Detailed explanation of the difference between construction method and method

 

Conclusion ！！！

Although there is no construction method in the student class But in the test code Student Object is also created . The reason is that when there is no constructor in the class, the system will construct a parameterless constructor by default  

The difference between construction method and common method structure is as follows ：

 [ List of modifiers ] Constructor name ( List of formal parameters ){
             Construct method body ;
            // You usually assign values to attributes in the constructor body , Complete the initialization of the property .
        }
         Be careful ：
             First of all ： The modifier list is currently uniformly written as ：public It must not be written as public static
             second ： Constructor name and class name must be consistent
             Third : The constructor does not need to specify the return value type , You can't write void, Writing words has become a common method .

The common method structure is ：
        [ List of modifiers ] return type Method name ( List of formal parameters ){
             Method body ;
        }
         Be careful : The modifier list is ： public static

How to call the constructor ？

The difference between a constructor and a normal method call ：

Common method ：

public class OOTest01
{
	public static void main(String[] args){
			//  Calling normal methods 
			dosome();   //  In the same class, there is no   Class name .dosome();  It's fine too 
            

	
	//  Common method 
		public static void dosome(){
			System.out.println("dosome");
		}
}

Construction method ：

Use new Operator to call the constructor .
         Grammar format :
            new Constructor name ( List of formal parameters );

        // Student s =new Student(); among Student(); It's actually a construction method

Students ： 

public class Student
{
	// Student number 
	int no;
	// full name 
	String name;
	// Age 
	int age;

	// Current Student There is no constructor defined in this class 
	// But the system will default to Student Class provides a parameterless constructor 

	//  The construction method without parameters ( Default constructor ) Write it down as follows ：
	public Student(){
		System.out.println(" The parameterless construction method is implemented ~");
	}

}

Test code ：

public class OOTest01
{
	public static void main(String[] args){
			//  Call constructor 
			new Student();	//  First execution 
			
			//  Calling normal methods 
			dosome();

		//  establish Student Object of type 
		Student s =new Student();	//  Second execution    new Student() 

		//System.out.println(s.name);   // null

		//  Output " quote s"
		//  As long as the output is not null  This indicates that this object must have been created .
	    //  Output results ： [email protected]
		System.out.println(s);
		}
	
	//  Common method 
		public static void dosome(){
			System.out.println("dosome");
		}
}

Running results ：

When defining a constructor with parameters ：

public class Student
{
	// Student number 
	int no;
	// full name 
	String name;
	// Age 
	int age;

	// Current Student There is no constructor defined in this class 
	// But the system will default to Student Class provides a parameterless constructor 

	// The construction method without parameters ( Default constructor ) Write it down as follows ：
	/*
	public Student(){
		System.out.println(" The parameterless construction method is implemented ~");
	}
	*/

	// Define a construction method with parameters 
	public Student(int i){
		System.out.println("~");
	}
}

Test code ：

public class OOTest01
{
	public static void main(String[] args){
			//  Call constructor 
			new Student();	//  First execution 
			
			//  Calling normal methods 
			dosome();

		//  establish Student Object of type 
		Student s =new Student();	//  Second execution    new Student() 

		//  As long as the output is not null  This indicates that this object must have been created .
	    //  Output results ： [email protected]
		System.out.println(s);
		}
	
	//  Common method 
		public static void dosome(){
			System.out.println("dosome");
		}
}

  The operation results are as follows ：

  When defining a parameterless constructor and a parameterless constructor ：【 Similar method overloading mechanisms 】

public class Student
{
	// Student number 
	int no;
	// full name 
	String name;
	// Age 
	int age;

	// Current Student There is no constructor defined in this class 
	// But the system will default to Student Class provides a parameterless constructor 

	// The construction method without parameters ( Default constructor ) Write it down as follows ：
	
	public Student(){
		System.out.println(" The parameterless construction method is implemented ~");
	}
	

	// Define a construction method with parameters 
	public Student(int i){
		System.out.println("~");
	}
}

Test code ：

public class OOTest01
{
	public static void main(String[] args){
			//  Call constructor 
			new Student();	//  First execution 
			
			//  Calling normal methods 
			dosome();

