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

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)

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

The basic syntax of PHP includes four key points: 1. The PHP tag must be ended, and the use of complete tags is recommended; 2. Echo and print are commonly used for output content, among which echo supports multiple parameters and is more efficient; 3. The annotation methods include //, # and //, to improve code readability; 4. Each statement must end with a semicolon, and spaces and line breaks do not affect execution but affect readability. Mastering these basic rules can help write clear and stable PHP code.

There are three common forms of Java for loops. 1. The basic for loop is suitable for cases where the number of loops is known. The syntax is for (initialization; conditional judgment; update), such as traversing arrays or counts; 2. The enhanced for loop (for-each) is used to simplify the traversal of arrays or collections, and the syntax is for (element type variable name: the object to be traversed), but the index cannot be accessed or the collection content cannot be modified; 3. Nested for loops are used to deal with two-dimensional structures such as matrices, outer control rows, and inner control columns, but performance issues need to be paid attention to.

The key to writing Python's ifelse statements is to understand the logical structure and details. 1. The infrastructure is to execute a piece of code if conditions are established, otherwise the else part is executed, else is optional; 2. Multi-condition judgment is implemented with elif, and it is executed sequentially and stopped once it is met; 3. Nested if is used for further subdivision judgment, it is recommended not to exceed two layers; 4. A ternary expression can be used to replace simple ifelse in a simple scenario. Only by paying attention to indentation, conditional order and logical integrity can we write clear and stable judgment codes.

ThetransientkeywordinJavaisusedtoexcludefieldsfromtheobject'sserializedstate;1.Itpreventsspecificfieldsfrombeingpartofthebytestreamduringserialization;2.Itisusefulforexcludingsensitivedata,cachedvalues,orunserializableresources;3.Transientfieldsarese

In Python, iterates through dictionaries can be achieved through keys, values or key-value pairs. The specific methods are as follows: 1. Use forkeyindict or my_dict.keys() when traversing keys; 2. Use my_dict.values() when traversing values; 3. Use my_dict.items() when traversing keys and values at the same time and unpack them into key and value; pay attention to avoid modifying dictionary size in the loop, considering version differences in insertion order, and performance problems when dealing with large dictionaries.