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What are Sealed Classes in Java?

Sealed classes are a feature introduced by Java 17 to limit which classes or interfaces can inherit or implement it. Its core role is to enhance control over inheritance by explicitly declaring allowed subclasses. Specifically: 1. Solve the problem that subclasses were not restricted at the language level before; 2. Support pattern matching (especially when combined with record classes); 3. Use sealed keywords and permits clauses to define allowed subclasses; 4. Subclasses must be declared final, sealed or non-sealed; 5. Applicable to closed type hierarchy, compile-time inspection and domain model design; 6. It is necessary to note that subclasses must be explicitly inherited in the same module or package. Sealing classes are suitable for scenarios that require strict inheritance control, but should not be abused.

Jul 08, 2025 am 02:42 AM

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Connecting to Databases Using Java JDBC

The key to connecting to a database with JavaJDBC is the driver, URL format and connection method. First, you need to introduce the corresponding database JDBC driver, such as MySQL uses mysql-connector-java, PostgreSQL uses postgresql.jar, Oracle uses ojdbc8.jar, and ensure that the version matches the database; second, you need to correctly configure the connection information, such as the URL format of MySQL is jdbc:mysql://hostname:port/database name? Parameter 1=value 1&parameter 2=value 2. Common problems include the time zone not set, SSL not closed, host name or port error; finally, pay attention to exception handling and resource release, use tr

Jul 08, 2025 am 02:41 AM

Database Connectivity

How to handle transactions in JDBC?

There are five steps to handle JDBC transactions: 1. Turn off automatic submission to start manual transactions; 2. Execute multiple database operations; 3. Submit transactions in normal times; 4 Rollback in abnormal times; 5. Use save points to control the intermediate state when necessary. By default, JDBC is in auto-commit mode and submits each SQL statement after execution. When multiple operations are involved in actual development, connect.setAutoCommit (false) should be called to turn off automatic submission so that all operations are in the same transaction. The subsequent operations can be submitted through connection.commit() or rollback back to ensure data consistency. It is recommended to place the key code at t

Jul 08, 2025 am 02:40 AM

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Deep Dive into the Java Virtual Machine Architecture

JVM is the core of Java program operation, including runtime data area, class loading mechanism, bytecode execution engine and garbage collection mechanism. 1. The runtime data area includes method area (metaspace after JDK8), heap (used to store object instances and perform garbage collection), stack (save thread method call information), local method stack (supports Native methods) and program counter (records the current instruction address). 2. The class loading mechanism consists of three ClassLoaders, Bootstrap, Extension and Application. It follows the parent delegation model and goes through five stages: loading, verification, preparation, parsing and initialization in turn to ensure the security and uniqueness of class loading. 3. Bytecode execution engine

Jul 08, 2025 am 02:38 AM

java virtual machine JVM architecture

Difference between `final`, `finally`, and `finalize` in Java.

In Java, final, finally and finalize are three keywords or methods that have different functions. 1. Final is used to restrict the modification or inheritance of variables, methods and classes to ensure immutability; 2. Finally used for code blocks in exception processing to ensure that resources release and other operations are always executed; 3. Finalize is an Object class method, which was used for cleaning before garbage collection, but has been deprecated. It is recommended to use AutoCloseable or try-with-resources instead. They are used to control invariance, ensure execution after exception handling, and replace resource cleaning methods.

Jul 08, 2025 am 02:30 AM

What is encapsulation?

EncapsulationinOOPisachievedbybundlingdataandmethodsintoasingleunitandcontrollingaccesstoanobject’sinternalstate.Itmattersbecauseithidesinternaldetails,allowsaccessonlythroughcontrolledmethods,andensuresdatavalidity.Toimplementit,fieldsaremadeprivate

Jul 08, 2025 am 02:29 AM

Implementing Producer-Consumer pattern using Java threads.

1. Using BlockingQueue is the most direct and recommended way to implement the Java producer-consumer model. It handles thread synchronization internally. The producer calls the put() method to block waiting queue space, and the consumer calls the take() method to block waiting data; 2. If the manual implementation requires the synchronized locking and wait/notify mechanism to coordinate thread behavior, the core is to loop check the conditions and operate the shared buffer within the synchronization block; 3. Notes include correct handling of interrupts, multi-threaded wake-up strategy to select notifyAll(), setting a reasonable buffer size, and elegantly closing threads.

