Add creational patterns, Interpreter; remove scripts; update README

01-creational/singleton/DatabaseConnection.java

@@ -0,0 +1,34 @@
+package singleton;
+
+/**
+ * Thread-safe Singleton using double-checked locking.
+ */
+public class DatabaseConnection {
+    private static volatile DatabaseConnection instance;
+    private final String url;
+    private int queryCount = 0;
+
+    private DatabaseConnection() {
+        this.url = "jdbc:mysql://localhost:3306/myapp";
+        System.out.println("Establishing connection to " + url);
+    }
+
+    public static DatabaseConnection getInstance() {
+        if (instance == null) {
+            synchronized (DatabaseConnection.class) {
+                if (instance == null) {
+                    instance = new DatabaseConnection();
+                }
+            }
+        }
+        return instance;
+    }
+
+    public String executeQuery(String sql) {
+        queryCount++;
+        return "Result #" + queryCount + " for: " + sql;
+    }
+
+    public int getQueryCount() { return queryCount; }
+    public String getUrl()     { return url; }
+}