Can an abstract class have a constructor

Question

Can an abstract class have a constructor   If so  how can it be used and for what purposes

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An abstract class can have a constructor BUT you can not create an object of abstract class so how do you use that constructor?

Thing is when you inherit that abstract class in your subclass you can pass values to its(abstract's) constructor through super(value) method in your subclass and no you don't inherit a constructor.

so using super you can pass values in a constructor of the abstract class and as far as I remember it has to be the first statement in your method or constructor.

User · Answer

Of Course  abstract class can have a constructor Generally class constructor is used to initialise fields So  an abstract class constructor is used to initialise fields of the abstract class  You would provide a constructor for an abstract class if you want to initialise certain fields of the abstract class before the instantiation of a child-class takes place  An abstract class constructor can also be used to execute code that is relevant for every child class  This prevents code duplication   We cannot create an instance of an abstract class But we can create instances of classes those are derived from the abstract class  So  when an instance of derived class is created  the parent abstract class constructor is automatically called   Reference  This Article

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Yes it can  abstract classes constructors are generally used for super calls for initialization events common to all the subclasses

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You would define a constructor in an abstract class if you are in one of these situations    you want to perform some initialization  to fields of the abstract class  before the instantiation of a subclass actually takes place you have defined final fields in the abstract class but you did not initialize them in the declaration itself  in this case  you MUST have a constructor to initialize these fields   Note that    you may define more than one constructor  with different arguments  you can  should   define all your constructors protected  making them public is pointless anyway  your subclass constructor s  can call one constructor of the abstract class  it may even have to call it  if there is no no-arg constructor in the abstract class    In any case  don t forget that if you don t define a constructor  then the compiler will automatically generate one for you  this one is public  has no argument  and does nothing

User · Answer

Of Course  abstract class can have a constructor Generally class constructor is used to initialise fields So  an abstract class constructor is used to initialise fields of the abstract class  You would provide a constructor for an abstract class if you want to initialise certain fields of the abstract class before the instantiation of a child-class takes place  An abstract class constructor can also be used to execute code that is relevant for every child class  This prevents code duplication   We cannot create an instance of an abstract class But we can create instances of classes those are derived from the abstract class  So  when an instance of derived class is created  the parent abstract class constructor is automatically called   Reference  This Article

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Not only can it  it always does  If you do not specify one then it has a default no arg constructor  just like any other class  In fact  ALL classes  including nested and anonymous classes  will get a default constructor if one is not specified  in the case of anonymous classes it is impossible to specify one  so you will always get the default constructor    A good example of an abstract class having a constructor is the Calendar class  You get a Calendar object by calling Calendar getInstance    but it also has constructors which are protected  The reason its constructors are protected is so that only its subclasses can call them  or classes in the same package  but since it s abstract  that doesn t apply   GregorianCalendar is an example of a class that extends Calendar

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Yes  Abstract Classes can have constructors     Here is an example using constructor in abstract class   abstract class Figure         double dim1              double dim2        Figure double a  double b                     dim1   a                   dim2   b                         area is now an abstract method      abstract double area          class Rectangle extends Figure        Rectangle double a  double b             super a  b                  override area for rectangle      double area              System out println  Inside Area for Rectangle              return dim1   dim2             class Triangle extends Figure        Triangle double a  double b             super a  b                  override area for right triangle      double area              System out println  Inside Area for Triangle              return dim1   dim2   2             class AbstractAreas        public static void main String args                  Figure f   new Figure 10  10      illegal now          Rectangle r   new Rectangle 9  5            Triangle t   new Triangle 10  8            Figure figref     this is OK  no object is created          figref   r           System out println  Area is     figref area              figref   t           System out println  Area is     figref area                 So I think you got the answer

