add a simple program under java_playground

Author: fishercoder1534

src/main/java/java_playground/MainApp.java

@@ -0,0 +1,8 @@
+package java_playground;
+
+public class MainApp {
+    public static void main(String... args) {
+        System.out.println("Hello world.");
+        System.out.println("Program finished.");
+    }
+}