Singleton Class

1. Singleton Class is a class which can only be intiated once.
2. It needs to have a private constructor ensuring noone can create the object from outside.

Eager Initialization

1. In this approach, the singleton instance is created at the time of class loading.
2. The instance is stored in a private static final field, ensuring it is available whenever required.
3. Since it is created upfront, it is inherently thread-safe.
4. Drawback: The instance is created even if it’s never used, which may waste resources if the object is heavy.

Lazy Initialization

1. In this approach, the singleton instance is created only when it is first requested.
2. The class provides a getInstance() method, which checks if the object already exists.
   1. If not, it creates a new instance.
   2. If yes, it returns the existing one.
3. This technique delays creation until needed, also known as Lazy Loading.
4. Drawback: In multi-threaded environments, special care (like synchronization or double-checked locking) is needed to ensure thread safety.

Code

Refer to the code-comments below for more insights:

// Config.java
package oops.singleton_class;

public class Config {

    /*
     * Method-A: Eager Intialisation, Instance is created as when class is loaded
     * Multi-threading Safe
     * Final keyword below ensures the variable is never reassigned (The value assigned now is the file)
     * We can't provide arguments to constructors on runtime
     */
    // private static final Config CLASS_INSTANCE = new Config("Eager");
    private static Config CLASS_INSTANCE = null;
    String id;
    /*
     * Private Constructor to avoid outside Intialisation
     */
    private Config(String id){
        this.id = id;

    }

    public static Config getInstance(String id){
        /*
         * Method-B: Lazy-Loading: Create Instance only when it is needed:
         * Not Thread-Safe for multi-threaded environment
         */
        if(CLASS_INSTANCE == null){
            CLASS_INSTANCE = new Config(id);
        }
        return CLASS_INSTANCE;
    }
    
    public String toString(){
        return "Config id: " + id;
    }
}

// Main.java
package oops.singleton_class;

public class Main {
    public static void main(String[] args){
        Config configObj1 = Config.getInstance("alpha");
        System.out.println(configObj1);
        Config configObj2 = Config.getInstance("beta");
        System.out.println(configObj2);
    }
}