Analysis of knowledge points for Java Development job interviews and java knowledge points for well-known Internet companies

Analysis of knowledge points for Java Development job interviews and java knowledge points for well-known Internet companies

If there is more damage than enough to make up for the problem, it is impossible to win the truth.

The author of this article has participated in many interviews within one year. All candidates are for Java development. During Continuous interviews, I classified and summarized some knowledge points in the Java Development job interview.

It mainly includes the following parts:

Interview is the first step from school to society.

Internet companies' campus recruitment is generally divided into two to three rounds of technical interviews and one round of HR interviews. However, some companies do not have an HR interview, but they simply have three rounds of technical support.

During a technical interview, the interviewer will first examine the relevant knowledge of the position you apply for, also known as basic knowledge and business logic interviews. As long as you don't have a very bad answer, the interviewer usually says, "Let's write a code." At this time, we started the algorithm interview.

That is to say, a round of technical interview = basic knowledge and business logic interview + algorithm interview.

In this article, we will talk about technical interviews. Technical interviews include: business logic and basic knowledge interviews.

The first is the business logic interview, that is, the project.

The interviewer will share some of the items on your resume with you. During this period, we will perform in-depth mining on what you have done.

Including: Why? What are you going to do if you try again? And so on. This step mainly checks whether we have a clear understanding of our own projects (practice projects or internal projects.

For business logic interview preparation, we recommend that you think more about and summarize the data sources and overall runtime framework of the project.

For example, you can summarize the project during your internship in a company without having to wait until you leave the company.

Next is the basic knowledge interview.

Java Development is a background development direction. Some people say that background development is very difficult because there are too many things to learn. That's right. This job requires learning a lot. Including: the basics of the language (Java/C ++/PHP), databases, network protocols, Linux systems, computer principles, and even front-end knowledge can all examine you, and is not beyond the outline.

Sometimes, you report back-end development posts and are familiar with the Java language, but the interviewer is in the C ++ development direction, so helpless ~

Well, let's talk less about it. Let's start to classify and explain common interview knowledge points.

(1) Basic Java knowledge points

1) What are the object-oriented features?

A: encapsulation, inheritance, and polymorphism.

2) What does overwriting and overloading mean in Java?

Analysis: Coverage and overloading are important basic knowledge points and are easy to confuse. Therefore, they are common in interviews.
A: Override refers to a subclass that overrides the parent class method. It can only throw fewer exceptions than the parent class, and the access permission cannot be smaller than the parent class.

The method to be overwritten cannot be private. Otherwise, a new method is defined in the subclass. Overload indicates that multiple methods with the same name can exist in the same class, however, the parameter lists of these methods are different.

Interviewer: What are the conditions for heavy load?

A: The parameter types, numbers, and order are different.

Interviewer: Can the return values of a function be different to that of an overload? Why?

A: No, because a value assignment is not required to call a function in Java. Example:

The following two methods:

　　　　void f(){}　　　　int f(){ return 1;}

As long as the compiler can clearly determine the semantics based on the context, for example, in int x = f ();, then it can indeed distinguish the overloaded method accordingly. However, sometimes you don't care about the return value of a method. What you want is other effects of a method call (this is often called "called for side effects "), in this case, you may call the method and ignore the returned value. Therefore, if you call the method as follows:

　　　　fun();

In this case, how can Java determine which f () is called? How do people understand this code? Therefore, it is impossible to distinguish between overloaded methods based on the return values of methods.

3) What are the differences between abstract classes and interfaces?

A:

Interviewer: How to Choose abstract classes and interfaces?

A:

4) differences between Java and C ++:

Analysis: although we do not know much about C ++, we will ask this question, especially during the interview on three sides (Director level.

A:

5) passing values and references in Java

Explanation: the interviewer will hand-write an example to show you the function execution result. For details, refer to my blog: Basic Analysis of Java value transfer and reference transfer.

A: value transfer means that the object is passed with a value, which means that a copy of the object is passed. Even if the copy is changed, the source object is not affected. Reference transfer refers to the transfer of an object by reference, which means that it is not a real object but a reference of an object.

Therefore, changes made to the referenced objects are reflected on all objects.

6) which of the following packages are commonly used in JDK?

A: java. lang, java. util, http://java.io, http://java.net, java. SQL.

