In the given code snippet, the + operator is used to concatenate two string literals: "2" and "3." Therefore, the output will be the concatenation of these two strings, which is "23." It's important to note that when the + operator is used with strings, it performs string concatenation, not arithmetic addition.

The Platform.runLater() method schedules tasks to be executed on the Main Application Thread in JavaFX. This is important because UI components should only be updated from the UI thread to maintain thread safety and avoid potential issues.

The primary purpose of using the Callable interface in Java is to run tasks asynchronously and return a result. Unlike the Runnable interface, Callable tasks can return a result or throw an exception. This is commonly used with the Executor framework for managing concurrent tasks that require results.

The Future interface in Java provides methods to check if a computation is complete, to wait for its completion, and to retrieve the result of the computation. It is commonly used in concurrent programming to manage asynchronous tasks and obtain results when they become available. Executor is not an interface for managing results, and the other options do not provide these specific methods.

In a switch case in Java, the default keyword is used to specify the code that should be executed if none of the case labels match the switch expression. It acts as a fallback or default option when no other case matches the switch expression.

Synchronized methods in Java are used primarily to prevent multiple threads from accessing and modifying shared resources simultaneously, ensuring thread safety. This is crucial in multithreading scenarios where multiple threads may otherwise interfere with each other when accessing shared data.

To establish a connection to a specified URL in Java, you use the openConnection() method provided by the URL class. This method returns a URLConnection object, which can be used to interact with the resource located at the URL.

The worst-case time complexity of the Quick Sort algorithm is O(n^2). However, on average, it has a time complexity of O(n log n), making it a very efficient sorting algorithm for large datasets. The worst-case scenario occurs when the pivot chosen is always the smallest or largest element, resulting in unbalanced partitions.

The concat() method in Java combines two strings, and in this case, it appends "Programming" to "Java," resulting in "JavaProgramming." Therefore, the correct output is "JavaProgramming."

In JDBC, the ResultSet interface provides methods like first(), last(), next(), and previous() to navigate through the result set obtained from a database query. The other options are not related to ResultSet navigation in JDBC.