High-frequency questions for Java interviews are mainly focused on basic syntax, object-oriented, multithreaded, JVM and collection frameworks. The most common questions include: 1. There are 8 basic Java data types, such as byte, short, int, long, float, double, char and boolean. It is necessary to note that String is not the basic data type; 2. Final is used to modify classes, methods or variables to represent immutable, and finally used to ensure code execution in exception processing. Finalize is an Object class method for cleaning before garbage collection; 3. Multi-thread synchronization can be implemented through synchronized keywords, ReentrantLock, volatile variables and concurrent tool classes; 4. The JVM memory model mainly includes heap, stack, method area and other areas. The garbage collection mechanism involves accessibility analysis, mark-cleaning algorithms, and collectors such as CMS and G1. Mastering the core principles and practical applications of these questions will help improve interview performance.

As a widely used programming language, Java is often highlighted during interviews. Many developers will encounter a common question when preparing for an interview: Which Java knowledge points are the easiest to be asked about? In fact, the real high frequency problems mainly focus on basic syntax, object-oriented, multi-threaded, JVM and collection frameworks. Let’s take a look at some of the most common Java interview questions and give some practical understanding and answer suggestions.

1. What are the basic data types in Java?

This is one of the most common entry-level questions in an interview. Although it seems simple, many people will answer incorrectly or miss details.

Java has 8 basic data types, namely:

• byte (1 byte)
• short (2 bytes)
• int (4 bytes)
• long (8 bytes)
• float (4 bytes)
• double (8 bytes)
• char (2 bytes)
• boolean (generally considered 1 bit, but depends on the JVM implementation)

Note: Many people mistakenly think that String is a basic data type, but in fact it is a reference type and belongs to a class.

Behind this question is actually to understand your basic understanding of the Java type system. If you can mention their size and default types (such as int and double ), that will add a lot of points.

2. What is the difference between final, finally and finalize?

This is a confusing but very typical Java basic problem.

• final : used to modify classes, methods or variables, indicating that they are not inheritable, cannot be rewritable or cannot be modified.
• finally : Usually used with try-catch , the code blocks in it will be executed regardless of whether an exception occurs (unless the JVM is forced to exit).
• finalize() : is a method of Object class. The garbage collector will call it before recycling the object to free resources. However, relying on this mechanism is not recommended because behavior is uncertain.

Tip: You can give examples of finally 's role in resource closure, such as closing file flow or database connection.

This question not only tests memory, but may also extend deeper content such as exception handling mechanisms and resource management.

3. How to implement thread synchronization in multithreading?

Concurrent programming in Java is the highlight of the interview, and thread synchronization is the core knowledge point.

Common implementation methods include:

• Use synchronized keyword: it can modify methods or code blocks to ensure that only one thread executes at the same time.
• Use ReentrantLock : More flexible than synchronized , supports advanced features such as trying locks and timeouts.
• Use volatile : suitable for scenarios where variable state changes need to be immediately visible, but cannot replace locks.
• Use concurrent tool classes: such as CountDownLatch , CyclicBarrier , Semaphore , etc.

In actual development, it is recommended to use the tool classes under java.util.concurrent package instead of manually implementing complex synchronization logic by yourself.

This problem may extend to deadlocks, thread pools, CAS, AQS, etc. It is recommended to prepare these relevant knowledge points in advance.

4. JVM memory model and garbage collection mechanism

JVM-related knowledge is almost a must-ask for mid-to-high-level Java development positions.

The main memory areas of the JVM include:

• Method area (metaspace)
• heap
• Stack
• Local method stack
• Program Counter

About Garbage Collect (GC):

• Garbage collection is mainly for heap memory.
• The main algorithms for determining whether the object is garbage include citation counting and accessibility analysis.
• Common garbage collection algorithms: mark-clear, copy, mark-organize.
• Different generations (Queen generation, old generation) use different GC algorithms and collectors (such as Serial, Parallel, CMS, G1).

If you can tell the situation and investigation ideas about the OOM or frequent Full GCs you have encountered based on actual projects, it will impress the interviewer even more.

Basically that's it. These questions seem common, but the knowledge points involved are very profound. When preparing, you must not only remember the answers, but also understand the principles and practical application scenarios behind them. Master these and be able to handle most Java interviews with ease.

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