Summary of Android knowledge points and common interview questions

The following is a summary of my knowledge points and some questions I encountered during the interview , Here's a summary , Please correct any mistakes found , thank you .

1. The difference between linked list and array

• The query efficiency ： Array query is more efficient （ Because it can be queried by subscript ）, The efficiency of linked list query is low . So the array is used when there are many queries .
• Efficiency of addition and deletion ： The efficiency of adding and deleting arrays is low （ Because you need to move, add or delete all the data behind the location ）, The efficiency of adding and deleting linked lists is high . Therefore, the linked list is used when adding and deleting operations are frequent .
• Memory space ： The memory space of the array is continuous , Fix . Dynamic allocation of linked list .
• Extensibility ： Poor array scalability , The linked list has strong scalability . Because when the array is created , Fixed the memory size , When the memory is full, it cannot be expanded , You can only recreate . The linked list is to add a data , Allocate a little content space .
• Random read rate ： High array random read rate , The random reading rate of linked list is low . Because the memory space of the array is continuous , And the linked list is scattered .
• Location ： Array elements are in the stack area , The list elements are in the heap area

2. List Hash The difference between arrays

• Store the object ：list Only objects can be stored , Arrays can store basic data types and objects ,hash Store key value pairs .
  Be careful ：

// There is no mistake in this case , It looks like an added basic data type , But it is automatically converted to the packaging type when adding .
ArrayList array = new ArrayList();
array.add(100);

• Storage order ：List The data stored in is orderly , It is stored in the order of adding data . Array order .hash There is no order , The order of output data is not the order of storage .
• Whether the data is repeatable ：List The data in is repeatable , The data in the array can be repeated ,hash The data in cannot be repeated , Because in HashMap Is data accessed through key value pairs , When key When I repeat , Will be the original value Replace the new value.
• Get the number of elements ：List adopt size() Method , Array pass length attribute ,HashMap adopt size() Method .

3. Which three parties have been used SDK

• Bmob： Back end cloud .
• Wind weather ： The interface .
• Aggregate data ： Various data interfaces .
• Face++： Face recognition related technologies .
• arcsoft ： Face recognition related technologies .

4. Android Four components

• Activity
• Service
• Broadcast Receiver
• Content Provider

5. The difference between heap and stack

• purpose ： Heap memory is used to store new Created objects or arrays , Stack memory is used to store methods or local variables .
• The order ： First in first out , last in last out . Stack in and out , Last in, first out .

6. Activity and Fragment The difference between

• Fragment yes Android3.0+ After that .
• Activity There can be multiple Fragment, and Fragment Can only be attached to one Activity.
• Fragment More flexible and lightweight .

7. Commonly used ADB command

Android Development commonly used ADB Command set

8. String,StringBuilder,StringBuffer The difference between

• Running speed ：StringBuilder > StringBuffer > String.String The string is always on , and StringBuilder and StringBuffer For string variables . namely String Once created, variables cannot be changed , and StringBuilder and StringBuffer Changeable .
• Thread safety ：StringBuffer Thread safety ,StringBuilder Thread unsafe .
  String by final Modified class , Non inheritable .
• Use scenario ： Use when the string does not change very often String, Used when a single thread has a large number of string operations StringBuilder, When multithreading has a large number of string operations, use StringBuffer.

9. The difference between threads and thread pools

• Threads Thread Every time new When the object , Poor performance . The thread pool reuses existing threads , Reduce the creation of objects 、 The cost of extinction , High performance .
• New threads lack unified management , There may be situations where threads are created indefinitely , Compete with each other , Finally, the system resources are occupied to cause a crash or OOM. The thread pool can effectively control the maximum number of concurrent threads , Improve system resource utilization , Avoid system congestion caused by excessive resource competition .
• Threads lack some functionality , Such as scheduled execution , Do it regularly , Interrupt threads, etc . Thread pools provide these functions .

10. Android Project directory structure

Android The role of each folder in the project directory structure

11. build.gradle file

Android build.gradle File,

12. Activity Life cycle

Reference resources google Official documents

13. heavy load

• Same method name
• Return value , Parameter type , The number of parameters is not exactly the same
• Overloaded methods can change modifiers

14. rewrite

• The parameter list is the same as the overridden method
• The return type can be different , But it must be a subclass of the return type of the parent class
• The access permission cannot be lower than that of the overridden method
• Member methods of a parent class can only be overridden by subclasses
• final Methods that modify cannot be overridden
• static Methods that modify cannot be overridden , But again
• Children and parents are in the same package , Subclasses can override the parent class except final and private All the ways to modify
• Child and parent are not in the same package , Subclasses can only override the parent class pubilc、protected He Fei final Method

15. Common abnormal

• Compile time exception
  SQLExecption
  IOExecption
  ClassNotFindExecption
  FileNotFindExecption
• Runtime exception
  NullPointerExecption
  StringIndexOutOfBoundsExecption
  ArrayIndexOutOfBoundsExecption
  ArithmeticExecption
  ClassCastExecption

16. Activity and Window,View The relationship between

Activity When it is created attach() Method initializes one PhoneWindow（ Inherited from Window）, every last Activity Both contain one and only PhoneWindow.
Activity call setContentView() It's actually called getWindow().setContentView(), take View Set to PhoneWindow On , and PhoneWindow Inside is through WindowManager Of addView(),removeView(),updateViewLayout() These three ways to manage View Of .
WindowManager The essence is the interface , In the end by the WindowManagerImpl Realization .

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