User · Answer

You would define a constructor in an abstract class if you are in one of these situations    you want to perform some initialization  to fields of the abstract class  before the instantiation of a subclass actually takes place you have defined final fields in the abstract class but you did not initialize them in the declaration itself  in this case  you MUST have a constructor to initialize these fields   Note that    you may define more than one constructor  with different arguments  you can  should   define all your constructors protected  making them public is pointless anyway  your subclass constructor s  can call one constructor of the abstract class  it may even have to call it  if there is no no-arg constructor in the abstract class    In any case  don t forget that if you don t define a constructor  then the compiler will automatically generate one for you  this one is public  has no argument  and does nothing

User · Answer

Yes it can have a constructor and it is defined and behaves just like any other class s constructor   Except that abstract classes can t be directly instantiated  only extended  so the use is therefore always from a subclass s constructor

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Consider this   abstract class Product        int value      public Product  int val             value  val            abstract public int multiply       class TimesTwo extends Product       public int mutiply            return value   2            The superclass is abstract and has a constructor

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As described by javafuns here  this is an example   public abstract class TestEngine      private String engineId     private String engineName      public TestEngine String engineId   String engineName            this engineId   engineId       this engineName   engineName            public gettors and settors    public abstract void scheduleTest        public class JavaTestEngine extends TestEngine       private String typeName      public JavaTestEngine String engineId   String engineName   String typeName             super engineId   engineName         this typeName   typeName           public void scheduleTest               do Stuff

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Since an abstract class can have variables of all access modifiers  they have to be initialized to default values  so constructor is necessary  As you instantiate the child class  a constructor of an abstract class is invoked and variables are initialized   On the contrary  an interface does contain only constant variables means they are already initialized  So interface doesn t need a constructor

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The purpose of the constructor in a class is used to initialize fields but not to  build objects   When you try to create a new instance of an abstract SuperClass  the compiler will give you an error  However  we can inherit an abstract class Employee and make use of its constructor by setting its variables See example below  public abstract class Employee     private String EmpName    abstract double calcSalary       Employee String name        this EmpName   name    constructor of abstract class super class        class Manager extends Employee   Manager String name        super name     setting the name in the constructor of sub class    double calcSalary         return 0

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Although there are many good answers  I would like to give my 2 cents   Constructor DOES NOT BUILD THE OBJECT  It is used to initialize an object   Yes  an Abstract class always has a constructor  If you do not define your own constructor  the compiler will give a default constructor to the Abstract class  Above holds true for all classes - nested  abstract  anonymous  etc    An abstract class  unlike interface  can have non-final non-static fields which need initialization  You can write your own constructor in the abstract class to do that  But  in that case  there won t be any default constructor   public abstract class Abs      int i      int j      public Abs int i int j           this i   i          this j   j          System out println i     j             Be careful while extending above abstract class  you have to explicitly call super from each constructor  The first line of any constructor calls to super    if you do not explicitly call super    Java will do that for you  Below code will not compile   public class Imp extends Abs   public Imp int i  int j int k  int l       System out println  2 arg          You have to use it like below example   public class Imp extends Abs   public Imp int i  int j int k  int l       super i j       System out println  2 arg

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Not only can it  it always does  If you do not specify one then it has a default no arg constructor  just like any other class  In fact  ALL classes  including nested and anonymous classes  will get a default constructor if one is not specified  in the case of anonymous classes it is impossible to specify one  so you will always get the default constructor    A good example of an abstract class having a constructor is the Calendar class  You get a Calendar object by calling Calendar getInstance    but it also has constructors which are protected  The reason its constructors are protected is so that only its subclasses can call them  or classes in the same package  but since it s abstract  that doesn t apply   GregorianCalendar is an example of a class that extends Calendar

User · Answer

Yes it can have a constructor and it is defined and behaves just like any other class s constructor   Except that abstract classes can t be directly instantiated  only extended  so the use is therefore always from a subclass s constructor

