Day_ eleven

Day11 Always review

1 Program basis

1.1 data type

The nature of data types ： Specify the size of the occupied memory space , Restrict the format type of data

Data types include ：

       Basic data types and reference data types

       Basic data types include integer floating-point Character Boolean type

       Integers include int byte short long;

       Floating point types include float( Single floating point 32 position )  double（ Double floating point 64 position ）;

       Character char;

       Boolean type boolean;

       Reference data types include   class Array Interface

ASCII code : Mapping between characters and integers

a = 97 ; A = 65 ; 0 = 48…….

Automatic type conversion ：

       Byte ->short -> int -> long -> float -> double

                     char ->int ->long -> float ->double

Be careful ：

When byte,short,char,int Of the four types , Any one or more operations , The result is int

1.2 Variable

Definition ： Variable quantity , It is convenient to operate the data in the space

Statement ： data type Variable name = value ;

       int i1 = 0;

Byte b1 = 0;

Short s1 = 10;

Long l1 = 10L;

Char c1 = ‘c’;

Double d1 = 2.2;

Float f1 = 1.2F;

Boolean b1 = true;

classification ：

       local variable ： Method , Can only be used in the current method , No default , The scope is a brace

       Static variables ： Use Static Decorated variable , And methods are level relations

       Member variables ： Not used static Decorated variable

call ：

       local variable ： Call the variable directly by name in the method , No default , Assignment must be made.

       Static variables ： adopt Class name . Static variable name call , Call the static variable of the current class in the current class , Class names can be omitted

       Member variables ： Class body through Object reference . Member variables call

The default value is ： Integers 0 ; decimal 0.0 ; Boolean type false ; Character \u0000 ; The reference type defaults to null

​​​​​​​1.3 Operator

i++ : Assign first , One more

++i : Add one first , To assign a value

Operator priority ：

 

An operator ：

       &  Bit and , It is true that both sides are true , Whether or not the first condition is false, The second condition will still be implemented

       && Short circuit and , Suppose the first condition is false, The second condition is not executed

       | : or , One on both sides is true That's true , Whether the first condition is false still true, The second condition is implemented

       || ： Short circuit or , If the first condition is true, Then the second condition is no longer executed

​​​​​​​1.4 Process control

Sequential structure ：

       Strictly from top to bottom , From left to right

Branching structure ：

       Through the specified judgment conditions , Selectively take different branches

If…else…. ：

Single branch ： There are cases of non implementation

       If( Boolean expressions ){

              Code executed when true

}

Double branch ： There must be a branch that performs

       If( Boolean expressions ){

              Code executed when true

}else{

       Code executed when is false

}

Switch

grammar ：

       Switch（ value ）{

       Case value ;

       Break;

}

​​​​​​​1.5  Loop structure

A loop is a code that is repeated many times

For loop

       for( expression 1, expression 2, expression 3){

The loop body

}

Three elements ： Starting value , Termination conditions , step

While loop

       while( Boolean type ){

       The loop body

}

Break ： Can be used in switch in , end case Branch , prevent case through

                It can also be used in loops , End the current cycle

Continue ： Skip the current cycle , Continue next time

​​​​​​​1.6  Method

A method is a collection of many statements , Put the code in the method , It can be used multiple times , The purpose is code reuse , Make the program shorter and clearer , Improve development efficiency

Statement ：

       List of modifiers return type Method name （ parameter list ）{ Method body }

classification ：

       Static methods ： Use static The method of decoration

       Member method ： No, static The method of decoration

       Construction method ： Modifier Method name （）, even void either

call ：

       Static methods ： Class name . Static method name （ Parameters ）

       Member method ： Object reference . Legal name of member （ Parameters ）

Method does not call, does not execute , Call to execute

1.7 Memory division

 

Static zone / Method area :

Save program files (class file ) And static data , Before the method is called , Also stored in the static area , There is also a runtime constant pool inside

VM Stack : Also called stack memory

Stack memory is a space opened up based on stack data structure , The feature is first in and last out

Stack : It's a data structure , First in, then out , Like a clip

The components of the stack

Stack space : Stack memory is stack space

Stack frame : Every stack element in the stack space It's called stack frame ( such as Every bullet in the magazine It's called stack frame )

Stack bottom element : The first stack frame put in

Top element of stack : The last stack frame put in

Stack operation

Pressing stack : It refers to the process of putting elements into stack space

Bomb stack : Is the process of ejecting elements

Stack memory , Is used to execute methods , Execution of all methods , Must be done in stack memory

Native Method Stack :

Used to execute some local methods , such as hashCode etc. , The model and operation are similar to VM Stack consistency

Heap memory : To save objects

​​​​​​​1.8  recursive

Recursion is to call the current method in a method

2 Array

Arrays are reference data types , Used to store multiple data

2.1 data structure

Data structure is computer storage 、 How to organize data . Data structure refers to the collection of data elements that have one or more specific relationships with each other . Usually , Well chosen data structure can bring higher operation or storage efficiency . Data structures are often related to efficient retrieval algorithms and indexing techniques

