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characteristic ： The elements are in order , And repeatable
Traverse ： Subscript ,foreach, iterator
Capacity expansion ：
Initial capacity 10, Load factor 0.5, Expansion increment 0.5 times
New capacity = Original capacity + Original capacity * 0.5 , Such as ArrayList The capacity of is 10, Then the capacity after one expansion is 15

List Implementation class of

1、ArrayList

Simple data structure , Over capacity Automatic expansion , The dynamic array
ArrayList Diagram of expansion principle ：
ArrayList The capacity expansion mechanism is a balance between efficiency and space ,
The internal implementation is based on the underlying array of objects
No Suitable for random addition or deletion ( Displacement phenomenon )
Apply to The maximum number of data is uncertain
Random access quick 、 It's fast
Thread unsafe ( When used as a member variable of a class )

2、LinkedList

LinkedList Provide additional get,remove,insert Method stay LinkedList Head or tail of
LinkedList Can be used as a stack （stack）【 It includes push,pop Method 】, queue （queue） Or two-way queue （deque）
With Double linked list Realization , The list has no capacity limit , The allowed element is null, Thread unsafe
Two way linked list atlas ：
fit Make random additions or deletions , Because the linked list structure will not shift when adding or deleting

3、Vector

Thread safety , But the parallel performance is slow ( Because it's locked ), Not recommended

4、CopyOnWriteArrayList

When writing copy
Thread safety
( Version of ) Final consistency , The real-time version cannot be consistent
Than Vector High performance
Suitable for Read more , Write less Scene
Realized List Interface , Usage and ArrayList similar
Copy a new array when writing , Finish inserting 、 Assign the new array to the original array after modification or removal

CopyOnWriteArrayList The illustration
Before looking at the picture, first understand the concept of reading and writing
read ： Only data is read , Do not modify data (CopyOnWriteArrayList The collection from the Central Plains is used to read )
Write ： Operate on the data to make the data change (CopyOnWriteArrayList in The new set is used to write )
stay CopyOnWriteArrayList For the new set all “ Write ” Once the operation of , The pointer to the original set will immediately point to the new set , Reflects the final consistency

Use ArrayList in remove Attention points of the method

remove characteristic ：
Remove subscript when passing in integer type
Delete the corresponding element when the object type is passed in ( Such as Integer.parseInt(null))

Data preparation ： For the convenience of demonstration , Need to have two or more identical elements next to each other

List<Integer> list=new ArrayList<Integer>();
  list.add(1);
  list.add(2);
  list.add(3);
  list.add(3);
  list.add(4);

Use ArrayList in remove Several different ways of writing methods ：

Examples of wrong writing

for(int i=0;i<list.size();i++){
   if(list.get(i)==3) list.remove(i);//list.get(i) Get the subscript i The elements of 
}

The reason for the error ：ArrayList When adding and deleting Displacement phenomenon , When two identical elements 3 Adjacent time , first 3 After judgment and deletion , the second 3 The subscript of all the elements following it will move forward by one bit , So the second one 3 It's the first one 3 The position of the subscript , But the pointer has already judged the first 3 Whether the element in the location is 3 了 , Therefore, the second... Will not be deleted 3

for(Integer i:list){
    if(i==3) list.remove(i);
}

wrong Error reason ： because ArrayList There's a variable in （modCount= The number of elements of the original array ） It also internally encapsulates an internal class （Itr）, This inner class implements the iterator , When using foreach Method traversal , It uses ArrayList Iterators of inner classes , The inner class defines a variable that changes the number of times （expectedModCount）, This variable is assigned an external value modcount Value , When using inner classes （Itr） When adding or modifying , Throw an exception , Its purpose is to prevent ArrayList The length changes .

In short , It is not recommended to use foreach Add and delete collections , It is more suitable for traversing data

Examples of correct writing

for(int i=0;i<list.size();i++){
   if(list.get(i)==3) list.remove(i--);
}

The right reason ： Used i--, That is to say After deleting The pointer will move forward one bit , Then go back to the deleted subscript position to judge , And this avoids because ArrayList The judgment omission caused by the displacement phenomenon of .

