[Java Silver] About initialization

Instance initialization

-Initialize with ** constructor ** -** Instance variable ** is initialized at ** instance creation **

** Constant (final) ** can also be ** initialized with the constructor ** Uninitialized constants defined in the class can be initialized in the constructor

Example)

public class Main{
   public static void main(String[] args){
       Sample a = new Sample(5);
       System.out.println(a.num);   //Is displayed as 5

   }
}

class Sample{
     final int num;  //The constant field is not initialized at this point
     Sample(int num){  //int num is a local variable with the same name as the instance variable
         this.num = num;  //Initialize constants in constructor
     }
}

Initialize the common processing of the constructor

-Use an initializer (instance initializer) called {} --If there are multiple constructors overloaded, common processing can be performed first in the instance initializer. --The instance initializer runs ** just before it is instantiated **

Example)

public class Main{
   public static void main(String[] args){
       Sample a = new Sample(5);    //"Run Instance Initializer"
       System.out.println(a.num);   //5
       
       Sample b = new Sample();    //"Run Instance Initializer"
       System.out.println(b.num);  //10

   }
}

class Sample{
     final int num;  //The constant field is not initialized at this point
     {
        System.out.println("Run Instance Initializer"); 
     }
     Sample(){
         this.num = 10;
     }
     Sample(int num){  
         this.num = num;  //Initialize constants in constructor
     }
}

Initialize static field

Static fields can be used without instantiating. Therefore, ** cannot be initialized by the constructor **

→ It is necessary to initialize with a static initializer (initializer)

Example)

public class Main{
   public static void main(String[] args){       
       System.out.println(Sample.num);  //100 and output
   }
}

class Sample{
     static final int num;  //Static constants have not been initialized at this point
     static {
        num = 100;  //Initialization of static constants
     }     
}

The order in which the initializers operate

** Initializer-> Instance Initializer-> Constructor **

Example)

public class Main{
   public static void main(String[] args){
       User a = new User();   //Output "Execution of initializer, execution of instance initializer, execution of constructor"
  }

}

class User{
  private static int count;
  static{
      User.count = 0;  //Initialize static variables
　　　 System.out.println("Run initializer");
  }
 
  {
     System.out.println("Run Instance Initializer");
  }

  User(){
     System.out.println("Executing the constructor");
  }
}