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Yes  Abstract classes can have constructors   Yes  when we define a class to be an Abstract Class it cannot be instantiated but that does not mean an Abstract class cannot have a constructor  Each abstract class must have a concrete subclass which will implement the abstract methods of that abstract class   When we create an object of any subclass all the constructors in the corresponding inheritance tree are invoked in the top to bottom approach  The same case applies to abstract classes  Though we cannot create an object of an abstract class  when we create an object of a class which is concrete and subclass of the abstract class  the constructor of the abstract class is automatically invoked  Hence we can have a constructor in abstract classes   Note  A non-abstract class cannot have abstract methods but an abstract class can have a non-abstract method  Reason is similar to that of constructors  difference being instead of getting invoked automatically we can call super    Also  there is nothing like an abstract constructor as it makes no sense at all

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Yes  an abstract class can have a constructor  Consider this   abstract class Product        int multiplyBy      public Product  int multiplyBy             this multiplyBy   multiplyBy             public int mutiply int val           return multiplyBy   val           class TimesTwo extends Product       public TimesTwo             super 2            class TimesWhat extends Product       public TimesWhat int what            super what             The superclass Product is abstract and has a constructor  The concrete class TimesTwo has a constructor that just hardcodes the value 2  The concrete class TimesWhat has a constructor that allows the caller to specify the value   Abstract constructors will frequently be used to enforce class constraints or invariants such as the minimum fields required to setup the class      NOTE  As there is no default  or no-arg  constructor in the parent   abstract class  the constructor used in subclass must explicitly call   the parent constructor

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Abstract class can have a constructor though it cannot be instantiated  But the constructor defined in an abstract class can be used for instantiation of concrete class of this abstract class  Check JLS      It is a compile-time error if an attempt is made to create an instance of an abstract class using a class instance creation   expression       A subclass of an abstract class that is not itself abstract may be   instantiated  resulting in the execution of a constructor for the   abstract class and  therefore  the execution of the field initializers   for instance variables of that class

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Yes surely you can add one  as already mentioned for initialization of Abstract class variables  BUT if you dont explicitly declare one  it anyways has an implicit constructor for  Constructor Chaining  to work

User · Answer

In order to achieve constructor chaining  the abstract class will have a constructor  The compiler keeps Super   statement inside the subclass constructor  which will call the superclass constructor  If there were no constructors for abstract classes then java rules are violated and we can t achieve constructor chaining

User · Answer

Yes it can  abstract classes constructors are generally used for super calls for initialization events common to all the subclasses

User · Answer

You would define a constructor in an abstract class if you are in one of these situations    you want to perform some initialization  to fields of the abstract class  before the instantiation of a subclass actually takes place you have defined final fields in the abstract class but you did not initialize them in the declaration itself  in this case  you MUST have a constructor to initialize these fields   Note that    you may define more than one constructor  with different arguments  you can  should   define all your constructors protected  making them public is pointless anyway  your subclass constructor s  can call one constructor of the abstract class  it may even have to call it  if there is no no-arg constructor in the abstract class    In any case  don t forget that if you don t define a constructor  then the compiler will automatically generate one for you  this one is public  has no argument  and does nothing

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package Test1   public class AbstractClassConstructor    public AbstractClassConstructor             public static void main String args             Demo obj   new Test  Test of code has started           obj test1               abstract class Demo      protected final String demoValue       public Demo String testName           this demoValue   testName             public abstract boolean test1       class Test extends Demo       public Test String name           super name               Override     public boolean test1            System out println  this demoValue     Demo test started           return true

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Yes  It is like any other class  It can have a constructor and it is called after creating object for the base class

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In a concrete class  declaration of a constructor for a concrete type Fnord effectively exposes two things    A means by which code can request the creation of an instance of Fnord A means by which an instance of a type derived from Fnord which is under construction can request that all base-class features be initialized    While there should perhaps be a means by which these two abilities could be controlled separately  for every concrete type one definition will enable both   Although the first ability is not meaningful for an abstract class  the second ability is just as meaningful for an abstract class as it would be for any other  and thus its declaration is just as necessary and useful

