std::chrono is used in C to process time, including obtaining the current time, measuring execution time, operation time point and duration, and formatting analysis time. 1. Use std::chrono::system_clock::now() to obtain the current time, which can be converted into a readable string, but the system clock may not be monotonous; 2. Use std::chrono::steady_clock to measure the execution time to ensure monotony, and convert it into milliseconds, seconds and other units through duration_cast; 3. Time point (time_point) and duration (duration) can be interoperable, but attention should be paid to unit compatibility and clock epoch (epoch)

HashMap implements key-value pair storage through hash tables in Java, and its core lies in quickly positioning data locations. 1. First use the hashCode() method of the key to generate a hash value and convert it into an array index through bit operations; 2. Different objects may generate the same hash value, resulting in conflicts. At this time, the node is mounted in the form of a linked list. After JDK8, the linked list is too long (default length 8) and it will be converted to a red and black tree to improve efficiency; 3. When using a custom class as a key, the equals() and hashCode() methods must be rewritten; 4. HashMap dynamically expands capacity. When the number of elements exceeds the capacity and multiplies by the load factor (default 0.75), expand and rehash; 5. HashMap is not thread-safe, and Concu should be used in multithreaded

CTE is a temporary result set in MySQL used to simplify complex queries. It can be referenced multiple times in the current query, improving code readability and maintenance. For example, when looking for the latest orders for each user in the orders table, you can first obtain the latest order date for each user through the CTE, and then associate it with the original table to obtain the complete record. Compared with subqueries, the CTE structure is clearer and the logic is easier to debug. Usage tips include explicit alias, concatenating multiple CTEs, and processing tree data with recursive CTEs. Mastering CTE can make SQL more elegant and efficient.

High-frequency questions in Java interviews are mainly focused on basic syntax, object-oriented, multithreaded, JVM and collection frameworks. The most common questions include: 1. There are 8 basic Java data types, such as byte, short, int, long, float, double, char and boolean. It is necessary to note that String is not the basic data type; 2. Final is used to modify classes, methods or variables to represent immutable, and finally is used to ensure code execution in exception processing. Finalize is an Object class method for cleaning before garbage collection; 3. Multi-thread synchronization can be achieved through synchronized keywords, ReentrantLock, and vo.

The method of using preprocessing statements to obtain database query results in PHP varies from extension. 1. When using mysqli, you can obtain the associative array through get_result() and fetch_assoc(), which is suitable for modern environments; 2. You can also use bind_result() to bind variables, which is suitable for situations where there are few fields and fixed structures, and it is good compatibility but there are many fields when there are many fields; 3. When using PDO, you can obtain the associative array through fetch (PDO::FETCH_ASSOC), or use fetchAll() to obtain all data at once, so the interface is unified and the error handling is clearer; in addition, you need to pay attention to parameter type matching, execution of execute(), timely release of resources and enable error reports.

Create and use SimpleDateFormat requires passing in format strings, such as newSimpleDateFormat("yyyy-MM-ddHH:mm:ss"); 2. Pay attention to case sensitivity and avoid misuse of mixed single-letter formats and YYYY and DD; 3. SimpleDateFormat is not thread-safe. In a multi-thread environment, you should create a new instance or use ThreadLocal every time; 4. When parsing a string using the parse method, you need to catch ParseException, and note that the result does not contain time zone information; 5. It is recommended to use DateTimeFormatter and Lo

WhensettingupMySQLtables,choosingtherightdatatypesiscrucialforefficiencyandscalability.1)Understandthedataeachcolumnwillstore—numbers,text,dates,orflags—andchooseaccordingly.2)UseCHARforfixed-lengthdatalikecountrycodesandVARCHARforvariable-lengthdata

BiConsumer is a functional interface in Java that handles operations that do not return results. It belongs to the java.util.function package and is suitable for scenarios where two data are required to operate at the same time, such as key-value pairs that traverse Map. A common usage is to iterate with Map's forEach method. Unlike other functional interfaces such as Consumer and BiFunction, BiConsumer does not generate a return value. The implementation methods include lambda expressions, method references and anonymous classes. When using them, you need to pay attention to the order of type parameters, non-returnable values, and exception handling.