Android TV development! Three years of experience in Android development, a summary of general popular frameworks

Preface

Since graduating in 18 years, I have worked for two companies and have done a few large and small projects. I am very grateful to my two bosses for giving me a lot of guidance on my android growth path and the relationship between teachers and friends.

I have participated in interviews with many companies since years ago, and have also received offers from several giants, as well as from other companies. Summarizing the experience is also a review and summary of the past.

Network: layered model, TCP, UDP, HTTP, HTTPS

Hierarchical model

• Application layer: responsible for handling specific application details, such as HTTP, FTP, DNS
• Transport layer: Provide end-to-end basic communication for two hosts, such as TCP, UDP
• Network layer: control packet transmission, routing, etc., such as IP
• Link layer: operating system device drivers, network card related interfaces

UDP

• UDP header structure: source port, destination port, length field, checksum
• Features: unreliable, disorderly, message-oriented, fast, lightweight
• Applicable scenarios: suitable for instant messaging, video calls, etc.
• Application: DHCP, DNS, QUCI, VXLAN, GTP-U, TFTP, SNMP

TCP

• TCP header structure: source port, destination port, sequence number, confirmation sequence number, SYN/ACK and other status bits, window size, checksum, emergency pointer
• Features: Oriented to byte stream, with congestion and flow control, reliable, orderly, slow, and heavier. Flow control and congestion control are realized through sliding window
• Applicable scenarios: file transfer, browser, etc.
• 应用：HTTP、HTTPS、RTMP、FTP、SMTP、POP3
• Three handshake:

1\. C->S：SYN，seq=x（你能听到吗？）
2\. S->C：SYN，seq=y，ack=x+1（我能听到，你能听到吗？）
3\. C->S：ACK，seq=x+1，ack=y+1（我能听到，开始吧）

两方都要能确保：我说的话，你能听到；你说的话，我能听到。所以需要三次握手
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• Wave four times:

1\. C->S：FIN，seq=p（我说完了）
2\. S->C：ACK，ack=p+1（我知道了，等一下，我可能还没说完）
3\. S->C：FIN，seq=q，ACK，ack=p+1（我也说完了）
4\. C->S：ACK，ack=q+1（我知道了，结束吧）

S 收到 C 结束的消息后 S 可能还没说完，没法立即回复结束标示，只能等说完后再告诉 C ：我说完了
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HTTP

• Hypertext transfer protocol, clear text transmission, default port 80
• POST and GET: Get parameters are placed in the url; Post parameters are placed in the request body
• Web page access process: DNS domain name resolution, TCP three-way handshake to establish a connection, initiate HTTP request

HTTPS

• The default port is 443, and the HTTP transmission data is encrypted using the SSL protocol, which is safe
• Encryption process: Client/Server generates a key through asymmetric encryption, and then uses this key to symmetrically encrypt the transmission data

Algorithms: data structure, commonly used algorithms

data structure

• Array, linked list
• Stack, queue
• Hash table
• Tree, pile, figure

Commonly used algorithms

• Sort
• Double pointer, sliding window, string
• Recursion, divide and conquer, dichotomy
• Backtracking, greedy, dynamic planning

Java basics: StringBuilder, generic erasure, Exception, IO, container

StringBuilder

• StringBuffer is thread-safe, StringBuilder is not thread-safe
• +In fact, it is realized by StringBuilder, so the non-loop body can be used directly +, but the loop body cannot, because StringBuilder will be created frequently
• String.concat is essentially a new String, which is inefficient and time-consuming to sort: StringBuilder <StringBuffer <concat <+

Generic erasure

• Generics related to class structure such as modified member variables will not be erased
• Container generics will be erased

Exception 和 Error

• Both Exception and Error inherit from Throwable
• Error mostly refers to unrecoverable error states, such as OOM, so there is no need to capture
• Exception 分为 CheckedException 和 UnCheckedException
  • CheckedException: must be explicitly caught and checked by the compiler, such as io operations
  • UnCheckedException: No need to display capture, such as null pointer, array out of bounds, etc.

IO 、 NIO 、 OKIO

• IO is stream-oriented, processing one byte at a time, NIO is buffer-oriented, generating or consuming one data block at a time
• IO is blocking, NIO is non-blocking
• NIO supports memory mapping
• Compared with io and nio, okio has simpler and easier api
• okio supports timeout mechanism
• okio introduces ByteString space for time to improve performance
• Okio uses the segment mechanism for memory sharing, saving copy time consumption

ArrayList、LinkedList

• ArrayList
  • Based on array implementation, fast search: o(1), slow addition and deletion: o(n)
  • The initial capacity is 10, and the capacity is expanded through the System.arrayCopy method
• LinkedList
  • Realization based on doubly linked list, slow search: o(n), fast addition and deletion: o(1)
  • Encapsulates calls to queues and stacks

HashMap 、HashTable、HashSet

• HashMap (allow key/value to be null)

  • Based on the implementation of arrays and singly linked lists, the array is the main body of the HashMap; the linked list exists to resolve hash conflicts and stores the entity that combines key and value
  • The array index is obtained by key.hashCode (also hashed twice), and indexed by key.equals on the linked list
  • When the hash collision falls in the same bucket, it is directly placed at the head of the linked list (after java1.8, it is placed at the tail)
  • When the number of linked lists in JAVA 8 is greater than 8, it will be converted to red-black tree storage, and the search time will change from O(n) to O(logn)
  • The length of the array is always 2 to the power of n: In this way, the remainder can be achieved through bit operations, so that the index can fall within the range of the array length
  • The load factor (default 0.75) indicates how much fill ratio is added to for expansion. The fill ratio is large: the linked list is longer and the search is slow; the fill ratio is small: the linked list is short and the search is fast
  • When expanding the capacity, directly create twice the length of the original array, and then hash the original object to find the new index, and re-place it

• HashTable (key/value is not allowed to be null)

  • The data structure is the same as HashMap
  • Thread safe

• HashSet

  • Based on the HashMap implementation, the element is the key of the HashMap, and Value is passed in a fixed value

ArrayMap、SparseArray

• ArrayMap

  • Implementation based on two arrays, one to store hash; one to store key-value pairs
  • The array storing the hash is ordered, and the binary search is used when searching
  • When a hash conflict occurs, the key-value pair array is stored continuously, and the search is also indexed by key.equals. When it is not found, the key-value pair array with the same hash value is traversed backwards and then forwards.
  • When expanding capacity, unlike HashMap directly double, the memory utilization is high; there is no need to rebuild the hash table, just call system.arraycopy array copy, which has higher performance
  • Not suitable for storing large amounts of data (below 1000), because binary search is much slower than red-black trees when the amount of data is large

• SparseArray

  • Based on ArrayMap, the key can only be a specific type

Concurrent collection

• ConcurrentHashMap
  • The data structure is the same as HashMap, or an array plus a linked list
  • Using segment lock technology, unlike HashTable, which directly synchronizes put and get operations without brain
  • The get operation is not locked, because the value is modified with volatile to ensure visible rows, and the performance is very high
  • After java1.8, the segment lock is removed, and CAS optimistic lock plus synchronized is used to achieve

LRUCache principle

• Implementation of LinkedHashMap based on the order of access, the most recently accessed will be ranked last

At last

Taking into account the length of the article, I made these questions and answers, as well as the questions I encountered in many years of interviews, and some interview materials into PDF documents. If you need a friend, you can privately mail me [Interview] to receive it for free

Click here to receive a summary of Android interview materials

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