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How the Java `equals()` and `hashCode()` Methods Work

The equals() and hashCode() methods must be rewrite correctly at the same time, otherwise the hash set (such as HashMap and HashSet) will be invalid; 2. Equals() is used to define the logical equality of objects, and the actual field values need to be compared instead of references; 3. HashCode() returns the object hash code, and it is necessary to ensure that the equal objects have the same hash value; 4. Violating the contract will make it impossible to find the stored object from the collection, because hash search first uses hashCode() to locate the bucket, and then uses equals() to confirm the match; 5. It is recommended to use Objects.equals() and Objects.hash() to implement null safe and consistent logic, and avoid objects used as keys.

Jul 23, 2025 am 02:02 AM

java equals()

Java Futures and Promises for Asynchronous Operations

Future is suitable for simple asynchronous tasks, while CompletableFuture provides more flexible chain calls and combination operations. 1. Future submits tasks through ExecutorService, using get() to get results, but the functions are limited; 2. CompletableFuture supports thenApply, thenAccept, exceptionally and other methods, which can realize chain processing and exception capture; 3. Complete() can be called manually to complete Future; 4. It is recommended to customize thread pools to optimize resource management to avoid blocking public thread pools; 5. Configure the number of threads reasonably according to the task type, and web applications can combine with frameworks

Jul 23, 2025 am 01:50 AM

Understanding Java Memory Model (JMM) Internals

Java Memory Model (JMM) is a set of specifications that solve the problems of visibility, orderliness and atomicity in concurrent programming. 1. The volatile keyword ensures the visibility of variables and prohibits instruction reordering, but does not guarantee the atomicity of composite operations; 2. Synchronized not only realizes mutually exclusive access, but also establishes happens-before relationships to ensure data consistency; 3. After the final field is assigned in the constructor, other threads can correctly see their initialization values, which is an important means to build thread-safe objects. Mastering the memory semantics of these keywords helps to write stable and reliable concurrent code.

Jul 23, 2025 am 01:08 AM

Advanced Java Logging Configuration with MDC

MDC is a thread binding context map provided by SLF4J, which is used to add custom information to the log to improve traceability. 1. Use MDC.put(key,value) to add context data, such as user ID and request ID; 2. Output these fields through %X{key} in the log configuration (such as Logback, Log4j2); 3. Automatically inject MDC information through interceptors or filters in web applications, and call MDC.clear() after the request is completed; 4. Manually pass the MDC context in multi-threaded or asynchronous tasks, which can be implemented by encapsulating the Executor or using a third-party library; 5. Configure the log framework (such as Logback, Log4j2) to ensure correct input

Jul 23, 2025 am 12:51 AM

How to use regular expressions in Java to validate an email?

The method to verify the mailbox format in Java is to use regular expressions to match the Pattern and Matcher classes. 1. Use Pattern and Matcher classes: generate Pattern objects by compiling regular expressions, and then create Matcher objects to match input strings; 2. Mailbox regular structure: including user name part, domain name part and top-level domain name part, which can cover most legal mailbox formats; 3. Notes: There is no need to pursue full compliance with RFC standards, front-end and back-end double-factor verification should be taken into account, and third-party libraries such as Apache CommonsValidator can be considered; 4. Sample test code: Write test methods to verify legal and illegal mailboxes to ensure accuracy.

Jul 23, 2025 am 12:50 AM

What is type erasure in Java generics?

Java's generic type erasure is the mechanism by which the compiler erases specific type information when processing generics. 1. Java will remove generic information during compilation, so that List and List are regarded as the same type at runtime; 2. This design is to be compatible with versions before Java 1.5; 3. Generic types will be replaced with boundary types, such as T is replaced with Object, TextendsNumber is replaced with Number, and the compiler inserts type conversion to ensure security; 4. Type erasure causes problems such as inability to create generic arrays, inability to check generic types with instanceof, and signature conflicts of different generic methods; 5. You can obtain parent class generic information through reflection or use anonymous internal classes to save generics to bypass

Jul 23, 2025 am 12:15 AM

java generics Type erasure

Java Native Memory Diagnostics and Tools

Confirm that the NativeMemory problem is manifested as normal heap memory but the total process memory is growing, the RES memory is far beyond the -Xmx setting, and an OOM error of Directbuffer or nativethread. 1. Use NMT (-XX:NativeMemoryTracking=summary) to track the native memory of JVM and view the memory trends of modules such as Thread and Internal through jcmd; 2. Pay attention to the DirectBuffer leakage, it is not released when using allocateDirect() or the MaxDirectMemorySize setting is unreasonable; 3. Check that too many threads lead to high stack space occupancy, which can be used

