What’s the point of having a private constructor?


This may be news to you: in Java, it is possible to have a private constructor. Why one would want to use one is explored below.

The private modifier when applied to a constructor works in much the same way as when applied to a normal method or even an instance variable. Defining a constructor with the private modifier says that only the native class (as in the class in which the private constructor is defined) is allowed to create an instance of the class, and no other caller is permitted to do so. There are two possible reasons why one would want to use a private constructor – the first is that you don’t want any objects of your class to be created at all, and the second is that you only want objects to be created internally – as in only created in your class.

Private constructors can be used in the singleton design pattern

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.

You can read more about the Singleton design pattern here: Design pattern interview question 1.

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Private constructors can prevent creation of objects

The other possible reason for using a private constructor is to prevent object construction entirely. When would it make sense to do something like that? Of course, when creating an object doesn’t make sense – and this occurs when the class only contains static members. And when a class contains only static members, those members can be accessed using only the class name – no instance of the class needs to be created.

Java always provides a default, no-argument, public constructor if no programmer-defined constructor exists. Creating a private no-argument constructor essentially prevents the usage of that default constructor, thereby preventing a caller from creating an instance of the class. Note that the private constructor may even be empty.

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