Data manipulation : Additions and deletions

2.2 Array

characteristic ：

       Continuous storage , Subscript from 0 Start , Use memory address offset

       Once the length is determined, it cannot be changed

       When adding or deleting , You should create a new array , Copy the required values into the new array

       Query changes are extremely efficient

Statement ：

       Static declaration ： When each element is known , Use static declarations

              data type [] Variable name = { value , value };

              data type [] Variable name = new data type { value };

       Dynamic statement ： When I don't know every element , Use dynamic declarations

              data type [] Variable name = new data type { length };

storage ：

 

Use ：

Inquire about : Array [ Subscript ]

change : Array [ Subscript ]= value

Sort ：

API : Arrays.sort( Array );

Bubbling : Comparing the two , Greater than swap position

choice : Suppose the first one is the smallest , Compare with the following in turn , Greater than the swap subscript swap value

Two points search ：

       Define the starting position , End position , In the middle

       If The goal is bigger than the middle be End unchanged start = middle +1

       If the target is smaller than the middle be Initial invariance end = middle -1

       Regenerate the middle position

       Start is greater than end

Two dimensional array ：

       Static declaration ：int[][] arr = { { The number };};

       Dynamic statement ：int[][] arr = new int[5][];

 

3 object-oriented

3.1 Object oriented and process oriented

Object oriented is a software development method , A programming paradigm . The concept of object orientation has gone beyond programming and software development , Extend to, for example, databases 、 Interactive interface 、 Application structure 、 Application platform 、 Distributed systems 、 Network management structure 、CAD technology 、 Artificial intelligence and other fields . Object oriented is a way to understand and abstract the real world , It is the product of the development of computer programming technology to a certain stage .

Process oriented : Focus on step by step

For example, when you want to do something , Analysis first

What should be done in the first step

What should be done in the second step

object-oriented : Focus on sub modules

For example, when you want to do something , Analyze who should do the job

summary :

Object - oriented has the advantage : Extensibility , Maintainability , flexibility , Reduce program coupling

shortcoming : Performance is relatively worse than process oriented

3.2 Classes and objects

class ： Describe the properties and characteristics of such things , Abstract the template

object ： Is a specific thing

Objects are implementations of classes , Saved the value of the attribute

If between objects Have the same attribute, the same value , Use Static variables

If between objects Have the same attribute but different values （ It can be the same ）, Use Member variables

​​​​​​​3.3  Construction method

Constructors are common to every class , The object used to create the class , By default, there is a parameterless construct

grammar ：

       Modifier Class name （ parameter list ）{ Method body }

       Out of commission static, no return value , even void None

​​​​​​​3.4  Instantiation

1 Load the corresponding class file

2 new Create space in heap memory

3 Execute construction , Initialize the heap memory object

4 Construction method: pop stack , And return the heap memory address to the variable

​​​​​​​​​​​​​​3.5  call

Object invokes static properties        

       1 You can use the class name to call

       2 You can use objects to call , Because the object will be converted into a class name call in the compilation phase

Static call member ：

       In the static method , Non static attributes cannot be used directly , An object call is required to call

Distinguish between member methods and constructor methods ：

         What has no return value is the constructor , even void None , What has a return value is the member method

​​​​​​​3.6 This

Definition ：

       Is in each object , A reference variable that holds its own address   this It means that the current object

function ：

       1 In a member method or constructor , Distinguish between member variables and local variables with the same name

       2 Used in construction methods , Overload calls other constructor methods in the current class , But it must be written in the first line of the constructor

       3 return this Returns the memory address of the current object , You can chain call

Be careful ：

       This Cannot appear in a static context

​​​​​​​3.7  encapsulation

Put all the components together , You can also hide the data through the permission control modifier , It can control the modification degree of class data by users

Proper encapsulation can make the code easier to understand , Easy to maintain , It improves the security of the code

Package

       Package mechanism , It mainly solves the problem of naming conflicts

Import

       Used to load other classes required in the current class , Must be in class above ,package Under the statement

​​​​​​​3.8  Access control

 

​​​​​​​3.9  Inherit

Definition ：

          Existing classes , Derive a new class , The new class is used for the properties and behaviors of the parent class

Purpose ：

       Code reuse , Increase of efficiency

grammar ：

       class Class name extends Parent class name { The class body }

Super ：

       Represents the characteristics of the parent class , It is used to distinguish methods and variables of the same name between parent and child classes in member methods and constructor methods super.xxx

       It can also be used in subclass methods , Call the specified parent class constructor super（xxx）, Must appear in the first line of the subclass constructor ​​​​​​​

3.10 overwrite

Overrides refer specifically to member methods , When the function of the parent class cannot meet the needs of the child class , Overwrite

Definition ：

       Write a method that is the same as the parent class , But the function is different

The rules ：

       1 Method name , Return value , parameter list Must be consistent with the parent class

       2 Cannot have lower access rights than the original method

       3 You cannot have a broader exception than the original method