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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

What is the exception hierarchy?

Exception hierarchy refers to the exception type organized in a tree structure in programming, and its core is the base class such as Python's Exception or Java's Throwable. 1. The exception hierarchy starts with BaseException, Exception, etc., and derives more specific exceptions such as IOException or NullPointerException. 2. Through the hierarchy, developers can accurately catch specific exceptions, such as first catching ValueError and then handling general exceptions. 3. When customizing exceptions, you should inherit the appropriate base class. For example, creating AppError as the basis for custom errors, and further refine DatabaseError.

Jun 28, 2025 am 12:55 AM

How to handle stack overflow errors?

Stack overflow errors are usually caused by recursion without termination or excessive local variables. When checking, you should first check whether the recursion logic is correct, ensure that there are clear termination conditions and gradually approach the termination point, and use loops when necessary; secondly, consider adjusting the thread stack size, but be careful to avoid wasting resources; finally, avoid excessively large local variables in the function and use dynamic allocation instead. 1. The main reason for stack overflow is infinite recursion or local variables occupy too much stack space. 2. Repair recursive logic requires termination judgment, such as adding n to factorial function

Jun 28, 2025 am 12:47 AM

What is the strictfp keyword?

The strictfp keyword is used to ensure that floating point operations in Java produce the same results on all platforms, which achieve consistency by forcing compliance with the IEEE754 standard. 1. It limits the accuracy of intermediate floating point results to float or double type to avoid slight errors caused by hardware differences; 2. It can be applied to class or method levels, but not to variables or constructors; 3. It is suitable for financial, scientific computing and other scenarios that require cross-platform consistency, which may slightly affect performance. This keyword is usually not required if precise control of floating point behavior is not required.

Jun 28, 2025 am 12:45 AM

java strictfp

What are bitwise operators?

Bitwiseoperatorsmanipulateindividualbitsofbinarynumbers.TheyperformoperationslikeAND(&),OR(|),XOR(^),NOT(~),leftshift(),enablingprecisecontroloverspecificbitswithoutaffectingothers.Theseoperatorsareusedinrealcodetoefficientlymanagesettings,flags,

Jun 28, 2025 am 12:26 AM

What is a package in Java?

PackagesinJavaorganizecodeintounitstopreventnamingconflictsandimproveorganization.1.Packagesgrouprelatedclasses,interfaces,andsub-packages.2.Theyavoidnameclashesbyallowingsame-namedclassesindifferentpackages.3.Theycontrolaccessusingpackage-privatevis

Jun 28, 2025 am 12:16 AM