MapReduce principle

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

Optional can clearly express intentions and reduce code noise for null judgments. 1. Optional.ofNullable is a common way to deal with null objects. For example, when taking values ​​from maps, orElse can be used to provide default values, so that the logic is clearer and concise; 2. Use chain calls maps to achieve nested values ​​to safely avoid NPE, and automatically terminate if any link is null and return the default value; 3. Filter can be used for conditional filtering, and subsequent operations will continue to be performed only if the conditions are met, otherwise it will jump directly to orElse, which is suitable for lightweight business judgment; 4. It is not recommended to overuse Optional, such as basic types or simple logic, which will increase complexity, and some scenarios will directly return to nu.

The core workaround for encountering java.io.NotSerializableException is to ensure that all classes that need to be serialized implement the Serializable interface and check the serialization support of nested objects. 1. Add implementsSerializable to the main class; 2. Ensure that the corresponding classes of custom fields in the class also implement Serializable; 3. Use transient to mark fields that do not need to be serialized; 4. Check the non-serialized types in collections or nested objects; 5. Check which class does not implement the interface; 6. Consider replacement design for classes that cannot be modified, such as saving key data or using serializable intermediate structures; 7. Consider modifying

To deal with character encoding problems in Java, the key is to clearly specify the encoding used at each step. 1. Always specify encoding when reading and writing text, use InputStreamReader and OutputStreamWriter and pass in an explicit character set to avoid relying on system default encoding. 2. Make sure both ends are consistent when processing strings on the network boundary, set the correct Content-Type header and explicitly specify the encoding with the library. 3. Use String.getBytes() and newString(byte[]) with caution, and always manually specify StandardCharsets.UTF_8 to avoid data corruption caused by platform differences. In short, by

JavaSocket programming is the basis of network communication, and data exchange between clients and servers is realized through Socket. 1. Socket in Java is divided into the Socket class used by the client and the ServerSocket class used by the server; 2. When writing a Socket program, you must first start the server listening port, and then initiate the connection by the client; 3. The communication process includes connection establishment, data reading and writing, and stream closure; 4. Precautions include avoiding port conflicts, correctly configuring IP addresses, reasonably closing resources, and supporting multiple clients. Mastering these can realize basic network communication functions.

In Java, Comparable is used to define default sorting rules internally, and Comparator is used to define multiple sorting logic externally. 1.Comparable is an interface implemented by the class itself. It defines the natural order by rewriting the compareTo() method. It is suitable for classes with fixed and most commonly used sorting methods, such as String or Integer. 2. Comparator is an externally defined functional interface, implemented through the compare() method, suitable for situations where multiple sorting methods are required for the same class, the class source code cannot be modified, or the sorting logic is often changed. The difference between the two is that Comparable can only define a sorting logic and needs to modify the class itself, while Compar

There are three common methods to traverse Map in Java: 1. Use entrySet to obtain keys and values at the same time, which is suitable for most scenarios; 2. Use keySet or values to traverse keys or values respectively; 3. Use Java8's forEach to simplify the code structure. entrySet returns a Set set containing all key-value pairs, and each loop gets the Map.Entry object, suitable for frequent access to keys and values; if only keys or values are required, you can call keySet() or values() respectively, or you can get the value through map.get(key) when traversing the keys; Java 8 can use forEach((key,value)-&gt

There are three common ways to parse JSON in Java: use Jackson, Gson, or org.json. 1. Jackson is suitable for most projects, with good performance and comprehensive functions, and supports conversion and annotation mapping between objects and JSON strings; 2. Gson is more suitable for Android projects or lightweight needs, and is simple to use but slightly inferior in handling complex structures and high-performance scenarios; 3.org.json is suitable for simple tasks or small scripts, and is not recommended for large projects because of its lack of flexibility and type safety. The choice should be decided based on actual needs.