One 、 object-oriented

java Is an object-oriented programming language , stay java in , Everything is an object .

Process oriented ： Process oriented is to analyze the steps needed to realize the requirements , Through some methods, step by step to achieve the requirements .

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 .

stay java in , There are three characteristics of object-oriented ：

1. encapsulation

2. Inherit

3. polymorphic

4. abstract

Member variables — attribute

· 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 （））

Define a member method

public： Access modifier , Later on

void： no return value

run()： Method name , identifier

{}： Method body . What does this method do

()： The parameter 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 parameters of the real incoming method , The actual parameter , Actual parameters . Parameters can have multiple , The middle is separated by commas Java Chuanshen in Chinese , You need to clarify the data type

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

When exactly should there be a return value , When there is no return value ？

In development , Most methods have return values

return keyword

1. return Keyword termination method continues
2. Returns the return value of the method

problem ： stay void The methods of , Can there be return？    

can

Call other methods in one method

public class Ch05 {

    String str;

   public void show(){
       //  Omit part of the code to be executed ...
       //  Call other methods in one method 
//       Ch05 c = new Ch05();
//       c.info();
       System.out.println(str);
       info();
   }

   public void info(){
       //  Omit part of the code to be executed ...

   }
}

Naming rules for identifiers ： Letter , Numbers , Underline , Dollar symbol , The number can't start 1、 Class name ： Hump 2、 Property name 、 Method name ： Little hump

 

  Focus on ：
  1、 We've learned before 8 Basic data types 
  2、 Starting today , We have more data types . All the classes we can meet ,
       Including what we just created Car It's a data type , A beauty is also a data type .
      , All classes are data types , All reference data types .
   From now on , We really understand class This keyword , Create a class , Used to create a data type .
   image car1,car2,car3....car100 It's called quoting , It points to an area in memory . Where these objects are stored 
   Space , We call it heap .

   We need to understand two memory spaces ：
  1、 Stack ： We store variables and basic data types （ characteristic ： First in, then out ） The address is a hexadecimal string 
  2、 Pile up 

  The initial value of the reference data type is null

public class Ch03 {

    public static void main(String[] args) {
//        Car car;  Declared a Car Properties of type car,
        //  assignment , except String Outside , Other reference data types need new
        Car car = new Car();
        //  Just see new, It's not null, It has an address 
        System.out.println(car);
    }
}

 

recursive

 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 

 

Method overloading

  Method overloading （Overload）
 1. Same method name 
 2. Different parameters （ Type of parameter , Number of parameters , The order of the parameters ）
 3. Occurs in the same class 

  Independent of return value type , Independent of access modifier , Independent of the method body 

  The benefits of method overloading ：  In order to use the same method with similar functions 

 Indefinite length parameter 
1. In the parameter list , At most one variable length parameter 
2. The variable length parameter must be in the last position 

· The essence of indefinite length parameter is array

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

 public int sum(int [] arr){
    result = 0;

practice ：

  demand ：
 * Write two overloaded methods
 * 1. One method is to find the maximum value of two numbers
 * 2. Another method is to find the maximum of three numbers

public class Ch03 {

    public int max(int num1,int num2) {

        return num1 > num2 ? num1:num2;
    }

    public int max(int num1,int num2,int num3){
        int a = (num1 > num2 ? num1 : num2);
        return a > num3 ? a : num3;
    }

    public static void main(String[] args) {
        Ch03 c = new Ch03();
        c.max(1,2,3);
    }
}

Access modifier

1. public Common , Public   Maximum authority , As long as it is public Declared method , As long as in the same project , Can be used .

· It can be placed in the class 、 attribute 、 Method 、 On the constructor

1. protected The protected ： Package can span , If the cross package must be its subclass

· Can be placed in the attribute 、 Method 、 On the constructor

1. default （ Write nothing ）default： Only the current package can be accessed （ You can't lead the bag in ）

· It can be placed in the class 、 attribute 、 Method 、 On the constructor

1. private Private   Minimum authority , Only the current class can be used

· Can be placed in the attribute 、 Method 、 On the constructor

 

Constructors  

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

characteristic （ characteristic ）：

1. 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 ）
2. Each class will have a default parameterless constructor
3. The constructor is called when the object is instantiated
4. We actually create objects through constructors
5. 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 .

1. public class Ch06 {
   
       public static void main(String[] args) {
           //  The process of creating objects , The process of instantiation , Create a Dog Class 
           //  Class name   Object name  = new  Constructors ;
           Dog dog = new Dog("");
   
       }
   }

Learning experience ：

Today's content is not very understood , You also need to watch videos to review and understand , Today's content is slightly understood , But you can't write the code yourself , This process needs more exercise , Think more ,