How do I call one constructor from another in Java

Question

Is it possible to call a constructor from another  within the same class  not from a subclass   If yes how  And what could be the best way to call another constructor  if there are several ways to do it

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The keyword this can be used to call a constructor from a constructor  when writing several constructor for a class  there are times when you d like to call one constructor from another to avoid duplicate code   Bellow is a link that I explain other topic about constructor and getters   and setters   and I used a class with two constructors  I hope the explanations and examples help you   Setter methods or constructors

User · Answer

Yes it is possible to call one constructor from another with use of this    class Example     private int a   1     Example            this 5     here another constructor called based on constructor argument         System out println  number a is   a              Example int b           System out println  number b is   b

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I will tell you an easy way  There are two types of constructors    Default constructor Parameterized constructor   I will explain in one Example  class ConstructorDemo          ConstructorDemo    Default Constructor                  System out println  D constructor                    ConstructorDemo int k   Parameterized constructor                  this     ------------- 1           System out println  P Constructor    k                         public static void main String   args                      this    error because  must be first statement in constructor          new ConstructorDemo     ------- 2           ConstructorDemo g new ConstructorDemo 3  --- 3                                        In the above example I showed 3 types of calling   this   call to this must be first statement in constructor This is Name less Object  this automatically calls the default constructor  3 This calls the Parameterized constructor    Note  this must be the first statement in the constructor

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I prefer this way         class User           private long id          private String username          private int imageRes       public User             init defaultID defaultUsername defaultRes             public User String username            init defaultID username  defaultRes                public User String username  int imageRes            init defaultID username  imageRes              public User long id  String username  int imageRes            init id username  imageRes               private void init long id  String username  int imageRes            this id id          this username   username          this imageRes   imageRes

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Originally from an anser by Mirko Klemm  slightly modified to address the question   Just for completeness  There is also the Instance initialization block that gets executed always and before any other constructor is called  It consists simply of a block of statements           somewhere in the body of your class definition   You can even have more than one   You can t call them  but they re like  shared constructor  code if you want to reuse some code across constructors  similar to calling methods   So in your case       System out println  this is shared constructor code executed before the constructor      field1   3      There is also a  static  version of this to initialize static members   static

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I know there are so many examples of this question but what I found I am putting here to share my Idea  there are two ways to chain constructor  In Same class you can use this keyword  in Inheritance  you need to use super keyword       import java util        import java lang         class Test                 public static void main String args                          Dog d   new Dog       Both Calling Same Constructor of Parent Class i e  0 args Constructor              Dog cs   new Dog  Bite       Both Calling Same Constructor of Parent Class i e  0 args Constructor                  You need to Explicitly tell the java compiler to use Argument constructor so you need to use  super  key word             System out println  ------------------------------                Cat c   new Cat                Cat caty   new Cat  10                 System out println  ------------------------------                   Self s   new Self                Self ss   new Self  self                         class Animal               String i           public Animal                         i    10               System out println  Animal Constructor     i                     public Animal String h                        i    20               System out println  Animal Constructor Habit     i                        class Dog extends Animal               public Dog                         System out println  Dog Constructor                      public Dog String h                        System out println  Dog Constructor with habit                         class Cat extends Animal               public Cat                         System out println  Cat Constructor                      public Cat String i                        super i      Calling Super Class Paremetrize Constructor              System out println  Cat Constructor with habit                         class Self               public Self                         System out println  Self Constructor                      public Self String h                        this       Explicitly calling 0 args constructor               System out println  Slef Constructor with value

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Using this keyword we can call one constructor in another constructor within same class  Example  -  public class Example             private String name            public Example               this  quot Mahesh quot                   public Example String name              this name   name

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Calling constructor from another constructor  class MyConstructorDemo extends ConstructorDemo       MyConstructorDemo                 this  calling another constructor              MyConstructorDemo String arg                System out print  This is passed String by another constructor    arg             Also you can call parent constructor by using super   call