7) connection and difference between JDK, JRE and JVM:

A: JDK is a java development kit and a core component of the java development environment. It provides all the tools required to compile, debug, and run a java program, including executable files and binary files, is a platform-specific software.

JRE is a java Runtime Environment and JVM implementation. It provides a platform for running java programs. JRE includes JVM, but does not include development tools such as java compiler/debugger.

JVM is a java Virtual Machine. When we run a program, JVM is responsible for converting bytecode into code of a specific machine. JVM provides memory management, garbage collection, and security mechanisms.

This is independent of hardware and operating systems, which is precisely why java programs can write multiple executions at a time.

Differences:

Others: limited space. The basic Java knowledge points in the interview include reflection, generics, and annotations.

Summary: This section describes the basic knowledge points of Java. These questions are mainly caused by the fact that the interviewer is not very difficult. It should be okay to review them properly.

(2) common Java collections

There are a lot of research efforts in this area. This part is a required knowledge point for the interview.

1) what are common collections?

A: The Map interface and the Collection interface are the parent interfaces of all Collection frameworks:

(2) What are the differences between HashMap and Hashtable? (Required)

A:

3) Do you know the underlying implementation of HashMap?

A: Before Java 8, its underlying implementation is array + linked list implementation. Java 8 uses array + linked list + red/black tree implementation. In this case, you can easily draw and analyze images on paper:

4) What is the difference between ConcurrentHashMap and Hashtable? (Required)

A: ConcurrentHashMap combines the advantages of HashMap and HashTable. HashMap does not consider synchronization. hashtable considers synchronization. However, hashtable locks the entire structure during each synchronization. The ConcurrentHashMap lock method is slightly fine-grained. ConcurrentHashMap divides the hash table into 16 buckets (default value). Common Operations such as get, put, and remove only lock the buckets currently used.

Interviewer: Do you know the specific implementation of ConcurrentHashMap?

A:

5) Why is the length of HashMap a power of 2?

A:

Worse, in this case, the positions available for the array are much smaller than the array length, which means the collision probability is further increased and the query efficiency is slowed down! This will result in a waste of space.

6) What is the difference between List and Set?

A: The List elements are ordered and can be repeated. The Set elements are unordered and cannot be repeated.

7) initial capacity and loading Factor of List, Set, and Map

A:

1. List

• The initial capacity of ArrayList is 10, the loading factor is 0.5, the expansion increment is 0.5 times of the original capacity + 1, and the length is 16 after one expansion.
• The initial Vector capacity is 10, and the loading factor is 1. Expansion increment: the original capacity is doubled. For example, the Vector capacity is 10, and the capacity is 20 after one expansion.

2. Set

HashSet: the initial capacity is 16, and the loading factor is 0.75. Expansion increment: 1 time of the original capacity. For example, the size of HashSet is 16, and the size is 32 after one expansion.

3. Map

HashMap, initial capacity 16, loading Factor 0.75; expansion increment: 1 time of the original capacity; for example, the HashMap capacity is 16, and the capacity is 32 after one expansion

8) What is the difference between the Comparable interface and the Comparator interface?

A:

9) The fast failure mechanism of the Java set "fail-fast"

A: It is an error detection mechanism for java sets. When multiple threads change the structure of the set, the fail-fast mechanism may be generated.

For example, if there are two threads (thread 1 and thread 2), thread 1 traverses the elements in set A through Iterator, at A time, thread 2 modifies the structure of set A (modification above the structure, rather than simply modifying the content of the Set element). At this time, the program will throw A ConcurrentModificationException, this produces the fail-fast mechanism.

Cause: the iterator directly accesses the content in the set during the traversal and uses a modCount variable during the traversal process. If the content of the Set changes during traversal, the value of modCount is changed.

Every time the iterator uses hashNext ()/next () to traverse the next element, it checks whether the modCount variable is the expectedmodCount value. If yes, it returns the traversal; otherwise, an exception is thrown and the traversal ends.

Solution:

Summary: This section describes the collection in Java and is a required knowledge point for Java job interviews. In addition to the above questions, it is recommended that you read the underlying implementation of each collection, deepen understanding.

(3) High concurrency programming-JUC package

In Java 5.0, java. util. the concurrent (JUC) package adds a common Utility Class for concurrent programming to define a thread-like custom subsystem, including thread pool, asynchronous IO, and lightweight task framework.

