In Java, when a subclass overrides a method from its superclass, it must adhere to the method signature, including the exceptions it can throw. If the superclass method declares exceptions, the overriding method can declare the same exceptions or subtypes of those exceptions. This rule ensures that the subclass does not throw unexpected exceptions.

Synchronized methods or blocks can lead to performance degradation because they introduce thread contention. When multiple threads try to access synchronized code, they may block, waiting for access, which can lead to slowdowns. While synchronization is necessary for thread safety, it should be used judiciously, especially in high-throughput scenarios, to avoid excessive contention and performance issues.

Serialization in Java is primarily used to convert objects into a byte stream so that they can be easily stored, transmitted over a network, or reconstructed at a later time. This is especially useful for saving object state, such as in file I/O or network communication.

To send a text message over a network, you should use DataOutputStream to convert the message to bytes and DataInputStream to read and decode it. This ensures proper encoding and decoding of the message. PrintWriter and Scanner deal with characters, FileInputStream/FileReader with bytes, and ObjectOutputStream/ObjectInputStream with object serialization, which aren't ideal for text messages.

Yes, constructors can be private in Java. A private constructor is often used in design patterns like Singleton to ensure that only one instance of a class can be created. It restricts external instantiation of the class.

The 'this' keyword in Java is primarily used to refer to the current instance of a class. It can be used to access instance variables and methods, and also to call other constructors of the same class. Option 3 is incorrect; 'this' does not create a new object but references the existing instance. Options 1, 2, and 4 are correct statements.

The substring() method of the String class in Java returns a new string that is a subset of the original string. It takes two parameters, the starting index and the ending index, and extracts the characters within that range, creating a new string. The other options do not describe the functionality of the substring() method.

It is recommended to create custom exceptions when you want to provide more specific information about the error. This helps in better error handling and debugging. Custom exceptions can also encapsulate application-specific logic related to the error.

In Java, primitive data types like int, char, and boolean are typically allocated on the stack because they have fixed sizes and are stored directly in the memory location of the variable. Reference data types, such as objects, are allocated on the heap because their sizes can vary, and they need to be dynamically managed. However, it's important to note that references to objects (not the objects themselves) can be stored on the stack. The allocation of memory depends on the context and whether the reference is local or part of an object.

Setting a reference to null in Java means that the object that was previously referenced by that variable becomes eligible for garbage collection. It is not immediately removed from memory, but it is marked as a candidate for garbage collection. When the Java garbage collector runs, it identifies objects with no active references (i.e., references set to null) and reclaims their memory. So, while setting a reference to null doesn't immediately remove the object, it initiates the process of cleaning up unreferenced objects.