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Yes it is possible to call one constructor from another  But there is a rule to it  If a call is made from one constructor to another  then      that new constructor call must be the first statement in the current constructor   public class Product        private int productId       private String productName       private double productPrice       private String category       public Product int id  String name            this id name 1 0              public Product int id  String name  double price            this id name price  DEFAULT               public Product int id String name double price  String category           this productId id          this productName name          this productPrice price          this category category            So  something like below will not work   public Product int id  String name  double price        System out println  Calling constructor with price        this id name price  DEFAULT        Also  in the case of inheritance  when sub-class s object is created  the super class constructor is first called   public class SuperClass       public SuperClass            System out println  Inside super class constructor            public class SubClass extends SuperClass       public SubClass               Even if we do not add  Java adds the call to super class s constructor like            super           System out println  Inside sub class constructor              Thus  in this case also another constructor call is first declared before any other statements

User · Answer

Using this keyword we can call one constructor in another constructor within same class  Example  -  public class Example             private String name            public Example               this  quot Mahesh quot                   public Example String name              this name   name

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Within a constructor  you can use the this keyword to invoke another constructor in the same class  Doing so is called an explicit constructor invocation    Here s another Rectangle class  with a different implementation from the one in the Objects section   public class Rectangle       private int x  y      private int width  height       public Rectangle             this 1  1             public Rectangle int width  int height            this  0 0 width  height             public Rectangle int x  int y  int width  int height            this x   x          this y   y          this width   width          this height   height             This class contains a set of constructors  Each constructor initializes some or all of the rectangle s member variables

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Yes  you can call constructors from another constructor  For example  public class Animal       private int animalType       public Animal             this 1     here this 1  internally make call to Animal 1              public Animal int animalType            this animalType   animalType           you can also read in details from Constructor Chaining in Java

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Yes it is possible public class User     private String name    quot  quot    private String surname    quot  quot    private int age   0   public User        this  quot name is undefined quot   quot surname is undefined quot  0          public User String name String surname         this name surname 0              public User String name  String surname  int age          this name   name        this surname   surname        this age   age

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It is called Telescoping Constructor anti-pattern or constructor chaining  Yes  you can definitely do  I see many examples above and I want to add by saying that if you know that you need only two or three constructor  it might be ok  But if you need more  please try to use different design pattern like Builder pattern  As for example     public Omar       public Omar a      public Omar a b      public Omar a b c      public Omar a b c d            You may need more  Builder pattern would be a great solution in this case  Here is an article  it might be helpful https   medium com  modestofiguereo design-patterns-2-the-builder-pattern-and-the-telescoping-constructor-anti-pattern-60a33de7522e

User · Answer

When I need to call another constructor from inside the code  not on the first line   I usually use a helper method like this   class MyClass      int field       MyClass           init 0            MyClass int value          if  value lt 0              init 0                  else              init value                   void init int x          field   x           But most often I try to do it the other way around by calling the more complex constructors from the simpler ones on the first line  to the extent possible  For the above example  class MyClass      int field      MyClass int value          if  value lt 0           field   0        else          field   value          MyClass           this 0

User · Answer

Yes  any number of constructors can be present in a class and they can be called by another constructor using this    Please do not confuse this   constructor call with this keyword   this   or this args  should be the first line in the constructor   Example   Class Test       Test             this 10      calls the constructor with integer args  Test int a            Test int a            this 10 5      call the constructor with double arg  Test double a            Test double a            System out println  I am a double arg constructor              This is known as constructor overloading  Please note that for constructor  only overloading concept is applicable and not inheritance or overriding

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Using this args   The preferred pattern is to work from the smallest constructor to the largest   public class Cons        public Cons                A no arguments constructor that sends default values to the largest         this madeUpArg1Value madeUpArg2Value madeUpArg3Value              public Cons int arg1  int arg2              An example of a partial constructor that uses the passed in arguments            and sends a hidden default value to the largest         this arg1 arg2  madeUpArg3Value                 Largest constructor that does the work     public Cons int arg1  int arg2  int arg3            this arg1   arg1          this arg2   arg2          this arg3   arg3            You can also use a more recently advocated approach of valueOf or just  of    public class Cons       public static Cons newCons int arg1                   This function is commonly called valueOf  like Integer valueOf                More recently called  of   like EnumSet of             Cons c   new Cons               c setArg1                return c             To call a super class  use super someValue   The call to super must be the first call in the constructor or you will get a compiler error

