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How to convert primitive to wrapper object?

In Java, the basic type to wrapping class is mainly achieved through automatic boxing and manual conversion. 1. Automatic boxing (such as Integerinteger=10) is automatically completed by the compiler, suitable for all basic types, but may affect performance; 2. Manual conversion uses valueOf method (such as Integer.valueOf(age)), which is more clear and often used for collection operations; 3. Pay attention to the cache mechanism, such as Integer caches objects from -128 to 127. .equals() should be used instead of == when comparing to avoid reference judgment errors. These mechanisms ensure that primitive types can be used in scenarios where objects are needed, especially in generics.

Jul 02, 2025 am 01:32 AM

What is a `CountDownLatch`?

CountDownLatch is used for thread synchronization in Java, and its core mechanism is to control thread execution through counters. It sets the count when initializing, calls countDown() to count down, and calls await() to make the thread wait until the count returns to zero. 1. Applicable to multi-threaded coordination task completion, unified startup signal and service readiness check; 2. Once the count is zeroed, it cannot be reset, and only a single time is valid; 3. In the example, two threads trigger countDown() after executing the task, and the main thread calls await() to wait for the two to complete to ensure that the task order is synchronized.

Jul 02, 2025 am 01:32 AM

What is a constructor?

Aconstructorisaspecialmethodusedtoinitializeobjectswhentheyarecreated.Itensuresthatnecessarysetuporpropertyassignmentshappenautomatically,preventingincompleteorinvalidstates.Keypointsinclude:1)Constructorshavethesamenameastheclassandnoreturntype.2)Th

Jul 02, 2025 am 01:32 AM

Constructor

What is the `enum` type in Java?

Enums in Java are special classes that represent fixed number of constant values. 1. Use the enum keyword definition; 2. Each enum value is a public static final instance of the enum type; 3. It can include fields, constructors and methods to add behavior to each constant; 4. It can be used in switch statements, supports direct comparison, and provides built-in methods such as name(), ordinal(), values() and valueOf(); 5. Enumeration can improve the type safety, readability and flexibility of the code, and is suitable for limited collection scenarios such as status codes, colors or week.

Jul 02, 2025 am 01:31 AM

java enum

What is a `CyclicBarrier`?

CyclicBarrierinJavaisasynchronizationtoolthatmakesmultiplethreadswaitatacommonpointuntilallhavearrived,thenreleasesthemtogether.1)Itworksbyinitializingwithanumberofthreads(parties),eachcallingawait()toblockuntilallreachthebarrier.2)Onceallarrive,they

Jul 02, 2025 am 01:31 AM

What is a TreeSet?

TreeSet is a collection class in Java, which realizes automatic sorting and deduplication of elements through red and black trees. Its internal is based on a self-balancing binary search tree (red and black tree), and supports insertion, deletion and search operations of O(logn) time complexity; automatically sorting according to natural order or custom comparator when adding elements; suitable for scenarios where elements are always ordered, quickly retrieve extreme values ??and avoid duplication, such as ranking lists or priority queues; basic operations include add, first, last, floor, ceiling, etc.; limitations include performance overhead greater than HashSet, non-thread-safe, and no null value (unless processed by custom comparator); and only apply to Java language.

Jul 02, 2025 am 01:31 AM

What is a soft reference?

Soft references are used in Java for memory-sensitive caches, allowing objects to be garbage collected to avoid memory overflow. When an object is reachable only by soft references, the JVM recycles the object when it is out of memory. For example, it is used to cache images, temporarily store calculation results, or manage resources that are difficult to reconstruct. Use the SoftReference class to create a soft reference, always check whether the get() return is null, and be ready to recreate the object. Unlike weak references, soft references are not recycled until memory is tight, while weak references are recycled the next time GC is recycled. When using it, you should avoid relying on it to save critical states, and do not use it excessively, and can be cleaned and tracked in combination with ReferenceQueue.

Jul 02, 2025 am 01:30 AM

What is an ArrayList?

AnArrayListinJavaisadynamiclistthatautomaticallyresizesitself,offeringflexibilityoverfixed-sizearrays.1)Itusesanunderlyingarraytostoreelementsandresizesautomaticallywhenfull.2)Youcaneasilyadd,remove,orreplaceelementsusingmethodslike.add(),.remove(),.

Jul 02, 2025 am 01:30 AM

Difference between soft weak and phantom references?

The difference between soft citations, weak citations and virtual citations in Java is their life cycle and purpose. 1. Soft References are used for cache and will be recycled when there is insufficient memory; 2. Weak References are recycled in the next GC and are suitable for short-term object tracking; 3. PhantomReferences cannot be retrieved and are only used for cleaning operations before object recycling. They are suitable for memory management requirements in different scenarios.

Jul 02, 2025 am 01:30 AM

What is a multidimensional array?

A multi-dimensional array is essentially an array of arrays that can store data from multiple dimensions. 1. It is suitable for representing complex data structures such as tables and matrices; 2. Two-dimensional arrays are common and shaped like grids, such as using rows to represent students and columns to represent subject scores; 3. Practical applications include game development, image processing, scientific computing and other fields; 4. When using them, you need to pay attention to the differences in index range and language implementation.

Jul 02, 2025 am 01:29 AM

What is the Decorator pattern?

The decorator pattern is used to dynamically add new functionality to objects, and its core lies in flexible expansion through composition rather than inheritance. When you need to combine functions in different ways (such as encryption, compressed messages), avoid code confusion caused by factor explosions. The decorator mode implements function overlay by wrapping the original objects layer by layer, while maintaining a unified interface. The specific steps are: 1. Define the public interface or base class (such as IMessage); 2. Create basic components (such as TextMessage); 3. Build abstract decorator classes, hold component references and implement the same interface; 4. Implement specific decorators (such as EncryptedMessageDecorator, CompressedMessageDecorator)

Jul 02, 2025 am 01:29 AM

Design Patterns

What is the Observer pattern?

The Observer pattern is a design pattern used to establish one-to-many dependencies between objects. Its core is that when an object's state changes, all dependencies (Observers) will automatically receive notifications and updates. This pattern contains four key components: Subject maintains the observer list and sends notifications; Observer defines the interface to receive updates (such as the update() method); ConcreteSubject implements specific notification logic; ConcreteObserver implements specific response behavior. Common application scenarios include user interface updates, event processing systems, and service notifications in distributed systems. The implementation steps are: 1. Define Obs

Jul 02, 2025 am 01:28 AM

How to generate Javadoc?

The key to generating Javadoc is to write good comments and master the basic commands and formats. 1. First, use the standard Javadoc comment block (starting with /* and ending with /) in the code, place it in front of the class, method, and field, and include summary sentences; 2. Use common tags such as @param, @return, @throws to describe parameters, return values ??and exceptions; 3. Use javadoc commands to generate documents through the command line. Common parameters include -d specifying the output directory, -author displaying the author, and -version displaying the version; 4. You can use the IDE to automatically generate documents, which is more convenient to operate. As long as you comment on specifications and be familiar with commands, you can easily generate clear API documents.

Jul 02, 2025 am 01:28 AM

How are equals and hashCode related?

Equals and hashCode must be rewritten at the same time, because if the two objects equals, their hashCodes must be the same, otherwise there will be a problem that the object cannot be located in HashMap or HashSet. 1. Both need to be rewritten based on the same field; 2. HashCode determines the storage location, and equals determines whether it matches; 3. After modifying the object field, it may cause inconsistent hash values, so try to use immutable fields to calculate hash; 4. Using Objects.hash can simplify the implementation and avoid null pointer exceptions.

Jul 02, 2025 am 01:27 AM