To prevent fall-through in a switch case in Java, you use the break keyword after each case block. This ensures that once a case is matched and executed, the control exits the switch statement and doesn't fall through to subsequent cases.

In Java, the throw keyword is used to manually throw an exception. It allows you to create custom exceptions or raise predefined exceptions when certain conditions are met. This is a fundamental part of Java's exception handling mechanism.

The primary advantage of using an ExecutorService to manage threads is better control over thread creation and reuse. It provides a higher-level abstraction for managing thread execution, which can lead to more efficient and scalable applications. The other options do not accurately describe the primary advantage of using an ExecutorService.

The correct answer is "URL." In Java, the URL class provides methods to get details of a URL and manipulate them. You can use URL class methods to retrieve various components of a URL, such as the protocol, host, port, path, and more. It is a fundamental class for working with URLs in Java.

The commit() method of the Connection interface in JDBC is used to make the changes made since the previous commit or rollback permanent. It effectively saves the changes to the database. Other options are not valid methods for this purpose.

In Java, a loop that contains another loop is called a "nested loop." Nested loops are used when you need to perform repetitive tasks within repetitive tasks. The inner loop is executed multiple times for each iteration of the outer loop. This nesting can be done with various loop types like for, while, or do-while.

Autoboxing (converting a primitive type to its corresponding wrapper type) and unboxing (converting a wrapper type to its corresponding primitive type) can have a significant impact on performance, especially in collections that deal with primitive data types (e.g., ArrayList). Each autoboxing and unboxing operation involves creating or extracting wrapper objects, which consumes memory and introduces overhead. This can lead to performance degradation in scenarios where these operations are frequent, such as large collections or loops. It's important to be aware of this when designing applications to avoid unnecessary autoboxing and unboxing.

When converting byte streams to character streams, you should use the InputStreamWriter class. It's used to bridge from byte streams to character streams and handles character encoding. ByteStreamReader and ByteToCharConverterStream are not standard classes in Java. InputStreamReader is used for byte-to-character conversion but not for handling the entire conversion process.

In a multi-transactional system, it's crucial to manage data consistency and prevent dirty reads. Database isolation levels help in achieving this. READ_COMMITTED ensures that a transaction can't read uncommitted changes from other transactions. SERIALIZABLE provides the highest level of isolation, preventing concurrent access to the same data. It's essential to understand these concepts for maintaining data integrity.

When you extend the Exception class (or its subclasses), your custom exception becomes a checked exception in Java. Checked exceptions must be either caught or declared in the method signature where they are thrown.