Singleton Design Pattern in Java

1. Definition

The Singleton Design Pattern ensures that a class has only one instance and provides a global point of access to that instance. It is designed to restrict the instantiation of a class to a single "unique" instance.

2. Problem Statement

In many software applications, there's a need to ensure that some classes have just one instance. Whether it's to manage configurations, manage database connections, or prevent the overhead of creating multiple objects when only one is required, we need a mechanism to ensure a single instance.

3. Solution

The Singleton pattern provides a solution by ensuring that:

1. The class itself is responsible for creating its one-and-only instance.

2. The class provides a method for other classes to access this instance.

3. The constructor of the class is made private to prevent other classes from instantiating it.

4. Structure

Key components of the Singleton pattern:

1. A private static variable that holds a single instance of the class.

2. A private constructor to ensure no other class can instantiate it.

3. A public static method that allows other classes to get the instance of the singleton class.

5. Implementation Steps

1. Declare the constructor of the class as private.

2. Create a private static variable to hold the instance of the class.

3. Provide a public static method to return the instance of the class.

6. Implementation

public class Singleton {

    // Step 2: Create a private static instance of the class.
    private static Singleton uniqueInstance;

    // Step 1: Declare the constructor as private.
    private Singleton() {
        // This will prevent instantiation from other classes.
    }

    // Step 3: Provide a public static method to return the instance of the class.
    public static synchronized Singleton getInstance() {
        if (uniqueInstance == null) {
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }
}

Output:

While there's no direct output, whenever you call Singleton.getInstance(), you are assured to always receive the same instance of the Singleton class.

Explanation

With the Singleton pattern:

1. The class guarantees that there's only a single instance by maintaining a private static reference (uniqueInstance).

2. Instantiation of the class from outside is prevented using a private constructor.

3. Access to the single instance is provided through a public static method, ensuring global access and the singular nature of the instance.

7. When to use?

The Singleton pattern is suitable for situations where:

1. A single instance of a class must be ensured because it's required to coordinate actions across the system.

2. The class requires direct access without creating an instance.

3. When you want to avoid the overhead of creating a new instance every time an object is required, such as in configuration managers or database connection pools.