Can a constructor in Java be private?

Question

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();
   

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.

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.

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.

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

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