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The purpose of the constructor in a class is used to initialize fields but not to  build objects   When you try to create a new instance of an abstract SuperClass  the compiler will give you an error  However  we can inherit an abstract class Employee and make use of its constructor by setting its variables See example below  public abstract class Employee     private String EmpName    abstract double calcSalary       Employee String name        this EmpName   name    constructor of abstract class super class        class Manager extends Employee   Manager String name        super name     setting the name in the constructor of sub class    double calcSalary         return 0

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Yes  It is like any other class  It can have a constructor and it is called after creating object for the base class

User · Answer

Since an abstract class can have variables of all access modifiers  they have to be initialized to default values  so constructor is necessary  As you instantiate the child class  a constructor of an abstract class is invoked and variables are initialized   On the contrary  an interface does contain only constant variables means they are already initialized  So interface doesn t need a constructor

User · Answer

Yes  an abstract class can have a constructor  Consider this   abstract class Product        int multiplyBy      public Product  int multiplyBy             this multiplyBy   multiplyBy             public int mutiply int val           return multiplyBy   val           class TimesTwo extends Product       public TimesTwo             super 2            class TimesWhat extends Product       public TimesWhat int what            super what             The superclass Product is abstract and has a constructor  The concrete class TimesTwo has a constructor that just hardcodes the value 2  The concrete class TimesWhat has a constructor that allows the caller to specify the value   Abstract constructors will frequently be used to enforce class constraints or invariants such as the minimum fields required to setup the class      NOTE  As there is no default  or no-arg  constructor in the parent   abstract class  the constructor used in subclass must explicitly call   the parent constructor

User · Answer

In a concrete class  declaration of a constructor for a concrete type Fnord effectively exposes two things    A means by which code can request the creation of an instance of Fnord A means by which an instance of a type derived from Fnord which is under construction can request that all base-class features be initialized    While there should perhaps be a means by which these two abilities could be controlled separately  for every concrete type one definition will enable both   Although the first ability is not meaningful for an abstract class  the second ability is just as meaningful for an abstract class as it would be for any other  and thus its declaration is just as necessary and useful

User · Answer

Abstract class can have a constructor though it cannot be instantiated  But the constructor defined in an abstract class can be used for instantiation of concrete class of this abstract class  Check JLS      It is a compile-time error if an attempt is made to create an instance of an abstract class using a class instance creation   expression       A subclass of an abstract class that is not itself abstract may be   instantiated  resulting in the execution of a constructor for the   abstract class and  therefore  the execution of the field initializers   for instance variables of that class

User · Answer

You would define a constructor in an abstract class if you are in one of these situations    you want to perform some initialization  to fields of the abstract class  before the instantiation of a subclass actually takes place you have defined final fields in the abstract class but you did not initialize them in the declaration itself  in this case  you MUST have a constructor to initialize these fields   Note that    you may define more than one constructor  with different arguments  you can  should   define all your constructors protected  making them public is pointless anyway  your subclass constructor s  can call one constructor of the abstract class  it may even have to call it  if there is no no-arg constructor in the abstract class    In any case  don t forget that if you don t define a constructor  then the compiler will automatically generate one for you  this one is public  has no argument  and does nothing

User · Answer

Consider this   abstract class Product        int value      public Product  int val             value  val            abstract public int multiply       class TimesTwo extends Product       public int mutiply            return value   2            The superclass is abstract and has a constructor

User · Answer

package Test1   public class AbstractClassConstructor    public AbstractClassConstructor             public static void main String args             Demo obj   new Test  Test of code has started           obj test1               abstract class Demo      protected final String demoValue       public Demo String testName           this demoValue   testName             public abstract boolean test1       class Test extends Demo       public Test String name           super name               Override     public boolean test1            System out println  this demoValue     Demo test started           return true

User · Answer

Yes it can have a constructor and it is defined and behaves just like any other class s constructor   Except that abstract classes can t be directly instantiated  only extended  so the use is therefore always from a subclass s constructor

