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Android interview theme collection

2022-04-23 13:49:00 【InfoQ】

2） The thread executes a piece of synchronization code , But can't get the related synchronization lock , It can only enter the blocking state , etc. Get synchronization lock , To resume execution ;

3） The thread executed the... Of an object wait() Method , Directly into the blocking state , Waiting for other threads to execute notify()/notifyAll() operation ;

4） Threads perform some IO operation , Due to waiting for related resources, it enters the blocking state , Such as System.in, But no input from the keyboard , Then enter the blocking state .

5） Thread comity ,Thread.yield() Method , Pause the currently executing thread object , Take execution opportunities Give up to threads of the same or higher priority , But it doesn't put the thread into blocking state , The thread is still executable state , At any time, you may get another share of CPU Time . Thread self closing ,join() Method , Call the... Of another thread in the current thread join() Method , The current thread is in blocking state , Until the end of another thread , At present The thread changes from blocking to ready .

6） Threads execute suspend() Put the thread into blocking state , must resume() Method is called , Can we make Thread re enters executable state .

[](()7? How does the thread close ？

1） Use flag bits

2） Use stop() Method , But it's like turning off the computer , Something unexpected may happen problem

3） Use interrupt interrupt()

public class Thread {

// Interrupt current thread

public void interrupt();

// Determine whether the current thread is interrupted

public boolen isInterrupt();

// Clear the interrupt status of the current thread , And return the previous value

public static boolen interrupted();

}

But the call interrupt() Method just passes the interrupt request message , Doesn't mean to stop the target thread immediately .

[](()8、 Let's talk about java The method of synchronization in

Why synchronization is needed , Because in multithreading concurrency control , When multiple threads operate on a shareable resource at the same time Source time , If there is no synchronization mechanism , It will lead to inaccurate data , So we need to add synchronization lock , indeed Ensure that the thread is called by other threads before the operation is completed , So as to ensure the uniqueness and accuracy of the variable .

1）synchronized Pertaining to a synchronous code block or method

because java Each object of has a built-in lock , When using this keyword to decorate a method , Built in lock will protect The whole method . Before calling this method , Need to get a built-in lock , Otherwise, it will be in a stuffy state .

2）volatile Modifying variables

Ensure the visibility of variables between processes , Every time a thread accesses volatile Modified variables are from memory Read , Instead of caching , In this way, the variables accessed by each thread are the same . And use a memory barrier .

3）ReentrantLock Reentrant lock , Its common methods are ReentrantLock()：

Create a ReentrantLock example lock() Gets the lock unlock() Release the lock

4） Use local variables ThreadLocal Thread synchronization , Each thread will save a copy of the variable copy , The copies are independent of each other , In this way, each thread can modify its own copy at will , Without affecting other threads . Common methods ThreadLocal() Create a thread local variable ;get() Returns this thread The current thread copy variable of the part ;initialValue() Returns the initial value of the current thread for this thread local variable value ;set(T value) Set the value in the current thread copy of this thread variable to value

5） Using atomic variables , Such as AtomicInteger, Common methods AtomicInteger(int value) gen Build a with a given initial value AtomicInteger Integers ;addAndGet(int data) In an atomic way Add the given value to the current value

6） Thread synchronization using blocking queue LinkedBlockingQueue

[](()9、 How to ensure thread safety ？

Thread security is embodied in three methods ：

1） Atomicity ： Provide exclusive access , Only one line and data can be operated at the same time .

JDK A lot of atomic class , Such as AtomicInteger\AtomicBoolean\AtomicLong, They are through CAS Complete atomicity . JDK There are two types of locks available ：synchronized rely on JVM Implementation lock , The of the keyword object Within the scope, only one thread can operate at the same time . The other is LOCK, yes JDK Code level locks provided , rely on CPU Instructions , The representative is ReentrantLock.

2） visibility ： One thread's modification to main memory is seen by other threads in time .

JVM Provides synchronized and volatile,volatile Visibility is through memory barriers and prohibitions The implementation of weight stop sorting ,volatile When writing operations , Add a... After the write operation store Barrier directive , Refresh shared variable values in local memory to main memory ; When reading operations ,

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Add a... Before reading load Instructions , Read shared variables from memory .

3） Orderliness ： Instructions are not reordered by the compiler .

