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Find duplicate elements in a Java array

To find duplicate elements in Java arrays, it can be achieved by loop counting, HashMap, or HashSet. 1. Use a nested loop to traverse the array and count, the time complexity is O(n2), which is suitable for small arrays; 2. Use HashMap to count the number of elements, the time complexity is O(n), which is suitable for large arrays; 3. Use HashSet to detect whether elements already exist, the time complexity is O(n), which is only judged whether there is duplication; 4. Pay attention to handling boundary situations such as empty arrays, and consider how to deal with the output form of multiple duplicate elements.

Jul 10, 2025 pm 12:17 PM

java array Repeating elements

Building RESTful APIs with Java Spring Boot

Using SpringBoot to build a RESTful API requires following resource naming specifications, HTTP method selection, Controller layer request processing, parameter binding method, unified response format and error handling mechanism. When designing an interface, you should focus on resource, such as /users represent user collection; select appropriate HTTP methods such as GET acquisition, POST creation, PUT update, DELETE deletion resources; use @RestController, @RequestMapping, @GetMapping, etc. to define interface paths and methods; bind through @PathVariable, @RequestParam, @RequestBody

Jul 10, 2025 pm 12:07 PM

How to profile a Java application for performance?

Java application performance analysis should first locate bottlenecks and then choose the appropriate method. 1. Use JDK's own tools such as jstat to view GC situation, jstack to troubleshoot thread problems, and jcmd for simple analysis; 2. Enable JFR to record runtime events, which is suitable for overall behavioral observation; 3. Use visual VM and other visual tools to intuitively view call stacks and hotspot methods; 4. Add monitoring buried points to the code to observe specific operations for a long time. Each method is suitable for different scenarios, and it is recommended to gradually and in-depth analysis from simple to traditional.

Jul 10, 2025 pm 12:06 PM

java Performance analysis

How to reverse a string in Java?

Inverting strings can be implemented in Java in a variety of ways. 1. The reverse() method of StringBuilder is the most recommended. The code is simple and efficient: newStringBuilder(original).reverse().toString(); 2. You can manually traverse the character array and exchange characters to achieve inversion, which helps you understand the underlying logic; 3. You can also use Java8Stream API to achieve functional style inversion, but the performance and readability are poor, which is only suitable for practice. The StringBuilder method is the first choice in actual development, and other methods can be selected and used according to specific needs.

Jul 10, 2025 am 11:58 AM

java String reverse

How to use Java Stream collect() with groupingBy?

The groupingBy collector of Stream in Java8 supports multiple grouping methods. ① Group by field: If you group by city, use Collectors.groupingBy(Person::getCity); ② Multi-level grouping: If you group by city first and then by age, use nested groupingBy; ③ Customize downstream operations: If you use Collectors.counting() to count the quantity, use Collectors.averagingInt() to calculate the average; ④ After grouping, merge data: If you splice the names into strings, use Collectors.mapping() to cooperate with Collectors.joini

Jul 10, 2025 am 11:53 AM

What are the best practices for writing concurrent Java code?

The following points should be followed by writing efficient and thread-safe concurrent Java code: 1. Use tool classes in the java.util.concurrent package, such as ConcurrentHashMap, CopyOnWriteArrayList and BlockingQueue, to improve performance and reliability; 2. Use thread pools (such as ExecutorService or ForkJoinPool) reasonably to manage thread resources, and set the appropriate number of threads according to the task type; 3. Avoid sharing mutable state, give priority to using immutable objects, and use atomic classes or locking mechanisms to ensure thread safety if necessary; 4. Pay attention to avoid deadlocks, live locks and resource hunger issues, and troubleshoot deadlocks can make it possible to

Jul 10, 2025 am 11:48 AM

java Concurrent programming

Exploring Concurrent Collections in Java util.concurrent

In a multi-threaded environment, using concurrent collections in the java.util.concurrent package can improve efficiency and security. 1.ConcurrentHashMap is suitable for high-concurrent read and write scenarios, and uses segmented locking or CAS mechanism to improve performance; 2.CopyOnWriteArrayList is suitable for List operations with more read and less read, such as event listener list; 3. BlockingQueue supports blocking operations and is often used in producer-consumer models; 4. Others such as ConcurrentSkipListMap, LinkedTransferQueue, etc. are also suitable for specific concurrent scenarios. When choosing, it should be based on read and write frequency, consistency requirements and other factors.

Jul 10, 2025 am 11:36 AM

java concurrent collection

What is Hibernate in Java?