1) Differences and connections between multithreading and a single thread:

A:

Conclusion: multithreading does not increase the program execution speed, but reduces the speed. However, it can reduce the user response time.

2) How to specify the execution sequence of multiple threads?

Explanation: the interviewer will give you an example. How can we make 10 threads print 0123456789 in order? (Write code implementation)

A:

3) differences between threads and processes: (required)

A:

• (1) During process switching, the CPU environment of the current process is saved and the CPU environment settings of the newly scheduled running process are involved.
• (2) For thread switching, only a small amount of register content needs to be saved and set, and storage management operations are not involved.

4) What are the four necessary conditions for multi-thread deadlock?

A:

Interviewer: How to avoid deadlocks? (I often continue to ask this question ~)

A: Specify the order in which the lock is obtained, for example:

5) differences between sleep () and wait (n) and wait:

A:

6) synchronized keyword:

Answer: underlying implementation:

Meaning: (monitor mechanism)

Synchronized is locking and applying the object lock. The object lock is a weight lock (monitor). The synchronized lock mechanism will biased towards the lock (single thread) During Running Based on thread competition) lightweight locks (multiple threads access the synchronized region), Object locks (weight locks, where multiple threads compete), and spin locks.

This keyword is the encapsulation of several locks.

7) volatile keywords

A: This keyword ensures that visibility does not guarantee atomicity.

Function:

Resolution: for the issue of command re-sorting, you can refer to the DCL double check lock failure related information.

8) ThreadLocal (thread local variable) Keyword:

A: When ThreadLocal is used to maintain a variable, it provides an independent copy of the variable for each thread that uses the variable. Therefore, each thread can change its own copy independently, it does not affect the copies corresponding to other threads.

ThreadLocal internal implementation mechanism:

9) Atomic keywords:

A: The basic data type can be atomic to perform auto-increment, auto-increment, and other operations. Refer to my blog: Use of class AtomicInteger under the concurrent. atomic package.

10) Do you know about the thread pool? (Required)

A: The java. util. concurrent. ThreadPoolExecutor class is a thread pool. The client calls ThreadPoolExecutor. submit (Runnable task) to submit the task. The number of worker threads maintained in the thread pool is the size of the thread pool in the thread pool. There are three forms:

• Current thread pool size: the actual number of worker threads in the thread pool;
• MaxinumPoolSize: indicates the maximum number of worker threads allowed in the thread pool;
• Core Thread size (corePoolSize): indicates the maximum number of worker threads not greater than the maximum thread pool size.

Summary: This section covers multi-thread programming and thread security in Java, which is a key and difficult part of the interview.

(4) JVM Memory Management

Since it is a Java Development interview, it is of course necessary to evaluate the JVM. The interviewer generally asks you if you have any knowledge of JVM?

I usually say everything I know, including JVM memory division, JVM garbage collection, and GC algorithms, the characteristics of the young generation and the old generation are described in detail.

1) JVM memory Division:

2) meanings of parameters similar to-Xms and-Xmn:

A:

Heap memory allocation:

Non-heap memory allocation:

3) What are the garbage collection algorithms?

A:

This algorithm suspends the entire application and generates memory fragments;

This algorithm only processes objects in use at a time. Therefore, the replication cost is relatively small, and the corresponding memory can be organized after the replication, without the "Fragmentation" problem. Of course, the disadvantage of this algorithm is also obvious, that is, it requires two times of memory space;

This algorithm avoids the "tag-clear" fragmentation problem, and also avoids the space problem of the "copy" algorithm.

4) which of the following root search algorithms can be used as root?

A:

• Classes loaded by the bootstrap loader and created objects;
• Referenced objects in JavaStack (Objects referenced in stack memory );
• Objects pointed to by static references in the method area;
• Objects referenced by constants in the method area;
• The object referenced by JNI in the Native method.

5) When does GC start?

A: the frequent occurrence of GC is the heap zone. The heap zone can also be divided into the new and old generations. The new generation is also divided into an Eden zone and two vor zones.