Be careful ： Not to be used --i, because --i Will be in remove Method in Before deleting perform

for(int i=list.size()-1;i>=0;i--){
 if(list.get(i)==3){
  list.remove(i);
 }
}

The right reason ： Traversal from the last element in the set to the first element is used Reverse traversal Method , Even though ArrayList The occurrence of the displacement phenomenon of cannot affect the deletion

The last way to write ：ArrayList The collection is being deleted 、 When adding, etc , The dynamic displacement characteristics should be considered , Iterators are recommended , It will be safer

Iterator<Integer> it=list.iterator();
 while(it.hasNext()){
  if(it.next()==3){
   it.remove();
  }
}

Be careful ： Of the above code it.remove Don't write as list.remove(i)

UML chart

What is? UML chart ？

Unified modeling language （UML） It's a kind of Modeling language
Most models are based on Chart In a way that shows
Modeling diagrams usually contain Several blocks or boxes , Connecting line And as models Additional information Text for , stay UML The rules relate and extend to each other .

Why use UML chart ？

（1）UML It unifies various methods for different types of systems 、 Different views of different development stages and different internal concepts spot , Thus, the unnecessary differences between various modeling languages are effectively eliminated . It is actually a general modeling language , It can be widely used by users of many object-oriented modeling methods .
（2）UML Modeling ability is better than others object-oriented The modeling method is stronger . It is not only suitable for the development of general systems , And for parallel 、 The modeling of distributed system is particularly suitable .
（3）UML It's a modeling language , Not a development process .

UML Illustration example

UML The relationship and representation in the figure

1、 relation (Association)

Relevance is a kind of The relationship you have , It makes One class knows the properties and methods of another class ; Such as ： Teachers and students , The relationship between husband and wife can be two-way , It could be one-way . A two-way association can have two arrows or no arrows , A one-way association has an arrow
The code is ： Member variables
Arrows and points ： With ordinary arrows solid , Point to the possessed
Above picture , Teachers and students are connected in two ways , The teacher has many students , Students may also have more than one teacher . But the relationship between students and a course is unidirectional , A student may take more than one course , Course is an abstract thing. He doesn't have students
The following figure shows self association ：

2、 rely on （Dependency）

Dependency is a kind of Relationship used , namely The implementation of one class requires the assistance of another class , So try not to use two-way interdependence , It's usually One way dependency
Code representation ： local variable 、 Method parameters or calls to static methods
For example, the method of one class uses the method of another class , Or local variables are used in the methods of a class
The stronger the relationship , The stronger the dependency
Arrows and points ： Dotted line with arrow , Point to the user

3、 Aggregation and combination

It's all about The relationship between the part in the whole and the whole The closeness of , they Are a kind of relationship

Combine （Composition）
It's the relationship between the whole and the part , but The part cannot exist alone without the whole . For example, the relationship between the company and the Department is the whole and part , There is no department without a company
synthetic relation It is a kind of relation , yes A relationship stronger than an aggregation relationship , It requires that the object representing the whole in the common aggregation relationship is responsible for the life cycle of the object representing the part
The code is ： Member variables
Arrows and points ： A solid line with a solid diamond , Rhombus pointing to the whole

polymerization （Aggregation）
It's the relationship between the whole and the part , And Parts can be separated from the whole and exist alone . For example, the relationship between the car and the tire is the relationship between the whole and the part , The tires can still be left in the car .
Aggregation is a kind of association , It's a strong correlation ; Association and aggregation are grammatically indistinguishable , We must examine the concrete logical relations
The code is ： Member variables
Arrows and points ： A solid line with a hollow diamond , Rhombus pointing to the whole

4、 Realization (Realization)

It's a kind of The relationship between class and interface , Express A class is an implementation of all the features and behaviors of an interface
The arrow points to ： Dotted line with triangular arrows , The arrow points to the interface

5、 generalization (Generalization)

It's a kind of Inheritance relationships , Express The relationship between general and special , It designates How subclasses specialize all the characteristics and behaviors of the parent class . for example ： Tiger is a kind of animal , There are not only tiger's characteristics, but also animal's common characteristics
The arrow points to ： A solid line with a triangular arrow , The arrow points to the parent class