You can use substr() or mb_substr() to get the first N characters in PHP. The specific steps are as follows: 1. Use substr($string,0,N) to intercept the first N characters, which is suitable for ASCII characters and is simple and efficient; 2. When processing multi-byte characters (such as Chinese), mb_substr($string,0,N,'UTF-8'), and ensure that mbstring extension is enabled; 3. If the string contains HTML or whitespace characters, you should first use strip_tags() to remove the tags and trim() to clean the spaces, and then intercept them to ensure the results are clean.

There are two main ways to get the last N characters of a string in PHP: 1. Use the substr() function to intercept through the negative starting position, which is suitable for single-byte characters; 2. Use the mb_substr() function to support multilingual and UTF-8 encoding to avoid truncating non-English characters; 3. Optionally determine whether the string length is sufficient to handle boundary situations; 4. It is not recommended to use strrev() substr() combination method because it is not safe and inefficient for multi-byte characters.

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

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

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

Method reference is a way to simplify the writing of Lambda expressions in Java, making the code more concise. It is not a new syntax, but a shortcut to Lambda expressions introduced by Java 8, suitable for the context of functional interfaces. The core is to use existing methods directly as implementations of functional interfaces. For example, System.out::println is equivalent to s->System.out.println(s). There are four main forms of method reference: 1. Static method reference (ClassName::staticMethodName); 2. Instance method reference (binding to a specific object, instance::methodName); 3.

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