A large object directly enters the old age, such as a long string array. The virtual machine provides a; XX: PretenureSizeThreadhold parameter, so that objects greater than this parameter value are directly allocated in the old age, avoid a large number of memory copies in the Eden zone and two vor zones;

6) Memory leakage and memory overflow

A:

Concept:

Cause Analysis of Memory leakage:

Summary: This section covers JVM virtual machines, including memory management and other knowledge. In addition to the above questions, the interviewer will continue to ask you some deep questions, probably to see where your limit is.
For example: Memory optimization, memory management, whether there are actual cases of Memory leakage, whether there is a real concern for memory, etc. Due to my lack of practical project experience, I have never been in touch with these deep-seated problems. You can check them online if necessary.

(5) Java 8 knowledge points

The interviewer will tell you how much you know about Java 8, which is explained below, and the new knowledge points of Java 8 answered during the interview.

1) The underlying implementation of HashMap has changed: HashMap is an array + linked list + red/black tree (the red/black tree section is added in JDK1.8.

2) In terms of JVM memory management, metadata replaces permanent generation.

Differences:

3) Lambda expressions (also called closures) allow us to pass functions as parameters to a method, or use code as data processing.

4) functional interface: refers to an interface with only one function, java. lang. runnable and java. util. concurrent. callable is an example of a functional interface. Java 8 provides a special annotation @ Functionallnterface to indicate that the interface is a functional interface.

5) Introduce repeated Annotations: Use the @ Repeatable annotation in Java 8 to define repeated annotations.

6) The default method can be implemented in the interface.

7) extended annotation application scenarios: Annotations can be used almost on any element: local variables, interface types, superclasses, and interface implementation classes, or even on function exception definitions.

8) New java. time package

Summary: some new features of Java 8, the interviewer generally does not require you to be proficient, mainly to see if you know something.

(6) network protocol Problems

In terms of network protocols, the most common scenarios include three handshakes between the server and client, status changes during four waves, network congestion control, and solutions.

1) three handshakes and four waves:

There are four statuses in total: active connection establishment, active disconnection, passive connection establishment, and passive disconnection

Two or four combinations:

2) Sliding Window Mechanism

In the three-way handshake phase, the sender and receiver tell each other the maximum data size they can receive. That is, the size of the Data receiving buffer pool. In this way, the other party can calculate whether the data volume can be sent.

During Processing, when the size of the buffer pool changes, you must send an update window notification to the recipient.

3) Congestion Avoidance Mechanism

Congestion: the demand for resources exceeds the available resources. If many resources in the network are insufficient at the same time, the network performance will deteriorate significantly, and the throughput of the entire network will decrease as the load increases.

Congestion Control: prevents excessive data from being injected into the network, so that the routers or links in the network are not overloaded.

Congestion Control Method:

• Slow Start + congestion avoidance;
• Fast retransmission + fast recovery.

4) What happened after entering "www.xxx.com" in the browser? Please elaborate.

Resolution: Classic Network Protocol problems.

A:

• The HTTP protocol is an application layer protocol based on TCP/IP. To request HTTP data, you must first establish a TCP/IP connection.
• It can be understood as follows: HTTP is a car that provides a specific form of encapsulation or display data; Socket is an engine that provides network communication capabilities.
• Communication between two computers is nothing more than data communication between two ports. The specific data will be displayed in what form is defined by different application layer protocols.

5) Common HTTP Status Codes

• 200 (successful)
• 304 (not modified): the requested webpage has not been modified since the last request. When the server returns this response, no webpage content is returned.
• 401 (unauthorized): authentication required for requests
• 403 (Forbidden): the server rejects the request.
• 404 (not found): the server cannot find the requested webpage

6) Differences Between TCP and UDP:

A:

For more information about network protocols, see my blog: FAQs about TCP/IP protocol interviews.

Conclusion: You must be familiar with the differences between TCP and UDP, three-way handshakes, and four-way switchover.

(7) database knowledge points

Since it is a backend development, database-related knowledge is also essential.

1) What are the differences between MySQL and MongoDB? How to choose?

2) What are the advantages and disadvantages of MongoDB?

(Ps I am not very familiar with this part, so I will not attach the reference answer. Please study it yourself ~)

3) Have you heard of transactions? (Required)

A: A series of operations performed as a single logical unit of work meet the following four features:

4) What are the concurrency issues of transactions?

A: updates, dirty reads, non-repeated reads, and Phantom reads are lost.

