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What are wildcards in generics?

WildcardsingenericsprovideflexibilitybyallowingunknowntypesinJava.1.Upper-boundedwildcards()areusedwhenthetypeisirrelevant,supportingallparameterizedtypes.Thesewildcardsensurecompile-timetypesafetywhileenablinggenericcodereuse,particularlyusefulincol

Jun 25, 2025 pm 02:05 PM

How to use enums?

Enumeration is suitable for scenarios such as finite states, control branches, classification options, etc. It is recommended to use PascalCase for naming, and the values ??are in full or first letter capital to avoid redundant prefixes. Different languages ??such as Python, TypeScript, and Java support for enumerations differently and require unified team specifications. When using it, you should avoid confounding irrelevant values, fix common values, consider data mapping, and separate common enumerations from sharing.

Jun 25, 2025 pm 01:49 PM

What is externalization?

Externalization is a psychological technique commonly used in narrative therapy, and its core is to help people separate problems from themselves. 1) By naming or giving a question its independent identity, such as "anxiety" or "Mr. Procrastination", individuals learn to look at the problem in a more objective way; 2) then describe how the problem affects life, when it occurs and the behaviors and emotions it brings; 3) ultimately challenge the impact of the problem and regain control. This approach reduces self-blame and shame, stimulates curiosity, prompts people to reflect on the root causes of the problem and take positive actions to promote change. It is suitable for anxiety, depression, addiction and other problems, and can also be used to deal with thinking patterns such as self-doubt or perfectionism.

Jun 25, 2025 pm 01:05 PM

What is an instance initializer block?

Instance initialization blocks are used in Java to run initialization logic when creating objects, which are executed before the constructor. It is suitable for scenarios where multiple constructors share initialization code, complex field initialization, or anonymous class initialization scenarios. Unlike static initialization blocks, it is executed every time it is instantiated, while static initialization blocks only run once when the class is loaded.

Jun 25, 2025 pm 12:21 PM

What is an Error in Java?

Errors in Java are uncontrollable runtime issues and should not be handled by programs. 1. Error indicates serious problems at the JVM or environment level, such as OutOfMemoryError, StackOverflowError, etc.; 2. Common types include VirtualMachineError, LinkageError, and ThreadDeath; 3. Errors should not be caught because they are usually unable to be restored; 4. Errors are different from exceptions, the former should not be caught, and the latter (Exception) can be handled; 5. When an error is encountered, you should analyze the stack trace, check resource limitations, review library compatibility, and monitor logs.

Jun 25, 2025 pm 12:06 PM

How to create custom annotations?

Custom annotations can enhance the code function through metadata. Java uses @interface definition and combines reflection reading. Examples include @Todo annotation annotation to annotate to do; Python uses decorators to simulate annotation behavior; when using it, it should reasonably design the scope of action, avoid abuse, strengthen document descriptions, and cooperate with tool chains to improve efficiency.

Jun 25, 2025 am 11:29 AM

What is the volatile keyword? (Rephrased)

volatiletellsthecompilernottooptimizeaccessestoavariablethatmaychangeunexpectedly,ensuringmemoryreads/writesoccurasintended.1.Itpreventscachinginregistersandenforcesmemoryaccessoneveryread/write.2.Itlimitsinstructionreorderingaroundthevariable(butnot

Jun 25, 2025 am 11:09 AM

Keywords volatile

What is a deadlock?

Adeadlockoccurswhenfourconditionscoexist:mutualexclusion,holdandwait,nopreemption,andcircularwait.1.Mutualexclusionmeansatleastoneresourcecannotbeshared.2.Holdandwaitoccurswhenaprocessholdsoneresourcewhilewaitingforanother.3.Nopreemptionmeansresource

Jun 25, 2025 am 10:21 AM

How does HashMap work internally?

HashMap realizes efficient storage and search through hash tables in Java. It uses an array linked list (or red and black tree) structure, first obtain the hash value through the hashCode of the key, and then map it to the array index after processing by the hash function to reduce conflicts; 1. When a hash collision occurs, use the linked list to connect multiple key-value pairs; 2. Since JDK8, when the length of the linked list exceeds 8, it is converted to a red and black tree to improve search efficiency; 3. The default initial capacity is 16 and the load factor is 0.75. When the number of elements exceeds the threshold, the capacity expansion is triggered, the array is doubled and the element position is recalculated; 4. Multi-threaded capacity expansion may lead to dead loops or data confusion. It is recommended to use ConcurrentHashMap in a concurrent environment.

Jun 25, 2025 am 09:49 AM

What is the `equals` method for strings?

Comparing string content in Java should use the equals() method instead of the == operator, because == only compares object references and not content. 1. Using == may lead to error results. For example, the newly created same string object will be judged to be unequal; 2. Equals() ensures the content consistency through character-by-character comparison, regardless of how the string is created; 3. Note that equals() is case sensitive and null exceptions must be avoided when processing null; 4. EqualsIgnoreCase() can be used for ignoring case comparison; 5. Others such as Objects.equals(a,b) can elegantly handle null values. Therefore, always use equals() to compare the actual inside of a string

Jun 25, 2025 am 09:21 AM

What is a List?

List in programming is a basic data structure used to store multiple ordered elements and supports different types and dynamic modifications. For example, lists in Python can be accessed through indexes and are suitable for frequent addition and deletion scenarios; in daily life, List is used to clearly list tasks or items, such as to-do items, and is often sorted by priority or time; the difference between List and Array and Set is its dynamicity, flexibility and allows for duplicate values.

Jun 25, 2025 am 09:01 AM

How to use a try-catch block?

The use of the try-catch block is to handle possible exceptions when the program runs, so that the program can handle errors gracefully instead of crashing directly. Common application scenarios include calling external interfaces, reading and writing files, parsing data formats, and user input verification. The basic structure is to execute code that may occur in the try block. The catch block catches and handles exceptions. Some languages ??support the use of multiple catch blocks according to the error type for differentiation processing. Notes include avoiding abuse, non-empty catch blocks, finally cleaning resources, and logging error logs. The example shows how JSON parsing failures and file reading errors are handled, and emphasizes that rational use can improve program robustness and debugging efficiency.

Jun 25, 2025 am 08:17 AM

What is the Factory pattern?

Factory mode is used to encapsulate object creation logic, making the code more flexible, easy to maintain, and loosely coupled. The core answer is: by centrally managing object creation logic, hiding implementation details, and supporting the creation of multiple related objects. The specific description is as follows: the factory mode handes object creation to a special factory class or method for processing, avoiding the use of newClass() directly; it is suitable for scenarios where multiple types of related objects are created, creation logic may change, and implementation details need to be hidden; for example, in the payment processor, Stripe, PayPal and other instances are created through factories; its implementation includes the object returned by the factory class based on input parameters, and all objects realize a common interface; common variants include simple factories, factory methods and abstract factories, which are suitable for different complexities.

Jun 24, 2025 pm 11:29 PM

What is type casting?

There are two types of conversion: implicit and explicit. 1. Implicit conversion occurs automatically, such as converting int to double; 2. Explicit conversion requires manual operation, such as using (int)myDouble. A case where type conversion is required includes processing user input, mathematical operations, or passing different types of values ??between functions. Issues that need to be noted are: turning floating-point numbers into integers will truncate the fractional part, turning large types into small types may lead to data loss, and some languages ??do not allow direct conversion of specific types. A proper understanding of language conversion rules helps avoid errors.

Jun 24, 2025 pm 11:09 PM