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object-oriented （ Behaviorisation ）： According to the characteristics of the whole demand 、 Functional division , Encapsulate these common parts into classes （ After class instantiation, the object ）, Let the object solve the problem

The three major characteristics

stay Java in , There are three aspects of object orientation （ Four ） Big features ：

encapsulation

Inherit

polymorphic

abstract

Member variables

If you declare variables directly in a class , Called member variable （ Global variables ）; If you declare variables in a method , It's called a local variable .

The process of creating objects , Class instantiation process , An object is also called an instance of a class

We created one Car Class object car1, This car1 Is the name of the object . Actually this car1 It's just an identifier .

Naming rules for object names ： Little hump

Syntax for creating objects ： Class name Object name = new Class name ( ); Class name Object name = new Constructors （ Class name （））

a key

We've learned before 8 Basic data type in

From today on, our data type More , All the classes we can meet , Including what we just created Car It's a data type .（ Except for basic data types , Others are reference data types ）. All classes are reference data types

· From now on , We really understand class This keyword , Used to create a class , Used to create a data type , Because a class is a data type . image car1,car2,car3......car100 It's called quoting , It points to an area in memory , Deposit Space , We call it heap .

We need to understand two memory spaces ：

Stack ： We store variables and basic data types （ characteristic ： First in, then out ） The address is a hexadecimal string

Pile up ： The initial value of the reference data type is null

Assignment except String outside , Other reference data types need new

Member method

public ： Access modifier

void ： no return value

run( )： Method name （ identifier ）

{ }： Method body （ What does this method do ）

( )： The parameter

Parameters can have multiple , The middle is separated by commas

Java The data type needs to be specified for the parameter transfer in

When declaring a method , Parameters in parentheses ： Formal parameters （ Shape parameter ）, It can be used as a local variable

When a method is called ,

The formal parameter is at the declaration , The argument is at the call

There are ways to return values

public

boolean： return type , After a series of operations, my method , To get a specific value , Return value , The type of return value is boolean

Return value with return xxx; Type to match

A method with a return value will get a specific value after execution , This value can be saved

In development , Most methods have return values

return keyword

return Keyword termination method continues

Returns the return value of the method

Naming rules for identifiers ：

（ Letter 、 Numbers 、 Underline 、 Dollar symbol 、 The number can't start ）

Class name ： Hump （ Capitalize all words ）

Property name 、 Method name ： Little hump （ Except for the first word , Other words are capitalized ）

Access modifier

public Common

protected The protected , If cross package , Must be its subclass Only attributes can be decorated Method Constructor defaults

Amicable , You can access under the current package

private Private Only attributes can be decorated , Method Constructors

Member method

Method names cannot be repeated

If one method has static, To call a non static Methods , You can only create objects , Use object . Method

Ch01 c = new Ch01( ) ;
c.run( );

Adjust yourself ： recursive

The problem of recursion ：StackOverflowError Stack overflow （ out of memory ）

Whether to use recursion in development ？ If you use it , What to pay attention to ？

Method overloading （OverLoad）

Same method name

Different parameters can constitute overloads of methods

Different types of parameters

The number of parameters is different

The order of the parameters is different

Independent of return value type ; Independent of access modifier ; Independent of the method body . Method overloading is only related to parameters

Method overloading must occur in the same class

The benefits of method overloading

In order to make the functions similar or even the same methods, we can use the same name

Indefinite length parameter

Only （ at most ） There is an indefinite length parameter

The variable length parameter must be in the last position

· The essence of indefinite length parameter is array

  public static int sum(int...arr){
        for (int i:
             arr) {
            System.out.println(i);
        }
        return 0;
    }

·java.lang All classes under the package do not need to be explicitly imported , The rest needs to be guided

Constructors （ Construction method ）

grammar ： Access modifier Class name ( parameter list ){ }

characteristic （ characteristic ）：

Java stay new When the object , First, check whether the class corresponding to the object has a constructor , If so, what kind of constructor ; If there is no constructor , Objects cannot be built （ Create objects ）
Each class will have a default parameterless constructor
The constructor is called when the object is instantiated
We actually create objects through constructors
If a constructor is declared as shown in a class , The original default built-in parameterless constructor is invalid （ Be overwritten ）. If there are multiple constructors , Must overload .