Use of construction methods

Construction method

 Constructor is a special method in class , It is usually created by the system （ Class instantiation ） Automatically call . The constructor is the first object to be 
 Method of execution , Mainly used to apply for memory 、 Initialize the member variables of the class . Although the constructor is also located in the class , But in many cases 
 Different from the ordinary member method , So some people think it is not a member method , And call it “ Constructors ”. The general form of the construction method is ：

 Constructor name ([ parameter list ]){
    
  [this([ parameter list ]);] |[super([ parameter list ]);]
   Statement sequence 
}

 among ,this Is to call other construction methods ,super Is to call the constructor of the parent class . They must be placed in front of other statements .
 The compilation construction method should pay attention to the following points ：

 1.  The name of the constructor must be exactly the same as the name of the class .
 2.  In addition to the access modifier , There cannot be any other modifiers , There can be no return value .
 3.  Although there is no return value , But it doesn't work “void” modification .
 4.  The construction method cannot be used static and final To modify . In general, it is not necessary to private modification , This will result in the inability to create objects externally .
 5.  Constructors cannot be explicitly called by objects . Usually by new Keyword to call , Or use this、super To call .
 6. this() and super() Can't exist at the same time 
 7.  The parameter list of the constructor can be empty , You can also have parameters . According to the presence or absence of parameters , It can be divided into the construction method without parameters and the construction method with parameters .
 8.  User defined classes can have multiple constructor methods , But the required parameter list is different .
 9.  If the user-defined class does not provide any constructor , The system will automatically provide a parameterless construction method .

1、 Definition and use of parameterless construction method

Examples of parameterless construction methods ：

 Calling the constructor uses “new Animal()”, This is an implicit call method . Notice the member variable , It has been given the beginning at the time of definition 
 value . In the constructor , First, output a message , And then again for x assignment . Because the execution of the construction method is after the member variable is defined , It will cover 
 The original initial value , So the value of the variable is 100.

If we add “void” It will look like this ：

 The output result is the initial value of the variable , It shows that the parameterless construction method we defined does not execute at all . This is because of the addition of “void” After the modifier ,
constructNoPara() It is no longer a construction method , It has become a common method . What we call is the default nonparametric constructor provided by the system itself 
 Law .

// The default nonparametric construction method provided by the system 
public Animal() {
    
}
// This method actually does nothing , Naturally, it will not change x Value .

2、 Definition and use of construction method with parameters

 In many cases , You need to assign different initial values to member variables according to different situations , This requires passing parameters to the constructor . therefore ,Java Zhongyunding 
 Construction method of semantic parameters , Moreover, this construction method with parameters can also define multiple （ The premise is that the parameter list is different ） This phenomenon is called the constructor 
 Overloading of law .

Java Regulations , If the programmer does not define a constructor , Then the system will automatically add a construction method without parameters . If the programmer goes to 
 Less than one construction method is defined , Then the system will no longer provide a construction method without parameters .

 When used new To create an object , You need to provide type compatible parameters , Otherwise, the compiler will report an error .

 Here, when defining the parameter construction method , At the same time, a parameter free construction method is defined , It is to let others create in the parameter free construction method that they habitually use 
 Object will not report an error , So even if you can't use this parameter free construction method, you have to define .

Be careful ：

 Method overloading ： The method names in a class are the same , Different parameter list , It has nothing to do with the return value   
 Different parameter lists ：
			1  The number of parameters is different 
			2  Different types of parameters 
			3  The order of the parameters is different 
 This phenomenon is caused by the construction method , But not only constructors can be overloaded , Methods defined by ourselves can also be overloaded .
 Advantages of overloading ： It can be used when the method is called , Automatically select the method to be executed according to the type of parameter 

3、this Keywords and constructor calls

this The role of keywords , Is used to call constructors explicitly . Its usage format is as follows ：

 Constructor name ([ parameter list ]){
    
  [this([ parameter list ]);]
   Statement sequence 
}

 The system will decide which constructor to call according to the parameter list . Use this You should also pay attention to the following points ：
 1.  use this When a constructor is called , This statement can only be used in constructor .
 2. this The statement must be the first statement in the constructor .
 3.  and new Different ,this Although you can call the constructor , But it just executes the statements in the constructor , Objects are not created .
 4. this() and super() Can't exist at the same time 

 stay main() Method uses a parameterless construction method to create objects , In the construction method without parameters , use this To call the constructor with parameters , however 
 Then output a message . Be careful this The call constructor can only be used as the first statement . The reason for this is , In many cases , Multiple structures 
 Method may do the same thing , It's just that the parameters are a little different , This can extract the same code separately into a construction method , Then make 
 use this To call it .

4、super Keywords and constructor calls

super The role of keywords , If the subclass and parent have the same method , You can use super call
If the class is on an inheritance chain , He can call public methods in his own parent class and above the parent class . Its usage format is as follows ：

 Constructor name ([ parameter list ]){
    
  [super([ parameter list ]);]
   Statement sequence 
}

adopt super（ Up ） key word , Verify the positional relationship between father and son . Use super You should also pay attention to the following points ：
1. this The statement must be the first statement in the constructor
2. super Variables of the parent class can be called , At this time, pay attention to the access permission modifier
3. Class's default nonparametric constructor , There is a sentence by default super()
4. this() and super() Can't exist at the same time

In the original Animal Class eat() Method , Override... In a subclass , In the subclass constructor , Call directly eat() Method and Application super Key word call eat() Method .

stay main() Use in method Cat Class to create objects , In the construction method without parameters , use suoer To call the constructor with parameters in the parent class , Then output a message . From the output, we can see that the parent object is superior to the child object , The construction method of the parent class is better than that of the child class. It executes first .