Jul 08, 2025 am 02:28 AM

java thread Producer and Consumer

Mocking dependencies for Java testing with Mockito.

Mockito is a commonly used mocking framework in Java unit testing, used to simulate dependency behavior to avoid side effects caused by real calls. 1. Mock is to create a "fake" object instead of real dependencies, which facilitates control of return values, verify calls, and avoid external influences; 2. Use Mockito to create Mock objects through annotations or manual methods; 3. The core functions include when(...).thenReturn(...) to define behavior and verify(...) to verify calls; 4. Precautions include avoiding excessive mock, requiring additional tools to handle static methods, ensuring correct initialization, and verifying the number of calls. Mastering these core content can effectively improve testing efficiency and reliability.

Jul 08, 2025 am 02:25 AM

mockito java test

Deep Dive into Java Concurrency Primitives

Java's concurrent primitives include synchronized, volatile, atomic classes, CAS and LockSupport, which are the basis for building high concurrency applications. 1. Synchronized ensures atomicity and visibility through monitor locks, and uses memory barriers to prevent instruction reordering; 2. volatile ensures variable visibility and prohibits instruction reordering, suitable for status flags and singleton modes; 3. Atomic classes such as AtomicInteger implement a lock-free mechanism based on CAS, which is suitable for scenarios that read more and write less, but need to pay attention to ABA issues; 4. LockSupport provides the underlying support for thread suspension and wake-up, which is more flexible than wait/notify and does not require locks. Understand this

Jul 08, 2025 am 02:17 AM

How to perform unit testing in Java using JUnit?

JUnit is a common framework for Java unit testing. The steps are as follows: 1. Introduce JUnit dependencies, add corresponding configurations to Maven or Gradle; 2. Write test classes and methods, use @Test, @Before, @After annotations; 3. Execute tests and view the results, which can be run through the IDE or command line; 4. Follow test suggestions, such as clear naming, independent testing, and overriding boundary conditions. By mastering these key points, you can quickly get started with JUnit tests.

Jul 08, 2025 am 02:07 AM

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Managing Transactions in Java JDBC Applications

Managing transactions in JavaJDBC applications requires manual control of commits and rollbacks to ensure data consistency. 1. Turn off automatic commit: connection.setAutoCommit(false), so that multiple SQL operations can be executed as one transaction; 2. Use try-catch block to handle transaction commit or rollback to ensure that data is not partially committed during exceptions; 3. Restore the automatic commit mode after commit or rollback: connection.setAutoCommit(true), preventing problems caused by connection pool reuse; 4. It is recommended to use try-with-resources to release resources, avoid making complex logical judgments in finally blocks, thereby effectively managing transaction processes.

Jul 08, 2025 am 01:54 AM

Safe Casting and Type Compatibility in Java

The key to the security of Java type conversion is to match the inheritance relationship and the actual object, and the upward transformation is automatic and safe; the downward transformation requires explicit and instanceof inspection; generics are unreliable due to type erasure; and the interface and implementation classes can be converted.

Jul 08, 2025 am 01:54 AM

java type conversion

Detecting and Avoiding Deadlocks in Java Multithreaded Programs

Deadlock refers to the phenomenon that multiple threads cannot continue to execute because they are waiting for each other's resources. Its generation requires four conditions: 1. Mutual exclusion, resources cannot be shared; 2. Hold and wait, threads do not release the occupied resources while waiting for other resources; 3. It cannot be preempted, resources can only be actively released by the holding thread; 4. Loop waiting, thread chains are waiting for each other. To detect deadlocks, you can view the "DEADLOCK" prompt in the thread stack through the jstack command, or use IDE tools, VisualVM and other visual tools to analyze. Methods to avoid deadlocks include: 1. Unify the locking order to break loop waiting; 2. Set a timeout mechanism, such as using tryLock(); 3. Reduce the granularity and range of locks; 4. Use Reent

Jul 08, 2025 am 01:43 AM

Deadlock avoidance java multithreading

What are the key features of Java 8?

Java8introducedmajorfeaturesthatenhancedcodeefficiencyandreadability.1.Lambdaexpressionsallowwritingconcisecodebytreatingfunctionalityasmethodarguments,reducingboilerplate.2.TheStreamAPIenablesdeclarativeprocessingofcollectionswithoperationslikefilte

Jul 08, 2025 am 01:18 AM

characteristic java 8