Constructor, static method, abstract method, default method in interface

One . Construction method ：

1. When creating an instance , In fact, it is created through the construction method .

2. The format of the construction method is ：public + Class name （ Parameters ）{ Method body }

3. Compared with ordinary methods , The constructor does not return a value , either void, When calling the constructor , Need to use new keyword

4. The format of creating an instance is ： Class name + quote （ object ） = new Class name （ Parameters / No arguments ）;

5. When no parameters are defined , The constructor has a parameterless constructor by default public + Class name （）{ Method body }

Be careful ：（ If we customize a construction method , Then there is no compiler default parameterless constructor ）！！！

6. If you want to use both parametric construction methods , And want to keep the parameterless construction method , Then we can only define both nonparametric methods and parametric methods .

7. When there is no initialized field in the constructor , The default value of the reference type is null,int The default value is 0,boolean The default value is false

8. In a class , Multiple construction methods can be defined , stay new() Invocation time , According to the type of parameter , Number , Automatic matching of sequence and other suitable construction methods

9. One constructor can call other constructors , In order to realize code reuse , The statements that call other constructor methods are this( Parameters )

Two . Static methods

1. use static Methods of keyword modification , Can be called directly by class name , Do not use instance objects

The format is public/protected/private + static + void + Method name （）{ Method body }

2. Static methods belong to class Class level , Does not belong to instance object , So I can't access this Variable , Also cannot access the member variables of the instance object .

3. Static methods can only access static methods or static member variables

4. Ordinary methods can access static methods or static member variables

5. You can also call static methods or static member variables through instance variables ,（ The compiler automatically rewrites the instance variable to the class name , It is not recommended to use ）, If you force this call , No mistake. , But a warning will be generated .

6. Execution order ： Static code block --> Building blocks of code --> Constructors --> Common code block

7. Static methods in the interface call directly with the interface name （ The member variable of an interface can only be public atatic final type ）

8. Static methods are often used in tool classes and auxiliary methods

3、 ... and . Common method ： Modifier （public、protected、private） + Return value + Method name （ Parameter type , Parameter name ）{ Method body , Realize specific functions }

Four . Abstract method ：① If the method of the parent class does not need to implement any function , Just sign the definition method , The purpose is to let the self class override the methods of the parent class , that , Methods of the parent class can be declared as abstract methods

② The format of abstract methods ：public/protected/private + abstract + void + Method name （）;

③ Abstract methods must be in abstract classes , Abstract classes don't necessarily have abstract methods

④ When inheriting an abstract parent class , The abstract method of the parent class must be overridden （ Unless the subclass is also an abstract class , Can not rewrite ）、

5、 ... and . Default method in interface ：① Ordinary methods and construction methods are not allowed in the interface , But you can define a default Method （ Equivalent to ordinary method ）

② The format is ：public + default + void + Method name （）{}