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Pretty simple   public class SomeClass       private int number      private String someString       public SomeClass            number   0          someString   new String               public SomeClass int number           this      set the class to 0         this setNumber number               public SomeClass int number  String someString           this number     call public SomeClass  int number           this setString someString              public void setNumber int number           this number   number            public void setString String someString           this someString   someString                   add some accessors     now here is some small extra credit   public SomeOtherClass extends SomeClass       public SomeOtherClass int number  String someString            super number  someString     calls public SomeClass int number  String someString                   Some other code      Hope this helps

User · Answer

Yes  any number of constructors can be present in a class and they can be called by another constructor using this    Please do not confuse this   constructor call with this keyword   this   or this args  should be the first line in the constructor   Example   Class Test       Test             this 10      calls the constructor with integer args  Test int a            Test int a            this 10 5      call the constructor with double arg  Test double a            Test double a            System out println  I am a double arg constructor              This is known as constructor overloading  Please note that for constructor  only overloading concept is applicable and not inheritance or overriding

User · Answer

You can call another constructor via the this      keyword  when you need to call a constructor from the same class  or the super      keyword  when you need to call a constructor from a superclass    However  such a call must be the first statement of your constructor  To overcome this limitation  use this answer

User · Answer

Originally from an anser by Mirko Klemm  slightly modified to address the question   Just for completeness  There is also the Instance initialization block that gets executed always and before any other constructor is called  It consists simply of a block of statements           somewhere in the body of your class definition   You can even have more than one   You can t call them  but they re like  shared constructor  code if you want to reuse some code across constructors  similar to calling methods   So in your case       System out println  this is shared constructor code executed before the constructor      field1   3      There is also a  static  version of this to initialize static members   static

User · Answer

Note  I just want to add one aspect  which I did not see in the other answers  how to overcome limitations of the requirement that this   has to be on the first line     In Java another constructor of the same class can be called from a constructor via this    Note however that this has to be on the first line   public class MyClass      public MyClass double argument1  double argument2        this argument1  argument2  0 0          public MyClass double argument1  double argument2  double argument3        this argument1   argument1      this argument2   argument2      this argument3   argument3          That this has to appear on the first line looks like a big limitation  but you can construct the arguments of other constructors via static methods  For example   public class MyClass      public MyClass double argument1  double argument2        this argument1  argument2  getDefaultArg3 argument1  argument2           public MyClass double argument1  double argument2  double argument3        this argument1   argument1      this argument2   argument2      this argument3   argument3         private static double getDefaultArg3 double argument1  double argument2        double argument3   0          Calculate argument3 here if you like       return argument3

User · Answer

Yes  you can call constructors from another constructor  For example  public class Animal       private int animalType       public Animal             this 1     here this 1  internally make call to Animal 1              public Animal int animalType            this animalType   animalType           you can also read in details from Constructor Chaining in Java

User · Answer

Yes  it is possible   public class Foo       private int x       public Foo             this 1              public Foo int x            this x   x            To chain to a particular superclass constructor instead of one in the same class  use super instead of this  Note that you can only chain to one constructor  and it has to be the first statement in your constructor body   See also this related question  which is about C  but where the same principles apply

User · Answer

You can a constructor from another constructor of same class by using  this  keyword  Example -  class This1       This1                 this  Hello            System out println  Default constructor                This1 int a                this            System out println  int as arg constructor                 This1 String s                System out println  string as arg constructor                   public static void main String args                  new This1 100             Output - string as arg constructor   Default constructor   int as arg constructor

User · Answer

Yes it is possible to call one constructor from another with use of this    class Example     private int a   1     Example            this 5     here another constructor called based on constructor argument         System out println  number a is   a              Example int b           System out println  number b is   b