User · Answer

Yes  an abstract class can have a constructor  Consider this   abstract class Product        int multiplyBy      public Product  int multiplyBy             this multiplyBy   multiplyBy             public int mutiply int val           return multiplyBy   val           class TimesTwo extends Product       public TimesTwo             super 2            class TimesWhat extends Product       public TimesWhat int what            super what             The superclass Product is abstract and has a constructor  The concrete class TimesTwo has a constructor that just hardcodes the value 2  The concrete class TimesWhat has a constructor that allows the caller to specify the value   Abstract constructors will frequently be used to enforce class constraints or invariants such as the minimum fields required to setup the class      NOTE  As there is no default  or no-arg  constructor in the parent   abstract class  the constructor used in subclass must explicitly call   the parent constructor

User · Answer

Yes  an Abstract Class can have a Constructor  You Can Overload as many Constructor as you want in an Abstract Class  These Contractors Can be used to Initialized the initial state of the Objects Extending the Abstract Class  As we know we can t make an object of an Abstract Class because Objects are Created by the  new  keywords and not by the constructors   they are there for only initializing the state of the subclass Objects

User · Answer

An abstract class can have a constructor BUT you can not create an object of abstract class so how do you use that constructor?

Thing is when you inherit that abstract class in your subclass you can pass values to its(abstract's) constructor through super(value) method in your subclass and no you don't inherit a constructor.

so using super you can pass values in a constructor of the abstract class and as far as I remember it has to be the first statement in your method or constructor.

User · Answer

Yes surely you can add one  as already mentioned for initialization of Abstract class variables  BUT if you dont explicitly declare one  it anyways has an implicit constructor for  Constructor Chaining  to work

User · Answer

Consider this   abstract class Product        int value      public Product  int val             value  val            abstract public int multiply       class TimesTwo extends Product       public int mutiply            return value   2            The superclass is abstract and has a constructor

User · Answer

yes it is  And a constructor of abstract class is called when an instance of a inherited class is created  For example  the following is a valid Java program      An abstract class with constructor abstract class Base   Base     System out println  Base Constructor Called      abstract void fun          class Derived extends Base   Derived     System out println  Derived Constructor Called      void fun     System out println  Derived fun   called             class Main   public static void main String args          Derived d   new Derived               This is the output of the above code   Base Constructor Called Derived Constructor Called  references  enter link description here

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Yes  an Abstract Class can have a Constructor  You Can Overload as many Constructor as you want in an Abstract Class  These Contractors Can be used to Initialized the initial state of the Objects Extending the Abstract Class  As we know we can t make an object of an Abstract Class because Objects are Created by the  new  keywords and not by the constructors   they are there for only initializing the state of the subclass Objects

User · Answer

Consider this   abstract class Product        int value      public Product  int val             value  val            abstract public int multiply       class TimesTwo extends Product       public int mutiply            return value   2            The superclass is abstract and has a constructor

User · Answer

Yes  Abstract Classes can have constructors     Here is an example using constructor in abstract class   abstract class Figure         double dim1              double dim2        Figure double a  double b                     dim1   a                   dim2   b                         area is now an abstract method      abstract double area          class Rectangle extends Figure        Rectangle double a  double b             super a  b                  override area for rectangle      double area              System out println  Inside Area for Rectangle              return dim1   dim2             class Triangle extends Figure        Triangle double a  double b             super a  b                  override area for right triangle      double area              System out println  Inside Area for Triangle              return dim1   dim2   2             class AbstractAreas        public static void main String args                  Figure f   new Figure 10  10      illegal now          Rectangle r   new Rectangle 9  5            Triangle t   new Triangle 10  8            Figure figref     this is OK  no object is created          figref   r           System out println  Area is     figref area              figref   t           System out println  Area is     figref area                 So I think you got the answer

