Add all 23 GoF design pattern implementations

01-creational/singleton/AppConfigEnum.java

@@ -0,0 +1,19 @@
+package singleton;
+
+public enum AppConfigEnum {
+
+    INSTANCE; // The JVM creates this exactly once, period.
+
+    // Enum instances can have fields and methods like any class
+    private final String configPath;
+
+    AppConfigEnum() {
+        // Enum constructors run once, when the constant is first used
+        this.configPath = System.getProperty("config.path", "application.properties");
+        System.out.println("Loading config from: " + configPath);
+    }
+
+    public String getProperty(String key) {
+        return System.getProperty(key, "(not set)");
+    }
+}