It can be done by volatile、synchronized、Lock To ensure order .

[](()10、 Both processes require writing or reading at the same time , Can we achieve ？ How to prevent process synchronization ？

I think it can be achieved , For example, there is no problem for both processes to read calendar process data , But at the same time write , Should be There will be conflict .

Shared memory can be used to share data between processes .

[](() Two 、Android Interview help

[](()1、 Make it as smooth as possible ListView, How do you optimize your work ？

①Item Layout , The fewer levels, the better , Use hierarchyview Tool view optimization .

② Reuse convertView

③ Use ViewHolder

④item When there are pictures in , Load asynchronously

⑤ When sliding fast , Don't load pictures

⑥item When there are pictures in , The picture should be compressed properly

⑦ Realize the paging loading of data

[](()2、 about Android Security issues , How much do you know

① Error exporting component

② Parameter verification is not strict

③WebView Introduce various security issues ,webview Medium js Inject

④ No confusion 、 No second packing

⑤ Clear text stores key information

⑦ Misuse HTTPS

⑧ Shanzhai encryption method

⑨ Abuse of authority 、 Memory leak 、 Use debug Signature

[](()3、 How to reduce it APK Bag size ？

Code to maintain good programming habits , Don't duplicate or unused code , Add carefully libs, Remove unused libs.

Use proguard Obfuscated code , It optimizes unused code , And confusion can also reduce safety The size of the package .

native code Part of , Most of the time, just support armabi And x86 The architecture of . If Not necessary , Think about taking it off x86 Part of .

resources

Use Lint Tools look for unused resources . Remove unused images ,String,XML wait . assets Please make sure there are no useless files in the directory .

Generate APK When ,aapt The tool itself will be right png Make optimization , But before that, you can also use its His tools are like tinypng The image is further compressed and preprocessed .

jpeg still png, Choose according to your needs , At some point jpeg Can reduce the size of the picture . about 9.png Pictures of the , Cut the stretchable area as small as possible , In addition, you can use 9.png Stretch to achieve big picture effect Try not to use the whole big picture .

Strategy

Selectively offer hdpi,xhdpi,xxhdpi Picture resources of . It is suggested that priority should be given to xhdpi Pictures of the , about mdpi,ldpi And xxxhdpi Provide different parts as needed .

Reuse existing image resources as much as possible . For example, symmetrical pictures , Just provide one , Another picture Slices can be implemented by code rotation .

The function that can be realized by code drawing , Try not to use too many pictures . For example, reduce the use of multiple picture groups become animate-list Of AnimationDrawable, This way provides multiple images, which takes up a lot of space .

[](()4、Android What is symmetric encryption and asymmetric encryption in the way you interact with the server ?

Symmetric encryption , That is, encryption and decryption of data are using the same key, There are algorithms in this area DES. Asymmetric encryption , Encryption and decryption are different key. Before sending data, make an agreement with the server Into public and private keys , Data encrypted by public key can be decrypted by private key , conversely . There are algorithms in this area RSA.ssh and ssl It's all typical asymmetric encryption .

[](() 3、 ... and 、Android Interview FAQs

**1、java in == and equals and hashCode The difference between **

The comparison values of basic data types are equal . The memory address of the comparison class , Is it the same object , Don't cover equals Under the circumstances , Compare memory address with , The original realization is == , Such as String Such as rewrite the equals Method . hashCode It's also Object class . Returns a discrete int Type integer . Operate on the collection class Use in , To improve query speed .（HashMap,HashSet Whether the comparison is the same ） If two objects equals,Java The runtime environment will think that they hashcode Must be equal . If two objects don't equals, their hashcode It could be equal . If two objects hashcode equal , They don't have to equals. If two objects hashcode It's not equal , They must not equals.

2、int And integer The difference between

int Basic types

integer object int The wrapper class

3、String、StringBuffer、StringBuilder difference

String: String constant It doesn't apply to situations where there is a constant need to change the value , Each change is equivalent to a new The object of

StringBuffer: String variable （ Thread safety ）

StringBuilder: String variable （ Thread unsafe ） Make sure it's available on a single thread , Efficiency is slightly higher than StringBuffer

4、 What is an inner class ？ The role of inner classes

Internal classes can directly access the properties of external classes

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