User · Answer

Using this args   The preferred pattern is to work from the smallest constructor to the largest   public class Cons        public Cons                A no arguments constructor that sends default values to the largest         this madeUpArg1Value madeUpArg2Value madeUpArg3Value              public Cons int arg1  int arg2              An example of a partial constructor that uses the passed in arguments            and sends a hidden default value to the largest         this arg1 arg2  madeUpArg3Value                 Largest constructor that does the work     public Cons int arg1  int arg2  int arg3            this arg1   arg1          this arg2   arg2          this arg3   arg3            You can also use a more recently advocated approach of valueOf or just  of    public class Cons       public static Cons newCons int arg1                   This function is commonly called valueOf  like Integer valueOf                More recently called  of   like EnumSet of             Cons c   new Cons               c setArg1                return c             To call a super class  use super someValue   The call to super must be the first call in the constructor or you will get a compiler error

User · Answer

When I need to call another constructor from inside the code  not on the first line   I usually use a helper method like this   class MyClass      int field       MyClass           init 0            MyClass int value          if  value lt 0              init 0                  else              init value                   void init int x          field   x           But most often I try to do it the other way around by calling the more complex constructors from the simpler ones on the first line  to the extent possible  For the above example  class MyClass      int field      MyClass int value          if  value lt 0           field   0        else          field   value          MyClass           this 0

User · Answer

There are design patterns that cover the need for complex construction - if it can t be done succinctly  create a factory method or a factory class   With the latest java and the addition of lambdas  it is easy to create a constructor which can accept any initialization code you desire   class LambdaInitedClass       public LamdaInitedClass Consumer lt LambdaInitedClass gt  init           init accept this            Call it with      new LambdaInitedClass l - gt       init l any way you want

User · Answer

The keyword this can be used to call a constructor from a constructor  when writing several constructor for a class  there are times when you d like to call one constructor from another to avoid duplicate code   Bellow is a link that I explain other topic about constructor and getters   and setters   and I used a class with two constructors  I hope the explanations and examples help you   Setter methods or constructors

User · Answer

I know there are so many examples of this question but what I found I am putting here to share my Idea  there are two ways to chain constructor  In Same class you can use this keyword  in Inheritance  you need to use super keyword       import java util        import java lang         class Test                 public static void main String args                          Dog d   new Dog       Both Calling Same Constructor of Parent Class i e  0 args Constructor              Dog cs   new Dog  Bite       Both Calling Same Constructor of Parent Class i e  0 args Constructor                  You need to Explicitly tell the java compiler to use Argument constructor so you need to use  super  key word             System out println  ------------------------------                Cat c   new Cat                Cat caty   new Cat  10                 System out println  ------------------------------                   Self s   new Self                Self ss   new Self  self                         class Animal               String i           public Animal                         i    10               System out println  Animal Constructor     i                     public Animal String h                        i    20               System out println  Animal Constructor Habit     i                        class Dog extends Animal               public Dog                         System out println  Dog Constructor                      public Dog String h                        System out println  Dog Constructor with habit                         class Cat extends Animal               public Cat                         System out println  Cat Constructor                      public Cat String i                        super i      Calling Super Class Paremetrize Constructor              System out println  Cat Constructor with habit                         class Self               public Self                         System out println  Self Constructor                      public Self String h                        this       Explicitly calling 0 args constructor               System out println  Slef Constructor with value

User · Answer

Yes it is possible public class User     private String name    quot  quot    private String surname    quot  quot    private int age   0   public User        this  quot name is undefined quot   quot surname is undefined quot  0          public User String name String surname         this name surname 0              public User String name  String surname  int age          this name   name        this surname   surname        this age   age

User · Answer

As everybody already have said  you use this       which is called an explicit constructor invocation   However  keep in mind that within such an explicit constructor invocation statement you may not refer to   any instance variables or  any instance methods or  any inner classes declared in this class or any superclass  or  this or  super    As stated in JLS    8 8 7 1

User · Answer

Yes it is possible to call one constructor from another  But there is a rule to it  If a call is made from one constructor to another  then      that new constructor call must be the first statement in the current constructor   public class Product        private int productId       private String productName       private double productPrice       private String category       public Product int id  String name            this id name 1 0              public Product int id  String name  double price            this id name price  DEFAULT               public Product int id String name double price  String category           this productId id          this productName name          this productPrice price          this category category            So  something like below will not work   public Product int id  String name  double price        System out println  Calling constructor with price        this id name price  DEFAULT        Also  in the case of inheritance  when sub-class s object is created  the super class constructor is first called   public class SuperClass       public SuperClass            System out println  Inside super class constructor            public class SubClass extends SuperClass       public SubClass               Even if we do not add  Java adds the call to super class s constructor like            super           System out println  Inside sub class constructor              Thus  in this case also another constructor call is first declared before any other statements

