Final answer:
A private constructor prevents other classes from creating instances of a class, a principle used in the Singleton design pattern to ensure there is only one instance of a class. Inside the class, however, nested or inner classes can create an instance because they have access to its private members.
Step-by-step explanation:
The constructor that prevents other classes from creating instances of a class is a private constructor. The use of a private constructor is a common design pattern in object-oriented programming known as the Singleton pattern. This pattern ensures that a class has only one instance and provides a global point of access to it.
When a constructor is declared as private, it cannot be accessed outside of the class, thereby preventing the creation of instances from outside classes. However, nested or inner classes within the enclosing class can instantiate the class because they have access to all its members, including private ones.
An example of a class with a private constructor might look like this:
\\public class Singleton {\\ private static Singleton instance = new Singleton();\\ private Singleton() {\\ // private constructor\\ }\\ public static Singleton getInstance() {\\ return instance;\\ }\\}\\
In this example, the Singleton class has a single instance which is created within the static initializer. This instance is returned by the getInstance method. Since the constructor is private, no other instances can be created outside this class, yet the nested class can still use getInstance to access the single instance.
It's important to note that this design pattern can be further enhanced to handle issues such as lazy instantiation, thread safety, and serialization.