Can a constructor in Java be private

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Can a constructor be private  How is a private constructor useful

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Private constructors prevent a class from being explicitly instantiated by callers see further information on PrivateConstructor

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Yes it can   A private constructor would exist to prevent the class from being instantiated  or because construction happens only internally  e g  a Factory pattern   See here for more information

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Can a constructor be private  How is a private constructor useful    Yes it can  I consider this another example of it being useful         ErrorType java public enum ErrorType       X      Y      Z          ErrorTypeException java import java util    import java lang    import java io       Translates ErrorTypes only abstract public class ErrorTypeException extends Exception       private ErrorTypeException            I don t want to expose thse     static private class Xx extends ErrorTypeException        static private class Yx extends ErrorTypeException        static private class Zx extends ErrorTypeException            Want translation without exposing underlying type     public static Exception from ErrorType errorType            switch  errorType                case X                  return new Xx                    case Y                  return new Yx                default                  return new Zx                            Want to get hold of class without exposing underlying type     public static Class lt   extends ErrorTypeException gt  toExceptionClass ErrorType errorType            switch  errorType                case X                  return Xx class                  case Y                  return Yx class              default                  return Zx class                      In the case here above  it prevents the abstract class from being instantiated by any derived class other than it s static inner classes  Abstract classes cannot be final  but in this case the private constructor makes it effectively final to all classes that aren t inner classes

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Yes  class can have a private constructor  It is needed as to disallow to access the constructor from other classes and remain it accessible within defined class   Why would you want objects of your class to only be created internally  This could be done for any reason  but one possible reason is that you want to implement a singleton  A singleton is a design pattern that allows only one instance of your class to be created  and this can be accomplished by using a private constructor

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Basic idea behind having a private constructor is to restrict the instantiation of a class from outside by JVM  but if a class having a argument constructor  then it infers that one is intentionally instantiating

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Private Constructors can be defnied in the Java for the following reasons   To have control on the instantiation of the Java objects  it wont allow you to create an instance of an object  It wont allow the class to be Subclassed This has a special advantage when implementing the singleton Pattern  Private contstructors are used for it and have a control on the creating the instance for the whole application  when you want to have a class with all constants defined and Does not require its instance any more  then we declare that class as a private constructor

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I expected that someone would ve mentioned this  the 2nd point   but   there are three uses of private constructors    to prevent instantiation outside of the object  in the following cases    singleton factory method static-methods-only  utility  class  constants-only class    to prevent sublcassing  extending   If you make only a private constructor  no class can extend your class  because it can t call the super   constructor  This is some kind of a synonym for final overloaded constructors - as a result of overloading methods and constructors  some may be private and some public  Especially in case when there is a non-public class that you use in your constructors  you may create a public constructor that creates an instance of that class and then passes it to a private constructor

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Yes  a constructor can be private  There are different uses of this  One such use is for the singleton design anti-pattern  which I would advise against you using  Another  more legitimate use  is in delegating constructors  you can have one constructor that takes lots of different options that is really an implementation detail  so you make it private  but then your remaining constructors delegate to it   As an example of delegating constructors  the following class allows you to save a value and a type  but it only lets you do it for a subset of types  so making the general constructor private is needed to ensure that only the permitted types are used  The common private constructor helps code reuse   public class MyClass        private final String value       private final String type        public MyClass int x            this Integer toString x    int                 public MyClass boolean x            this Boolean toString x    boolean                 public String toString             return value               public String getType             return type               private MyClass String value  String type            this value   value           this type   type             Edit Looking at this answer from several years later  I would like to note that this answer is both incomplete and also a little bit extreme  Singletons are indeed an anti-pattern and should generally be avoided where possible  however  there are many uses of private constructors besides singletons  and my answer names only one   To give a couple more cases where private constructors are used    To create an uninstantiable class that is just a collection of related static functions  this is basically a singleton  but if it is stateless and the static functions operate strictly on the parameters rather than on class state  this is not as unreasonable an approach as my earlier self would seem to suggest  though using an interface that is dependency injected often makes it easier to maintain the API when the implementation requires larger numbers of dependencies or other forms of context   When there are multiple different ways to create the object  a private constructor may make it easier to understand the different ways of constructing it  e g   which is more readable to you new ArrayList 5  or ArrayList createWithCapacity 5   ArrayList createWithContents 5   ArrayList createWithInitialSize 5    In other words  a private constructor allows you to provide factory function s whose names are more understandable  and then making the constructor private ensures that people use only the more self-evident names  This is also commonly used with the builder pattern  For example   MyClass myVar   MyClass      newBuilder        setOption1 option1       setOption2 option2       build

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Well  if all of your methods in a class are static  then a private constructor is a good idea

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Yes  Class can have private constructor  Even abstract class can have private constructor   By making constructor private  we prevent the class from being instantiated as well as subclassing of that class   Here are some of the uses of private constructor      Singleton Design Pattern  To limit the number of instance creation  To give meaningful name for object creation using static factory method  Static Utility Class  or Constant Class To prevent Subclassing Builder Design Pattern and thus for creating immutable classes

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Yes and it is used to prevent instantiation and subsequently overriding  This is most often used in singleton classes

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According to me we can declare constructor as a private and also we can get the instance of that class in the subclass by using static method in class in which we declare constructor and then return class object  we class this method from to the sub class by using classname method name bcz it is static method and the we will get the instance of class in which we declare const

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Yes   This is so that you can control how the class is instantiated  If you make the constructor private  and then create a visible constructor method that returns instances of the class  you can do things like limit the number of creations  typically  guarantee there is exactly one instance  or recycle instances or other construction-related tasks   Doing new x   never returns null  but using the factory pattern  you can return null  or even return different subtypes   You might use it also for a class which has no instance members or properties  just static ones - as in a utility function class

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yes a constructor can be private  A private Constructor prevents any other class from instantiating example of private constructor   public class CustomHttpClient   private static HttpClient customHttpClient       A private Constructor prevents any other class from instantiating     private CustomHttpClient

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Yes   A private constructor is used to prevent instance initializing  such as the Math final class you use in java  Singleton also use private constructor

[java] Can a constructor in Java be private?

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