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What is the Adapter pattern?

TheAdapterpatternsolvestheproblemofincompatibleinterfacesinsoftwaredevelopmentbyactingasabridgebetweenthem.Itallowsexistingclassesorthird-partylibrarieswithmismatchedinterfacestoworkseamlesslywithinasystemwithoutmodifyingtheiroriginalcode.Forexample,

Jun 28, 2025 am 01:41 AM

What is covariant return type?

Covariant return types allow subclasses to use more specific return types when rewriting parent class methods, improving code readability and polymorphic support. The core points are as follows: 1. It makes the return type of the subclass method more specific than the parent class (such as Dog instead of Animal); 2. It is available in Java 1.5 and C, but C#, Python, and JavaScript are not directly supported; 3. It is often used in factory methods, smooth interfaces and other scenarios to reduce casting; 4. When using it, it is necessary to ensure that there is an inheritance relationship between the return type and is not suitable for basic types and generic erasing environments.

Jun 28, 2025 am 01:39 AM

What is connection pooling?

Connection pooling improves performance by reusing database connections. It avoids frequent creation and destruction of connections, reduces latency, reduces server load, and prevents connection limits from being exceeded during peak traffic. Its working principle is: when applying the requested connection, if there is an available and matching connection in the pool, it will be returned directly, otherwise a new connection will be created; after use, the connection is marked as available rather than closed. Commonly found in Web applications and microservice architectures, such as Django, RubyonRails, Node.js and other frameworks. Pay attention to pool size configuration, idle connection timeout and connection leakage issues.

Jun 28, 2025 am 01:39 AM

What is a ListIterator?

ListIterator is a special iterator for traversing and modifying lists in Java. Its main advantage is that it can traverse in both directions. 1. It allows forward and backward traversal using next() and previous() methods; 2. Provide index tracking function to obtain the current position through nextIndex() and previousIndex(); 3. Support safely adding, deleting or replacing elements during the traversal process; 4. The initial position is located before the first element, always between elements rather than directly pointing to the element, so you can use the add() method to insert a new element at the current position. Note when using: you must check hasNext() or hasPrevious() before

Jun 28, 2025 am 01:37 AM

java

What is a functional interface?

Functional interface refers to an interface with only one abstract method in Java, which lays the foundation for the use of lambda expressions and method references. Its core significance is to allow functions to be processed as method parameters or code as data, so that the code is more concise, readable and flexible. The key to determining whether an interface is a functional interface is not the total number of methods, but the number of abstract methods it has: ? There is only one abstract method → ??functional interface; ?Two or more abstract methods → non-functional interfaces. Even if the interface contains default methods or static methods, these do not count to the total number of abstract methods. 1. It can be clearly identified through the @FunctionalInterface annotation, but not required. 2. Java has built-in multiple common

Jun 28, 2025 am 01:36 AM

Can an enum have methods constructors or fields?

Yes, enums in Java can have methods, constructors, and fields. Specifically, it includes: 1. Enumeration can add field values ??to each constant through a private constructor, such as adding an abbreviation name to the date of each week; 2. Enumeration can define constructors, which must be private or package-private, and be called once for each constant when the class is loaded, and parameters can be passed; 3. Enumeration can define methods like ordinary classes, such as custom comparison methods or overriding the toString method; 4. Enumeration cannot inherit other classes but can implement interfaces, and static auxiliary methods can be added for search operations. These features make Java enumeration powerful and flexible.

Jun 28, 2025 am 01:35 AM

Difference between String StringBuffer and StringBuilder?

The difference between String, StringBuffer and StringBuilder in Java is that: 1. String is immutable, and a new object is created every time it is modified, suitable for unchanged data; StringBuffer and StringBuilder are variable, suitable for frequent modifications. 2.StringBuffer is thread-safe but has low performance, suitable for multi-threaded environments; StringBuilder is non-threaded but faster, suitable for single-threaded scenarios. 3. The three share append, insert, delete and other methods, which are easy to switch when using. 4. Use suggestions: Use String when the data remains unchanged; Use StringBuil for frequent modification of single thread

Jun 28, 2025 am 01:33 AM

What is PermGen space? (Note: Mentioning it's removed in newer Java versions might be needed for a full answer but keep the question simple).