User · Answer

Calling constructor from another constructor  class MyConstructorDemo extends ConstructorDemo       MyConstructorDemo                 this  calling another constructor              MyConstructorDemo String arg                System out print  This is passed String by another constructor    arg             Also you can call parent constructor by using super   call

User · Answer

It is called Telescoping Constructor anti-pattern or constructor chaining  Yes  you can definitely do  I see many examples above and I want to add by saying that if you know that you need only two or three constructor  it might be ok  But if you need more  please try to use different design pattern like Builder pattern  As for example     public Omar       public Omar a      public Omar a b      public Omar a b c      public Omar a b c d            You may need more  Builder pattern would be a great solution in this case  Here is an article  it might be helpful https   medium com  modestofiguereo design-patterns-2-the-builder-pattern-and-the-telescoping-constructor-anti-pattern-60a33de7522e

User · Answer

You can call another constructor via the this      keyword  when you need to call a constructor from the same class  or the super      keyword  when you need to call a constructor from a superclass    However  such a call must be the first statement of your constructor  To overcome this limitation  use this answer

User · Answer

As everybody already have said  you use this       which is called an explicit constructor invocation   However  keep in mind that within such an explicit constructor invocation statement you may not refer to   any instance variables or  any instance methods or  any inner classes declared in this class or any superclass  or  this or  super    As stated in JLS    8 8 7 1

User · Answer

Pretty simple   public class SomeClass       private int number      private String someString       public SomeClass            number   0          someString   new String               public SomeClass int number           this      set the class to 0         this setNumber number               public SomeClass int number  String someString           this number     call public SomeClass  int number           this setString someString              public void setNumber int number           this number   number            public void setString String someString           this someString   someString                   add some accessors     now here is some small extra credit   public SomeOtherClass extends SomeClass       public SomeOtherClass int number  String someString            super number  someString     calls public SomeClass int number  String someString                   Some other code      Hope this helps

User · Answer

There are design patterns that cover the need for complex construction - if it can t be done succinctly  create a factory method or a factory class   With the latest java and the addition of lambdas  it is easy to create a constructor which can accept any initialization code you desire   class LambdaInitedClass       public LamdaInitedClass Consumer lt LambdaInitedClass gt  init           init accept this            Call it with      new LambdaInitedClass l - gt       init l any way you want

User · Answer

Within a constructor  you can use the this keyword to invoke another constructor in the same class  Doing so is called an explicit constructor invocation    Here s another Rectangle class  with a different implementation from the one in the Objects section   public class Rectangle       private int x  y      private int width  height       public Rectangle             this 1  1             public Rectangle int width  int height            this  0 0 width  height             public Rectangle int x  int y  int width  int height            this x   x          this y   y          this width   width          this height   height             This class contains a set of constructors  Each constructor initializes some or all of the rectangle s member variables

User · Answer

I will tell you an easy way  There are two types of constructors    Default constructor Parameterized constructor   I will explain in one Example  class ConstructorDemo          ConstructorDemo    Default Constructor                  System out println  D constructor                    ConstructorDemo int k   Parameterized constructor                  this     ------------- 1           System out println  P Constructor    k                         public static void main String   args                      this    error because  must be first statement in constructor          new ConstructorDemo     ------- 2           ConstructorDemo g new ConstructorDemo 3  --- 3                                        In the above example I showed 3 types of calling   this   call to this must be first statement in constructor This is Name less Object  this automatically calls the default constructor  3 This calls the Parameterized constructor    Note  this must be the first statement in the constructor