5) What are the locks in the database?
A: exclusive lock, exclusive lock, and update lock.

6) What are the transaction isolation levels?

A: Read uncommitted, Read committed, Repeatable read, and serialization.

Extended question: Which is the default isolation level for MySQL transactions?

A: It can be repeated.

Resolution: For a detailed answer to question (4) (5) (6), please refer to my blog: detailed explanation of database concurrency mechanism and transaction isolation level

(Ps. in-depth research on database transactions involves distributed transactions, that is, two-segment commit and three-segment commit. Please study at your own level)

7) What is the function of database indexing? (Required) What is the underlying data structure and why is this data structure used?

A:

Expansion question: What is the difference between clustered index and non-clustered index?

8) What are the differences between MyISAM and InnoDB?

A:

• MyISAM does not support transactions. InnoDB is a storage engine for transactions;
• MyISAM only supports table-level locks. BDB supports page-level locks and table-level locks. The default value is page-level locks. InnoDB supports row-level locks and table-level locks. The default value is Row-level locks;
• The MyISAM engine does not support foreign keys. InnoDB supports foreign keys;
• Tables of the MyISAM engine are often damaged in the case of a large number of highly concurrent reads and writes;
• MyISAM is more advantageous for count () queries;
• InnoDB is designed for the maximum performance when processing a large amount of data. Its CPU efficiency may be unmatched by any other disk-based relational database engine;
• MyISAM supports full-text indexing (FULLTEXT), not InnoDB;
• The efficiency of querying, updating, and inserting tables in the MyISAM engine is higher than that in InnoDB.

The main difference is that MyISAM tables do not support transactions, row-level locks, and foreign keys. InnoDB tables support transactions, row-level locks, and foreign keys. (You can answer this directly)

9) Where, group by, and having keywords in the database:

Answer: Keyword:

Having and where:

When the where clause, group by clause, having clause, and aggregate function are included at the same time, the execution sequence is as follows:

10) there are still some problems, such as the usage differences between MySQL and SQL Server, and the use of limit keywords.

Summary: The transaction mechanism and isolation level of the database are important. Of course, database indexes are mandatory. Occasionally, you will be given several tables, allowing you to write SQL statements on site, mainly investigating keywords such as group by and having.

(8) MVC Framework knowledge points

I used Spring MVC and MyBatis frameworks in the project, so I wrote only these two frameworks on my resume. The interviewer asked questions about these two frameworks. The following questions are for your reference.

Typical three-layer framework for Java Web development: Web layer, Service layer (business logic layer), and Dao layer (data access layer)

• Web layer: includes JSP, Servlet, and other Web-related content;
• Business Layer: focus only on business logic;
• Data Layer: encapsulates access details to the database.

Spring knowledge point

1) Do I know about Spring IOC and AOP?

A:

• IOC: Control inversion. (decoupling) the dependency between objects is handed over to the Spring container. The configuration file is used to create the dependent objects, and the active creation object is changed to the passive mode;
• AOP: This function separates functional code from the business logic code.

2) How does one implement AOP? How to choose? (Required)

A: JDK dynamic proxy implementation and cglib implementation.

Select:

Extension: How to Implement JDK dynamic proxy? (Plus points)

A: JDK dynamic proxy can only generate a proxy for the class that implements the interface, instead of for the class. All interfaces implemented for this target type will be proxies. The principle is to create an interface implementation class during running to complete the proxy for the target object.

Analysis: For more information about IOC and AOP, see my blog: Spring core AOP (for Aspect-Oriented Programming) Summary, Spring framework learning-control inversion (IOC)

3) What is the core controller of Spring MVC? What are the message processing processes?

A: The core controller is DispatcherServlet. The message process is as follows:

4) Other problems include the differences between redirection and forwarding, dynamic proxy and static proxy.

Mybatis knowledge point

For MyBatis, the difference between placeholder # And $ is evaluated as follows:

Summary: limited by the author's level, the MVC Framework does not know much about it and lacks practical capabilities. The interviewer Occasionally asks little about the underlying implementation principles of the framework, and the competent partner can learn more.

(9) big data-related knowledge points

Big Data is related because I wrote KafKa-related projects on my resume, so the interviewer will ask questions about KafKa-related knowledge points. I also summarized some simple concepts, the deep implementation principle is unknown because there is not much practical experience.