The main reasons for the problems with PermGen are its fixed size limitations and excessive class loading. In Java7 and previous versions, PermGen is a fixed area in JVM heap memory used to store class metadata, static variables, etc. When applications frequently redeploy, use reflection or dynamic generation classes (such as Spring, Hibernate) or third-party libraries to load a large number of classes, it is easy to raise java.lang.OutOfMemoryError:PermGenspace error. 1. Increasing PermSize and MaxPermSize parameters can alleviate the problem; 2. Reduce unnecessary class loading and duplicate deployment; 3. Use CMS garbage collector and enable class unloading mechanism; 4. Check

Jun 28, 2025 am 01:31 AM

What is the heap space?

Heapspace is a memory area where data is stored dynamically when a program is run, especially in languages ??such as Java. ① It is different from the stack and is used to manage more complex and longer life-cycle objects such as strings, arrays and custom data structures. ② The heap memory is automatically managed through the garbage collection mechanism. When the object is no longer referenced, the memory it occupies will be released. ③If the object is continuously created without releasing the old object, it may result in OutOfMemoryError. ④ You can configure the maximum heap size through command line parameters (such as Java's -Xmx), but too small the heap will affect performance, and too large the heap will waste resources. ⑤ Common reasons for insufficient heap space include memory leaks, unlimited caches and excessive loading of large data at one time. ⑥ Optimization method includes using

Jun 28, 2025 am 01:29 AM

How to use `Lock` interface?

Compared with synchronized, the Lock interface provides more flexible thread synchronization control. 1. Common implementation classes include ReentrantLock (reentrant lock), ReentrantReadWriteLock and WriteLock (read-write separation lock) and StampedLock (efficient read-write lock that supports optimistic reading). 2. The steps to use are: create a Lock instance, call lock() to add a lock, execute critical area code, and finally call unlock() to release the lock. 3. Compared with synchronized, Lock supports trying to add locks (tryLock), timeout waiting (tryLock(time)

Jun 28, 2025 am 01:20 AM

java concurrency Lock interface

What is JUnit?

JUnit is a testing framework mainly used in Java applications, and its core role is to support automated unit testing. Reasons for using JUnit include: 1. Supports automated testing to facilitate discovering regression problems caused by code changes; 2. Simple writing and define testing methods through @Test annotation; 3. Good integration with mainstream IDEs and build tools; 4. Have extensive community support. JUnit's key components include @Test, assertion methods (such as assertEquals), and annotations for pre- and post-test execution (such as @BeforeEach and @BeforeAll). It is suitable for unit testing scenarios, such as in TDD development, in continuous integration processes, or in regression testing.

Jun 28, 2025 am 01:16 AM

When does the `finally` block execute?

Finally blocks will be executed in programming regardless of whether an exception is thrown or not. The main function is to ensure that the cleanup code has a chance to run. 1. The finally block will run after the execution of the try and catch blocks. It will be executed even if an exception occurs and is processed, no exception occurs, or is returned from the try/catch. 2. If there is a return statement in a try or catch, finally will still be executed before the method actually returns, but the return in it may overwrite the original return value and should be avoided. 3. The most common use is resource cleaning, such as closing files, database connections, etc. to prevent resource leakage. 4. Before Java7, you need to manually write try-catch-finally for resource management.

Jun 28, 2025 am 01:05 AM

Why do we need wrapper classes?

Java uses wrapper classes because basic data types cannot directly participate in object-oriented operations, and object forms are often required in actual needs; 1. Collection classes can only store objects, such as Lists use automatic boxing to store numerical values; 2. Generics do not support basic types, and packaging classes must be used as type parameters; 3. Packaging classes can represent null values ??to distinguish unset or missing data; 4. Packaging classes provide practical methods such as string conversion to facilitate data parsing and processing, so in scenarios where these characteristics are needed, packaging classes are indispensable.

Jun 28, 2025 am 01:01 AM

Can a class have multiple main methods?

Yes,aclasscanhavemultiplemainmethodsthroughmethodoverloading,butonlyonewiththeexactsignaturepublicstaticvoidmain(String[]args)servesastheentrypoint.Otheroverloadedversionslikemain(Stringargs)ormain(int[]args)aretreatedasregularstaticmethodsandmustbec

Jun 28, 2025 am 12:58 AM