Singleton Pattern

What

Ensures a class has only one instance and provides a global point of access to it. Ensures one and only one object is created.

When

You need exactly one instance of a class (and it must be globally accessible). You need one shared instance (e.g., config, logger).

class Logger {
    public void log(String msg) {
        System.out.println(msg);
    }
}
 
class App {
    public static void main(String[] args) {
        Logger l1 = new Logger();
        Logger l2 = new Logger(); // multiple loggers, state inconsistency risk
        l1.log("Start");
        l2.log("End");
    }
}
 

How

• Hide constructor.
• Provide a static method to get the instance, creating it lazily if necessary.

class Singleton {
    private static Singleton instance;
    private Singleton() {}
    public static Singleton getInstance() {
        if (instance == null) instance = new Singleton();
        return instance;
    }
}
 

Examples

• Configuration managers.
• Logging services.
• Database connection pool.

Why

Advantages

• Controlled access to the sole instance.
• Lazy instantiation possible.

Disadvantages

• Global state makes testing harder.
• Can lead to hidden dependencies.
• Breaks the Single Responsibility Principle.