Java uses wrapper classes because basic data types cannot directly participate in object-oriented operations, and object forms are often required in actual needs; 1. Collection classes can only store objects, such as Lists use automatic boxing to store numerical values; 2. Generics do not support basic types, and packaging classes must be used as type parameters; 3. Packaging classes can represent null values ??to distinguish unset or missing data; 4. Packaging classes provide practical methods such as string conversion to facilitate data parsing and processing, so in scenarios where these characteristics are needed, packaging classes are indispensable.

Enums in Java are special classes that represent fixed number of constant values. 1. Use the enum keyword definition; 2. Each enum value is a public static final instance of the enum type; 3. It can include fields, constructors and methods to add behavior to each constant; 4. It can be used in switch statements, supports direct comparison, and provides built-in methods such as name(), ordinal(), values() and valueOf(); 5. Enumeration can improve the type safety, readability and flexibility of the code, and is suitable for limited collection scenarios such as status codes, colors or week.

Java supports asynchronous programming including the use of CompletableFuture, responsive streams (such as ProjectReactor), and virtual threads in Java19. 1.CompletableFuture improves code readability and maintenance through chain calls, and supports task orchestration and exception handling; 2. ProjectReactor provides Mono and Flux types to implement responsive programming, with backpressure mechanism and rich operators; 3. Virtual threads reduce concurrency costs, are suitable for I/O-intensive tasks, and are lighter and easier to expand than traditional platform threads. Each method has applicable scenarios, and appropriate tools should be selected according to your needs and mixed models should be avoided to maintain simplicity

There are three main differences between Callable and Runnable in Java. First, the callable method can return the result, suitable for tasks that need to return values, such as Callable; while the run() method of Runnable has no return value, suitable for tasks that do not need to return, such as logging. Second, Callable allows to throw checked exceptions to facilitate error transmission; while Runnable must handle exceptions internally. Third, Runnable can be directly passed to Thread or ExecutorService, while Callable can only be submitted to ExecutorService and returns the Future object to

JavaNIO is a new IOAPI introduced by Java 1.4. 1) is aimed at buffers and channels, 2) contains Buffer, Channel and Selector core components, 3) supports non-blocking mode, and 4) handles concurrent connections more efficiently than traditional IO. Its advantages are reflected in: 1) Non-blocking IO reduces thread overhead, 2) Buffer improves data transmission efficiency, 3) Selector realizes multiplexing, and 4) Memory mapping speeds up file reading and writing. Note when using: 1) The flip/clear operation of the Buffer is easy to be confused, 2) Incomplete data needs to be processed manually without blocking, 3) Selector registration must be canceled in time, 4) NIO is not suitable for all scenarios.

In Java, enums are suitable for representing fixed constant sets. Best practices include: 1. Use enum to represent fixed state or options to improve type safety and readability; 2. Add properties and methods to enums to enhance flexibility, such as defining fields, constructors, helper methods, etc.; 3. Use EnumMap and EnumSet to improve performance and type safety because they are more efficient based on arrays; 4. Avoid abuse of enums, such as dynamic values, frequent changes or complex logic scenarios, which should be replaced by other methods. Correct use of enum can improve code quality and reduce errors, but you need to pay attention to its applicable boundaries.

Interface Isolation Principle (ISP) requires that clients not rely on unused interfaces. The core is to replace large and complete interfaces with multiple small and refined interfaces. Violations of this principle include: an unimplemented exception was thrown when the class implements an interface, a large number of invalid methods are implemented, and irrelevant functions are forcibly classified into the same interface. Application methods include: dividing interfaces according to common methods, using split interfaces according to clients, and using combinations instead of multi-interface implementations if necessary. For example, split the Machine interfaces containing printing, scanning, and fax methods into Printer, Scanner, and FaxMachine. Rules can be relaxed appropriately when using all methods on small projects or all clients.

Java's class loading mechanism is implemented through ClassLoader, and its core workflow is divided into three stages: loading, linking and initialization. During the loading phase, ClassLoader dynamically reads the bytecode of the class and creates Class objects; links include verifying the correctness of the class, allocating memory to static variables, and parsing symbol references; initialization performs static code blocks and static variable assignments. Class loading adopts the parent delegation model, and prioritizes the parent class loader to find classes, and try Bootstrap, Extension, and ApplicationClassLoader in turn to ensure that the core class library is safe and avoids duplicate loading. Developers can customize ClassLoader, such as URLClassL