User · Answer

yes it is  And a constructor of abstract class is called when an instance of a inherited class is created  For example  the following is a valid Java program      An abstract class with constructor abstract class Base   Base     System out println  Base Constructor Called      abstract void fun          class Derived extends Base   Derived     System out println  Derived Constructor Called      void fun     System out println  Derived fun   called             class Main   public static void main String args          Derived d   new Derived               This is the output of the above code   Base Constructor Called Derived Constructor Called  references  enter link description here

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In order to achieve constructor chaining  the abstract class will have a constructor  The compiler keeps Super   statement inside the subclass constructor  which will call the superclass constructor  If there were no constructors for abstract classes then java rules are violated and we can t achieve constructor chaining

User · Answer

Although there are many good answers  I would like to give my 2 cents   Constructor DOES NOT BUILD THE OBJECT  It is used to initialize an object   Yes  an Abstract class always has a constructor  If you do not define your own constructor  the compiler will give a default constructor to the Abstract class  Above holds true for all classes - nested  abstract  anonymous  etc    An abstract class  unlike interface  can have non-final non-static fields which need initialization  You can write your own constructor in the abstract class to do that  But  in that case  there won t be any default constructor   public abstract class Abs      int i      int j      public Abs int i int j           this i   i          this j   j          System out println i     j             Be careful while extending above abstract class  you have to explicitly call super from each constructor  The first line of any constructor calls to super    if you do not explicitly call super    Java will do that for you  Below code will not compile   public class Imp extends Abs   public Imp int i  int j int k  int l       System out println  2 arg          You have to use it like below example   public class Imp extends Abs   public Imp int i  int j int k  int l       super i j       System out println  2 arg

User · Answer

Yes it can have a constructor and it is defined and behaves just like any other class s constructor   Except that abstract classes can t be directly instantiated  only extended  so the use is therefore always from a subclass s constructor

User · Answer

As described by javafuns here  this is an example   public abstract class TestEngine      private String engineId     private String engineName      public TestEngine String engineId   String engineName            this engineId   engineId       this engineName   engineName            public gettors and settors    public abstract void scheduleTest        public class JavaTestEngine extends TestEngine       private String typeName      public JavaTestEngine String engineId   String engineName   String typeName             super engineId   engineName         this typeName   typeName           public void scheduleTest               do Stuff

User · Answer

Yes  Abstract classes can have constructors   Yes  when we define a class to be an Abstract Class it cannot be instantiated but that does not mean an Abstract class cannot have a constructor  Each abstract class must have a concrete subclass which will implement the abstract methods of that abstract class   When we create an object of any subclass all the constructors in the corresponding inheritance tree are invoked in the top to bottom approach  The same case applies to abstract classes  Though we cannot create an object of an abstract class  when we create an object of a class which is concrete and subclass of the abstract class  the constructor of the abstract class is automatically invoked  Hence we can have a constructor in abstract classes   Note  A non-abstract class cannot have abstract methods but an abstract class can have a non-abstract method  Reason is similar to that of constructors  difference being instead of getting invoked automatically we can call super    Also  there is nothing like an abstract constructor as it makes no sense at all

User · Answer

Yes  an abstract class can have a constructor  Consider this   abstract class Product        int multiplyBy      public Product  int multiplyBy             this multiplyBy   multiplyBy             public int mutiply int val           return multiplyBy   val           class TimesTwo extends Product       public TimesTwo             super 2            class TimesWhat extends Product       public TimesWhat int what            super what             The superclass Product is abstract and has a constructor  The concrete class TimesTwo has a constructor that just hardcodes the value 2  The concrete class TimesWhat has a constructor that allows the caller to specify the value   Abstract constructors will frequently be used to enforce class constraints or invariants such as the minimum fields required to setup the class      NOTE  As there is no default  or no-arg  constructor in the parent   abstract class  the constructor used in subclass must explicitly call   the parent constructor

[java] Can an abstract class have a constructor?

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