors never return a value    Constructor is always called when object is created  example - Default constructor  class Student3        int id        String name        void display   System out println id     name          public static void main String args               Student3 s1 new Student3              Student3 s2 new Student3              s1 display              s2 display                   Output   0 null 0 null   Explanation  In the above class you are not creating any constructor so compiler provides you a default constructor Here 0 and null values are provided by default constructor   Example of parameterized constructor  In this example  we have created the constructor of Student class that have two parameters  We can have any number of parameters in the constructor   class Student4        int id        String name        Student4 int i String n             id   i            name   n                void display   System out println id     name          public static void main String args               Student4 s1   new Student4 111  Karan              Student4 s2   new Student4 222  Aryan              s1 display              s2 display                    Output   111 Karan 222 Aryan

[java] Purpose of a constructor in Java?

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