Singleton Pattern Docs

Author: PrathameshDhande22

content/Singleton Pattern.md

@@ -0,0 +1,99 @@
+---
+title: Singleton Pattern
+created: 2025-04-29
+tags:
+  - creational
+---
+## Definition
+
+The **Singleton Pattern** ensures that a class has only one instance and provides a global point of access to it.
+
+---
+## Real-World Analogy
+
+Imagine you have a database that every part of your application needs to access. Creating a new database connection each time can be expensive and requires repetitive configuration connection strings, usernames, passwords, and so on. Instead, you can use a single shared database connector that’s created once and used everywhere. This is exactly where the Singleton Pattern is useful.
+### Implementation in Java
+
+First, create a `DatabaseConnector.java` class with a private constructor so that no one outside the class can directly create an instance. Then define a private static field to hold the single instance of the class. Provide a public static method, `getInstance()`, which returns the instance. If the instance doesn’t exist yet, it’s created; otherwise, the existing instance is returned.
+
+```java title="DatabaseConnector.java"
+public class DatabaseConnector {
+    // Holds the single instance
+    private static DatabaseConnector instance;
+
+    private String connectionString;
+
+    // Private constructor prevents external instantiation
+    private DatabaseConnector(String connectionString) {
+        this.connectionString = connectionString;
+    }
+
+    @Override
+    public String toString() {
+        return "DatabaseConnector[connectionString=" + connectionString + "]";
+    }
+
+    // Synchronized to avoid race conditions in multithreaded environments
+    public static synchronized DatabaseConnector getInstance() {
+        if (instance == null) {
+            instance = new DatabaseConnector("PostgresSQL");
+        }
+        return instance;
+    }
+}
+```
+
+Notice that `getInstance()` is synchronized to prevent multiple threads from creating separate instances simultaneously.
+
+To use the connector:
+```java title="SingletonPatternExample.java"
+public class SingletonPatternExample {
+    public static void main(String[] args) {
+        DatabaseConnector conn1 = DatabaseConnector.getInstance();
+        System.out.println(conn1);
+
+        DatabaseConnector conn2 = DatabaseConnector.getInstance();
+        System.out.println(conn2);
+    }
+}
+```
+
+**Output:**
+```
+DatabaseConnector[connectionString=PostgresSQL]
+DatabaseConnector[connectionString=PostgresSQL]
+```
+Both calls return the same instance.
+
+---
+## Design Diagram
+
+```mermaid
+classDiagram
+    class DatabaseConnector {
+        - static DatabaseConnector instance
+        - String connectionString
+        - DatabaseConnector(String connectionString)
+        + static DatabaseConnector getInstance()
+        + String toString()
+    }
+```
+
+---
+## Real-World Example in Java
+
+The `Runtime` class in Java also uses the Singleton Pattern. There is only one JVM per machine, so Java provides a single Runtime instance:
+
+```java title="RuntimeExample.java"
+public class RuntimeExample {
+    public static void main(String[] args) {
+        Runtime rt = Runtime.getRuntime();
+        System.out.println("Available processors: " + rt.availableProcessors());
+    }
+}
+```
+
+**Output:**
+```
+Available processors: 8
+```

content/index.md

@@ -122,6 +122,7 @@ Consider an `SQLManager` class that performs CRUD operations. It has an `ILogger
 3. [[Decorator Pattern]]
 4. [[Factory Method Pattern]]
 5. [[Abstract Factory Pattern]]
+6. [[Singleton Pattern]]
 
 ---
 > [!Note]