The following concepts are summarized for your reference.

1) Basic Features of KafKa:

A: it supports fast persistence, batch reading and writing of messages, and message partitions. It improves the concurrency capability, online addition of partitions, and creation of multiple copies for each partition.

Extension: why can I achieve fast persistence?

A: KafKa stores messages on disks and reads and writes messages to disks in sequence. This avoids the performance bottleneck caused by random read/write to disks (too long seeking time; the sequential read/write speed of the disk exceeds the random read/write speed of the memory.

2) core concepts:

A:

• Producer: generates messages and pushes them to the partition of the Topic according to certain rules.
• Consumer: pull messages from topics and consume them.
• Topic: a logical concept used to store messages. It is a set of messages.
• Partition ):

Copy (replica ):

Consumer Group: each consumer group belongs to a consumer group. Each message can only be consumed by one consumer in the Consumer group, but can be consumed by multiple consumer groups.

Broker:

Cluster & Controller:

Retention Policy and log compression:

About the copy mechanism: (additional points)

ISR set: a set of copies that are currently "available" and have similar message volumes to the Leader. The following conditions are met:

HW & LEO:

ISR, HW, and LEO work together:

Disaster Tolerance Mechanism of KafKa: The replica Leader and Follower are used to improve disaster tolerance.

Summary: Please prepare your own big data-related knowledge points based on your resume.

(10) common Linux commands

The author is not very proficient in this aspect. The knowledge points come from the network summary and the interviewer's questions. They are only for reference by small partners.

1) grep, sed, and awk commands

Analysis: awk commands can be mastered, which is a plus point in the interview.

2) files and directories:

Pwd displays the current directory

Ls displays the files and directories in the current directory:

3) File Processing Commands include touch, cp, In, mv, rm,

4) command for directory processing: mkdir

5) View file Content: file, cat, more, less, tail, head

6) Monitoring Program command: ps, top

Eg. Find all processes whose names contain java: ps-ef | grep java

Top Command real-time monitoring process

The first part of the top command output shows a summary of the system.

7) differences between ps and top commands:

8) Data Compression

9) terminate the process: kill PID or kill all

So far, I have explained the important knowledge points involved in the Java Development interview from ten different aspects. In addition to my last chat on algorithm interview, I will summarize and share my experience in the last year's interview test.

Next, I would like to talk to you about your resume to provide more help, including preparing your resume and posting your resume.

Prepare a resume:

First, I would like to introduce what my resume includes:

My resume should highlight my outstanding position, so I put my personal technical blog and github address in a relatively top position so that the interviewer or HR can see it at a glance.

If you have participated in any major competition and won the award, you can write it clearly.

I would like to give you a small reminder that you must have a certain understanding of the knowledge points written on your resume. If you do not understand it, do not write it, because you will be asked badly (even if one interviewer does not ask, ).

For example, if you write A technology A on your resume and say that you have used technology A in the project, the interviewer will ask the underlying implementation and principle of technology.

If you answer the question, it is true that this is a plus point, but we cannot answer it many times.

Submit resume

Posting resumes is equally important. For small friends who are not from a prestigious school, when posting resumes to some internet companies, they may face the possibility of being flushed. At this time, the importance of internal push is reflected.

Intranet push refers to internal recommendation. There are two benefits of Intranet push:

Internal push time point:

How to push data internally?

Internal push means you need to find internal staff to make recommendations. The internal push channel is mainly to find your senior brother and sister. In addition, you can always pay attention to the beiyou forum, the "largest university forum in China.

The beiyou forum has abundant resources and countless posts are pushed in the recruitment season each year. If you need it, please pay attention to it. There are also a large number of internal push messages in the discussion areas of Niuke network and the competition code network.

Conclusion

This is a long article. However, a long article does not cover all my experience in the interview test during the year. Finding a job is a long-running battle, and the winner is the final one.

For those who are devoted to Java development, the knowledge points mentioned in this article are definitely the focus of the interview. I hope you can master them effectively.

(Limited by the author level. If there are any mistakes in this article, please note that we will make progress together .)

I have a public account, and I often share some Java-related dry goods. If you like my share, you can search for "Java head" or "javatuanzhang" to follow up.