preface : These are very basic , There's still a lot to write , The focus is on programming understanding .

Chapter I :

* Java It is a high-level language with compilation and interpretation ;
* Its source code operation mechanism :
*  java The generated bytecode file is platform independent , The reason is JRE, He is a java The program creates a separate JVM Container running program ;Java Language communication
Install the corresponding on different architecture systems JRE, realization Java Cross platform operation of programs ( One time compilation , Run everywhere )
* JDK compile java The source code is bytes ;JRE,Java Runtime environment ;JVM,java virtual machine ;
* adopt JDK Compiler tools for (javaac.exe) take java Source code (.java) Compile as JVM Executable bytecode (.class)
Chapter II :

identifier : An identifier is a name ,

The identifier consists of letters , Underline , Dollar sign and number composition , Unlimited length .(Java Using 16 Bit Unicode character set )        

The first character of the identifier cannot be a numeric character .

Identifier cannot be a keyword

stay Java in “ quote ” Is to point to the location of an object in memory , In essence, it is a pointer with strong integrity and security restrictions


  Memory space problem :

java The default integer is int type , statement long Type Xujia l

java The default floating point is double type , Want to declare float Xujia f;

boolean The default value of type data is flase;


  When the value with small capacity is assigned to the value with large capacity ( The system completes automatically );

When the value with large capacity is assigned to the value with small capacity ( Use display type conversion );

take int Value assigned to byte and short Time , If the value range of these variables is exceeded, type conversion must be carried out ;


Multidimensional array in use new Operator , At least give the size of the highest dimension ;

When calculating the value of an arithmetic expression :

Operate with the highest accuracy ;

If the highest precision of the expression is less than int, Then press int Perform operations ;

char The precision of the operation result of type data is int;

Logical operator :

& Logic and    && Short circuit and ;

| Logical or      || Short circuit or ;


Chapter III :

* Combine the abstract attributes and methods , Data types that form an overall abstraction are called classes
* class (class) And objects (object) Is the core concept of object-oriented ; An object is an individual of such things that actually exist , Therefore, it is also called an instance ;
* The naming of the class must comply with the provisions of the identifier ;
* Construction method
* His name must be exactly the same as the name of the class ;
* No data types are returned ;
* Allow several constructors in a class , However, the parameters of these construction methods must be different ;
* When we do not define a constructor, we will define a default constructor ;
* After declaring object variables , The object does not yet reference an entity , At this time, the object is an empty object ;
* Member variables and class variables :
* Class variable static modification , Class variables are variables associated with all objects created by this class , Therefore, class variables can be accessed through objects and class names ;
* Instance variables are simply variables associated with the corresponding objects , Instance variables of different objects are different from each other , That is, allocate different memory space , It must be accessed through objects ;
* Instance method and class method :
* Instance methods can manipulate instance variables and class variables , But instance methods must be called with objects ;
* Class methods cannot manipulate instance variables , Can be called by object and class name ;
* this keyword :
* use this Refers to the current object ;
* Cannot be used in class methods this;
* this Distinguish between local variables and member variables , If member variable and local variable have the same name , If you want to use member variables plus this;
* Access rights :
Chapter IV :

*   abstract class :
* use abstract Modified class ;
* out-of-service new Create this kind of object , You can only create subclasses first
* You can declare objects , Upper transformation object as a subclass ;
* Abstract method not available static,final,private modification ;
* Abstract classes can have abstract methods , Or not , A class containing abstract methods must be an abstract class
* Interface :
* use Interface Define an interface ;
* All variables in the interface are automatically public static final;
* Methods in the interface are automatically public abstract;
* A class that implements the interface can implements Declare yourself
* Interface callback
  The focus is on programming understanding .

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