		//  establish Student Object of type 
		Student s =new Student();	//  Second execution    new Student() 

		//  As long as the output is not null  This indicates that this object must have been created .
	    //  Output results ： [email protected]
		System.out.println(s);
		}
	
	//  Common method 
		public static void dosome(){
			System.out.println("dosome");
		}
}

Running results ：

  When giving test code Student(); When parameters are passed ：

public class OOTest01
{
	public static void main(String[] args){
			//  Call constructor 
			new Student(100);	//  First execution 
			
			//  Calling normal methods 
			dosome();

		//  establish Student Object of type 
		Student s =new Student(100);	//  Second execution    new Student() 

		//  As long as the output is not null  This indicates that this object must have been created .
	    //  Output results ： [email protected]
		System.out.println(s);
		}
	
	//  Common method 
		public static void dosome(){
			System.out.println("dosome");
		}
}

Running results ：

  reflection ：【 In the future, try to write the parameterless construction method 】

    1、 reflection ： When the instance variable is not assigned manually , In fact, the system will assign values by default ,
     So when is the default assignment performed ？
         Do you assign values to these instance variables when the class is loaded ？
             No ( Class loading occurs in the method area At this time, it is not in the stacking area new An object )
             Instance variables are initialized during the execution of the constructor , Complete the assignment .
            【 Because when new Object time Such as new Student(); Student(); It's actually a construction method 】

public class User
{
	int id;
	String name;
	int age;
	
	//  Parameterless construction method 
	public User(){
		// There are actually three lines of code that you can't see .
		// Although there is no code written in the body of the parameterless constructor ,
		// But in fact, the default value initialization of instance variables is carried out in this method body ( Instance variables can be modified )
		/*
		id =0;
		name ="junker";
		age =0;
		*/
	}

	//  There are parameter construction methods 
	public User(int i){
		/*
		id =0;
		name =null;
		age =0;
		*/
	}
}

public class OOTest02
{
	public static void main(String[] args){
		
		User u =new User();
		System.out.println(u.name);//junker
		User a =new User(100);
		System.out.println(a.name);//null
	}
}

Constructor code walkthrough

public class Vip
{
	// Membership number 
	long no;
	// Membership name 
	String name;
	// Birthday 
	String birth;
	// Gender 
	boolean sex;

	// Parameterless construction method 
	public Vip(){
		
	}
	
	// There are parameter construction methods 
	public Vip(long huiyuanHao){
		no =huiyuanHao;
		// Default name =null;birth =null;sex =false; 
		
	}
	public Vip(long huiyuanHao,String xingMing){
		no =huiyuanHao;
		name =xingMing;
	}
	public Vip(long huiyuanHao,String xingMing,String shengRi){
		no =huiyuanHao;
		name =xingMing;
		birth =shengRi;
	}
	public Vip(long huiyuanHao,String xingMing,String shengRi,boolean xingBie){
		no =huiyuanHao;
		name =xingMing;
		birth =shengRi;
		sex =xingBie;
	}	
}

public class OOTest03
{
	public static void main(String[] args){
		
		Vip v1 =new Vip();
		System.out.println(v1.no);//0
		System.out.println(v1.name);//null
		System.out.println(v1.birth);//null
		System.out.println(v1.sex);//false

		Vip v2 =new Vip(11111L);
		System.out.println(v2.no);//11111L
		System.out.println(v2.name);//null
		System.out.println(v2.birth);//null
		System.out.println(v2.sex);//false

		Vip v3 =new Vip(22222L," Little sheep ");
		System.out.println(v3.no);//22222L
		System.out.println(v3.name);// Little sheep 
		System.out.println(v3.birth);//null
		System.out.println(v3.sex);//false

		Vip v4 =new Vip(33333L," The Wolf ","60-6-6");
		System.out.println(v4.no);//33333L
		System.out.println(v4.name);// The Wolf 
		System.out.println(v4.birth);//60-6-6
		System.out.println(v4.sex);//false

		Vip v5 =new Vip(44444L," Iron man ","10-1-1",false);
		System.out.println(v4.no);//44444L
		System.out.println(v4.name);// Iron man 
		System.out.println(v4.birth);//10-1-1
		System.out.println(v4.sex);//false
		
	}
}