HibernatesolvestheproblemofwritingandmaintainingrepetitivedatabasecodeinJavabyprovidinganobject-relationalmapping(ORM)framework.1.ItallowsdeveloperstoworkwithJavaobjectsinsteadofwritingrawSQLqueries.2.ItautomaticallymapsJavaclassestodatabasetables.3.

Jul 09, 2025 am 02:42 AM

How the Java JIT Compiler Optimizes Code

The JIT compiler improves performance through method inline, hotspot code recognition, escape analysis and scalar replacement, lock optimization and other means. 1. Method inlines the small method directly embedded in the call, reducing call overhead and promoting other optimizations; 2. Hotspot code identification uses counter to find high-frequency execution code for compilation, and centralize resources to optimize the key paths; 3. Escape analysis determines whether the object is escaping, and combines scalar replacement to reduce memory allocation; 4. Lock optimization includes mechanisms such as lock elimination, lock coarseness and bias locking, improving multi-thread synchronization efficiency. These optimizations enable Java programs to achieve higher performance at runtime.

Jul 09, 2025 am 02:42 AM

How to handle out of memory errors?

When encountering insufficient memory error (OOM), you should first check the resource usage, optimize the code structure, and then configure the operating environment reasonably. Specific methods include: 1. Avoid creating a large number of temporary variables in the loop, use a generator instead of list comprehension, and close file handles and database connections in time; 2. For Python programs, you can manually call gc.collect() to assist garbage collection; 3. Appropriately adjust the runtime memory limit under the premise that the code is free of problems, such as Java setting JVM parameters, Python setting a reasonable memory limit using 64-bit interpreter, Docker setting; 4. Use tracemalloc, memory_profiler, VisualVM and other tools to locate memory bottlenecks,

Jul 09, 2025 am 02:42 AM

How to connect to a database using JDBC in Java?

The core steps in Java to use JDBC to connect to a database include: loading the driver, establishing a connection, executing SQL, processing results, and closing resources. The specific operations are as follows: 1. Add database driver dependencies. For example, MySQL requires mysql-connector-java; 2. Load and register JDBC drivers, such as Class.forName("com.mysql.cj.jdbc.Driver"); 3. Use DriverManager.getConnection() to establish a connection, pass in the URL, username and password; 4. Create a Statement or PreparedStatemen

Jul 09, 2025 am 02:26 AM

jdbc

What is a ThreadLocal in Java?

ThreadLocal is used in Java to create thread-private variables, each thread has an independent copy to avoid concurrency problems. It stores values ??through ThreadLocalMap inside the thread. Pay attention to timely cleaning when using it to prevent memory leakage. Common uses include user session management, database connections, transaction context, and log tracking. Best practices include: 1. Call remove() to clean up after use; 2. Avoid overuse; 3. InheritableThreadLocal is required for child thread inheritance; 4. Do not store large objects. The initial value can be set through initialValue() or withInitial(), and the initialization is delayed until the first get() call.

Jul 09, 2025 am 02:25 AM

java

What causes OutOfMemoryError: Java heap space?

OutOfMemoryError in Java applications: The main reason for Javaheapspace is insufficient heap memory, which is usually caused by low heap size settings, memory leaks, frequent creation of large objects or unreasonable GC configuration. 1. When the heap memory is insufficient, the -Xmx parameter can be adjusted to increase the maximum heap; 2. Memory leaks can be used to analyze the heap dump to find unreleased references; 3. Avoid creating large objects in the loop and use streaming or cache pools instead; 4. Select the appropriate GC type according to the load and optimize the parameter configuration. The solution order should be: increase the heap, troubleshoot leakage, optimize object usage, and adjust GC strategy.

Jul 09, 2025 am 02:19 AM

How does the Java Enum type work internally?

Java enums are essentially classes inherited from java.lang.Enum. Each enum value is a static final instance of the class and is initialized when the class is loaded. 1. Enumeration supports adding fields, construction methods, ordinary methods and abstract methods to enable them to carry data and encapsulate behavior; 2. Each enum instance implies two fields name and ordinal, representing the name and declaration order respectively. The ordinal value is determined by the declaration order and is not recommended for business judgment; 3. Enumerations rely on ordinal values ??in switch, changing the declaration order will affect logic; 4. Enumeration provides values() and valueOf() methods to obtain all instances and find instances based on names; 5.

Jul 09, 2025 am 02:09 AM

internal principles