Add all 23 GoF design pattern implementations (2026-06-13)

02-structural/proxy/DatabaseConnection.java

@@ -0,0 +1,11 @@
+package proxy;
+
+/**
+ * Subject interface — defines what both the real object and proxy expose.
+ * Clients depend on this, not on the concrete class.
+ */
+public interface DatabaseConnection {
+    void connect();
+    String executeQuery(String sql);
+    void disconnect();
+}

02-structural/proxy/LazyConnectionProxy.java

@@ -0,0 +1,52 @@
+package proxy;
+
+/**
+ * Virtual Proxy — delays creating the RealDatabaseConnection until
+ * the first actual query is made. If no query is ever made (e.g.,
+ * the service is initialized but never used in this request),
+ * the expensive connection is never opened.
+ *
+ * This is exactly how Hibernate proxies work: entities are not
+ * loaded from the database until you access a field on them.
+ */
+public class LazyConnectionProxy implements DatabaseConnection {
+
+    private final String url;
+    private RealDatabaseConnection real;  // null until first use
+
+    public LazyConnectionProxy(String url) {
+        this.url = url;
+        System.out.println("[Proxy] Created for " + url + " (real connection NOT opened yet)");
+    }
+
+    // Lazy initialization — create and connect only on first real need
+    private void initIfNeeded() {
+        if (real == null) {
+            System.out.println("[Proxy] First access — initializing real connection...");
+            real = new RealDatabaseConnection(url);
+            real.connect();
+        }
+    }
+
+    @Override
+    public void connect() {
+        // Proxy absorbs the connect() call — real connection opened lazily
+        System.out.println("[Proxy] connect() called — deferring to first query");
+    }
+
+    @Override
+    public String executeQuery(String sql) {
+        initIfNeeded();  // NOW we actually need the connection
+        return real.executeQuery(sql);
+    }
+
+    @Override
+    public void disconnect() {
+        if (real != null) {
+            real.disconnect();
+            real = null;
+        } else {
+            System.out.println("[Proxy] disconnect() called but connection was never opened");
+        }
+    }
+}

02-structural/proxy/LoggingProxy.java

@@ -0,0 +1,41 @@
+package proxy;
+
+import java.time.Instant;
+
+/**
+ * Logging Proxy — adds timing and audit logging around every query
+ * without touching RealDatabaseConnection or any of its callers.
+ *
+ * This is the "cross-cutting concern" use case of Proxy,
+ * the same mechanism behind Spring AOP's @Around advice.
+ */
+public class LoggingProxy implements DatabaseConnection {
+
+    private final DatabaseConnection target;
+
+    public LoggingProxy(DatabaseConnection target) {
+        this.target = target;
+    }
+
+    @Override
+    public void connect() {
+        System.out.println("[Log] connect() at " + Instant.now());
+        target.connect();
+    }
+
+    @Override
+    public String executeQuery(String sql) {
+        long start = System.currentTimeMillis();
+        System.out.println("[Log] QUERY START: " + sql);
+        String result = target.executeQuery(sql);
+        long elapsed = System.currentTimeMillis() - start;
+        System.out.println("[Log] QUERY END: " + elapsed + "ms | result: " + result);
+        return result;
+    }
+
+    @Override
+    public void disconnect() {
+        System.out.println("[Log] disconnect() at " + Instant.now());
+        target.disconnect();
+    }
+}

02-structural/proxy/Main.java

@@ -0,0 +1,51 @@
+package proxy;
+
+/**
+ * Proxy Design Pattern — Runnable Demo
+ *
+ * Shows two proxy types:
+ *  1. Virtual Proxy (lazy connection)
+ *  2. Logging Proxy (cross-cutting concern)
+ *  3. Proxy chaining (both together)
+ *
+ * Run: javac proxy/*.java && java proxy.Main
+ * Article: https://ankurm.com/proxy-design-pattern-java/
+ */
+public class Main {
+
+    public static void main(String[] args) throws InterruptedException {
+        System.out.println("=== Proxy Design Pattern Demo ===\n");
+
+        // --- Virtual Proxy: lazy connection ---
+        System.out.println("-- Virtual Proxy (lazy loading) --");
+        DatabaseConnection lazy = new LazyConnectionProxy("jdbc:postgresql://localhost/mydb");
+        lazy.connect();  // absorbed by proxy, no real connection yet
+        System.out.println("(no real connection yet — saved startup time)");
+        System.out.println("Result: " + lazy.executeQuery("SELECT * FROM users WHERE id=1"));
+        System.out.println("Result: " + lazy.executeQuery("SELECT COUNT(*) FROM orders"));
+        lazy.disconnect();
+
+        System.out.println();
+
+        // --- Logging Proxy: wraps the real connection ---
+        System.out.println("-- Logging Proxy --");
+        DatabaseConnection real = new RealDatabaseConnection("jdbc:mysql://localhost/shopdb");
+        real.connect();
+        DatabaseConnection logged = new LoggingProxy(real);
+        logged.executeQuery("SELECT * FROM products LIMIT 10");
+        logged.disconnect();
+
+        System.out.println();
+
+        // --- Proxy chaining: lazy + logging ---
+        System.out.println("-- Chained Proxies: Lazy + Logging --");
+        DatabaseConnection chain =
+            new LoggingProxy(
+                new LazyConnectionProxy("jdbc:oracle://localhost/warehouse"));
+        chain.connect();
+        chain.executeQuery("SELECT SUM(quantity) FROM inventory");
+        chain.disconnect();
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}

02-structural/proxy/RealDatabaseConnection.java

@@ -0,0 +1,33 @@
+package proxy;
+
+/**
+ * Real Subject — the actual, expensive database connection.
+ * Opening it takes time. We want to delay this until truly needed.
+ */
+public class RealDatabaseConnection implements DatabaseConnection {
+
+    private final String url;
+
+    public RealDatabaseConnection(String url) {
+        this.url = url;
+    }
+
+    @Override
+    public void connect() {
+        System.out.println("[Real DB] Connecting to " + url + " (expensive operation)...");
+        // Simulate connection setup time
+        try { Thread.sleep(100); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
+        System.out.println("[Real DB] Connected.");
+    }
+
+    @Override
+    public String executeQuery(String sql) {
+        System.out.println("[Real DB] Executing: " + sql);
+        return "ResultSet{rows=42}";  // simulated result
+    }
+
+    @Override
+    public void disconnect() {
+        System.out.println("[Real DB] Disconnecting from " + url);
+    }
+}