User · Answer

Using this args   The preferred pattern is to work from the smallest constructor to the largest   public class Cons        public Cons                A no arguments constructor that sends default values to the largest         this madeUpArg1Value madeUpArg2Value madeUpArg3Value              public Cons int arg1  int arg2              An example of a partial constructor that uses the passed in arguments            and sends a hidden default value to the largest         this arg1 arg2  madeUpArg3Value                 Largest constructor that does the work     public Cons int arg1  int arg2  int arg3            this arg1   arg1          this arg2   arg2          this arg3   arg3            You can also use a more recently advocated approach of valueOf or just  of    public class Cons       public static Cons newCons int arg1                   This function is commonly called valueOf  like Integer valueOf                More recently called  of   like EnumSet of             Cons c   new Cons               c setArg1                return c             To call a super class  use super someValue   The call to super must be the first call in the constructor or you will get a compiler error

User · Answer

I prefer this way         class User           private long id          private String username          private int imageRes       public User             init defaultID defaultUsername defaultRes             public User String username            init defaultID username  defaultRes                public User String username  int imageRes            init defaultID username  imageRes              public User long id  String username  int imageRes            init id username  imageRes               private void init long id  String username  int imageRes            this id id          this username   username          this imageRes   imageRes

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Yes  it is possible   public class Foo       private int x       public Foo             this 1              public Foo int x            this x   x            To chain to a particular superclass constructor instead of one in the same class  use super instead of this  Note that you can only chain to one constructor  and it has to be the first statement in your constructor body   See also this related question  which is about C  but where the same principles apply

User · Answer

You can a constructor from another constructor of same class by using  this  keyword  Example -  class This1       This1                 this  Hello            System out println  Default constructor                This1 int a                this            System out println  int as arg constructor                 This1 String s                System out println  string as arg constructor                   public static void main String args                  new This1 100             Output - string as arg constructor   Default constructor   int as arg constructor

User · Answer

Yes  it is possible   public class Foo       private int x       public Foo             this 1              public Foo int x            this x   x            To chain to a particular superclass constructor instead of one in the same class  use super instead of this  Note that you can only chain to one constructor  and it has to be the first statement in your constructor body   See also this related question  which is about C  but where the same principles apply

User · Answer

Using this args   The preferred pattern is to work from the smallest constructor to the largest   public class Cons        public Cons                A no arguments constructor that sends default values to the largest         this madeUpArg1Value madeUpArg2Value madeUpArg3Value              public Cons int arg1  int arg2              An example of a partial constructor that uses the passed in arguments            and sends a hidden default value to the largest         this arg1 arg2  madeUpArg3Value                 Largest constructor that does the work     public Cons int arg1  int arg2  int arg3            this arg1   arg1          this arg2   arg2          this arg3   arg3            You can also use a more recently advocated approach of valueOf or just  of    public class Cons       public static Cons newCons int arg1                   This function is commonly called valueOf  like Integer valueOf                More recently called  of   like EnumSet of             Cons c   new Cons               c setArg1                return c             To call a super class  use super someValue   The call to super must be the first call in the constructor or you will get a compiler error

User · Answer

Note  I just want to add one aspect  which I did not see in the other answers  how to overcome limitations of the requirement that this   has to be on the first line     In Java another constructor of the same class can be called from a constructor via this    Note however that this has to be on the first line   public class MyClass      public MyClass double argument1  double argument2        this argument1  argument2  0 0          public MyClass double argument1  double argument2  double argument3        this argument1   argument1      this argument2   argument2      this argument3   argument3          That this has to appear on the first line looks like a big limitation  but you can construct the arguments of other constructors via static methods  For example   public class MyClass      public MyClass double argument1  double argument2        this argument1  argument2  getDefaultArg3 argument1  argument2           public MyClass double argument1  double argument2  double argument3        this argument1   argument1      this argument2   argument2      this argument3   argument3         private static double getDefaultArg3 double argument1  double argument2        double argument3   0          Calculate argument3 here if you like       return argument3

User · Answer

Yes  it is possible   public class Foo       private int x       public Foo             this 1              public Foo int x            this x   x            To chain to a particular superclass constructor instead of one in the same class  use super instead of this  Note that you can only chain to one constructor  and it has to be the first statement in your constructor body   See also this related question  which is about C  but where the same principles apply

[java] How do I call one constructor from another in Java?

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