[java] Can a constructor in Java be private?

Can a constructor be private? How is a private constructor useful?

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:

1. 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).

2. 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();
   

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.

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 :

1. Singleton Design Pattern
2. To limit the number of instance creation
3. To give meaningful name for object creation using static factory method
4. Static Utility Class or Constant Class
5. To prevent Subclassing
6. Builder Design Pattern and thus for creating immutable classes

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.

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.

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<? extends ErrorTypeException> 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

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.

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

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.

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.

Private Constructors can be defnied in the Java for the following reasons

1. To have control on the instantiation of the Java objects, it wont allow you to create an instance of an object.

2. It wont allow the class to be Subclassed

3. 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.

4. 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.

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

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

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() {
}}

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