Understanding Classes with No Public Constructor in Java

Every class in Java requires a constructor to be defined, as it is the initializer that sets up the object when it is created. However, some classes are designed to be non-instantiable or abstract, leading to situations where a public constructor is not available. This article explores the scenarios and implications of defining, or attempting to define, a class with no public constructor.

What Is a Non-Instantiable Class?

A non-instantiable class is a class that cannot be instantiated directly from outside the class itself. This is commonly achieved by defining a private constructor, which prevents any other code from creating an instance of the class. Such classes are often utility classes or helper classes that contain only static methods. By encapsulating them within a non-instantiable class, you ensure that the methods and the data they operate on remain protected and accessible only through the static methods.

Example: Utility Class with Private Constructor

// Utility Class with Private Constructor
public class UtilityClass {
    // Private constructor prevents instantiation
    private UtilityClass() {
        // Prevent instantiation
    }
    // Static utility methods
    public static void utilityMethod() {
        // Method implementation
    }
}

What Is an Abstract Class?

On the other hand, an abstract class in Java is a class that cannot be instantiated and is usually used for defining some common properties and methods for derived classes (subclasses). An abstract class can have one or more abstract methods (methods with no implementation). Additionally, an abstract class can have constructors (including private, protected, public, etc.), but none of the constructors can be called to create an object of the abstract class directly. Instead, it serves as a blueprint for inheritance.

Example: Abstract Class with Public Constructor

// Abstract Class with Public Constructor
public abstract class AbstractClass {
    // Public constructor
    public AbstractClass() {
        // Constructor implementation
    }
    // Abstract method that must be implemented by sub-classes
    public abstract void abstractMethod();
}

Composing with Public Static Methods

When a class is designed without a public constructor, one common approach is to provide public static methods to access the private constructor. These methods allow users to create an instance of the class or access specific methods within the class in a controlled manner.

Example: Accessing a Private Constructor Class

// Private Constructor Class with Public Static Method
public class ClassToAccess {
    public static void main(String[] args) {
        // Access the private constructor through a public static method
        ClassWithPrivateConstructor obj  ();
    }
}
// Class with Private Constructor and Public Static Method
public class ClassWithPrivateConstructor {
    private ClassWithPrivateConstructor() {
        // Private constructor
    }
    // Public method with static return type
    public static ClassWithPrivateConstructor getObj() {
        return new ClassWithPrivateConstructor();
    }
    // Other methods
    public void printsomething() {
        // Method implementation
    }
}

Best Practices and Considerations

Avoid unnecessary restrictions on instantiation. Use a private or protected constructor only when it truly benefits the design of your class.

Ensure that any static methods you provide for accessing private constructors are well-documented and used appropriately.

Remember that abstract classes provide a way to enforce certain behaviors in subclasses, whereas non-instantiable classes are more about encapsulation and utility.

In conclusion, while a class without a public constructor can complicate object creation, it can also serve useful purposes in terms of encapsulation and utility. The choice between a non-instantiable class and an abstract class depends on the specific requirements and design decisions of your project.