Jul 23, 2025 am 12:09 AM

java programming

Developing Serverless Java Functions on AWS Lambda

JavaissuitableforAWSLambdainspecificscenariosdespitenotbeingthemostcommonchoice.TodevelopJava-basedLambdafunctionseffectively,firstsetupyourenvironmentusingMavenorGradle,installAWSSAMCLIorServerlessFramework,useJava8or11,configureanIDEwithAWSToolkitp

Jul 22, 2025 am 04:37 AM

how to override toString method in java

The main purpose of overriding the toString() method is to return a more meaningful representation of the object string. The default toString() outputs class name and hash code, such as com.example.Person@1b6d3586, which is not conducive to debugging and log analysis, and after rewriting, you can output such as Person{name='Alice',age=30}, which is convenient for quick understanding of object status. When rewriting, you need to use @Override annotation to return clear format and avoid null or complex logic. Suitable for debugging, logging, unit testing, and collection output. Mainstream IDEs such as IntelliJ and Eclipse provide the function of automatically generating toString(), L

Jul 22, 2025 am 04:37 AM

Understanding Java Class Loader Leakage

The main reasons for the Java class loader leak are that the thread context class loader is not reset, the static variable holds the class loader or class instance, the listener and callback are not logged out, and the JDBC driver registration is not cleared. 1. The thread context class loader has not been restored after use, so it should be set and restored manually; 2. Static variables hold the class loader or the classes it loads for a long time, so it cannot be recycled. It is recommended to replace strong references with weak references; 3. The listener and callback are not logged out, which will cause the class loader to be unable to be released, and it should be explicitly unregistered when closed; 4. The JDBC driver is not removed from DriverManager, which will also cause leakage, and should be actively cleaned before the application is closed. Such problems can be effectively prevented through code specifications, resource management and memory analysis tools.

Jul 22, 2025 am 03:57 AM

java

Java Security for SQL Injection Prevention

The core methods to prevent SQL injection include: 1. Use PreparedStatement precompiled statements to ensure that the input is processed as data; 2. Whitelist verification, length limit and special character escape of the input; 3. Correctly use ORM frameworks such as Hibernate and MyBatis to avoid splicing SQL; 4. Do not expose error information, scan for vulnerabilities regularly, and restrict database permissions. These measures jointly ensure the SQL security of Java applications.

Jul 22, 2025 am 03:56 AM

java sql injection

Java Event Buses and Reactive Programming

EventBus is suitable for simple publish-subscribe scenarios, and ReactiveProgramming is good at complex data stream processing. 1. EventBus is an event distributor, used to decouple component communication, and is suitable for simple scenarios such as inter-page notifications and log triggers. Its advantage is that it is easy to use but difficult to manage complex links. 2. ReactiveProgramming is based on data flow and supports transformation, merge and other operations. It is suitable for complex scenarios such as real-time processing and asynchronous combination. It has characteristics such as backpressure and error processing, but has high learning costs. 3. When choosing, you should judge based on the needs: EventBus is used for simple notifications, and Reactive is used for complex stream processing. The two can also coexist.

Jul 22, 2025 am 03:54 AM

Reactive programming Java事件總線

Java ForkJoinPool for Parallel Computing

ForkJoinPool is a parallel computing tool used in Java to efficiently handle splitable tasks. Its core feature is that it supports fork/join mode and work theft algorithm. 1. It is suitable for "dividing and conquer" problems, such as recursion and big data processing; 2. When using it, you need to inherit RecursiveTask (with return value) or RecursiveAction (without return value), and implement task splitting logic in the compute() method; 3. Submit tasks through ForkJoinPool and coordinate execution; 4. Pay attention to avoid blocking operations, reasonably divide granularity, reduce shared state and correctly handle exceptions; 5. Compared with ordinary thread pools, its scheduling is more efficient and more suitable for parallel splitting tasks.

Jul 22, 2025 am 03:54 AM

Understanding Java Object Lifecycle and Memory Management

The Java object life cycle goes through three stages: creation, use and recycling. 1. Create: trigger class loading through new keywords, the JVM allocates memory in the heap and executes constructor initialization of the object; 2. Use: The object is accessed by reference, and frequently creates large objects can consider the optimization performance of the object pool; 3. Recycling: When the object is unreachable, it is recycled by GC. Common algorithms include mark-clearing, copying, mark-collation, and different recyclers adapt to different scenarios; determine whether the object can be recycled dependent reference accessibility, such as explicit null, end of scope, weak reference failure, etc.; methods to avoid memory leaks include reducing global variables, timely logging out listeners, using memory analysis tools, and paying attention to the problem of internal classes holding external class references. Understanding these mechanisms can help

Jul 22, 2025 am 03:51 AM

java Memory management