Effective 자바 - 사유 구조 기 강화 단일 속성 최 적 화 된 솔 루 션 매 거

 public class Elvis {
     public static final Elvis INSTANCE = new Elvis();
     private Elvis() { }
     public void leaveTheBuilding() {
         System.out.println("Who a baby, I'm outta here!");
     }
     // This code would normally appear outside the class!
     public static void main(String[] args) {
         Elvis elvis = Elvis.INSTANCE;
         elvis.leaveTheBuilding();
     }
   }

public class Elvis {
         private static final Elvis INSTANCE = new Elvis();
         private Elvis() { }
         public static Elvis getInstance() { return INSTANCE; }
         public void leaveTheBuilding() {
             System.out.println("Who  a baby, I'm outta here!");
         }
         // This code would normally appear outside the class!
         public static void main(String[] args) {
             Elvis elvis = Elvis.getInstance();
             elvis.leaveTheBuilding();
         }
       }  

private int add4(int x1, int x2, int x3, int x4) {  
        return add2(x1, x2) + add2(x3, x4);  
    }  

    private int add2(int x1, int x2) {  
        return x1 + x2;  
    }
//         jvm             
private int add4(int x1, int x2, int x3, int x4) {  
        return x1 + x2 + x3 + x4;  
    }  

enum SingletonDemo{
        INSTANCE;
        public void otherMethods(){
            System.out.println("Something");
        }
    }

enum Color{
    RED,GREEN,BLUE;
}
public class Hello {
    public static void main(String[] args){
        Color color=Color.RED;
        int counter=10;
            switch (color){
                case RED:
                    System.out.println("Red");
                    color=Color.BLUE;
                    break;
                case BLUE:
                    System.out.println("Blue");
                    color=Color.GREEN;
                    break;
                case GREEN:
                    System.out.println("Green");
                    color=Color.RED;
                    break;
            }
        }
    }
}

  enum Color{
    RED(1),GREEN(2),BLUE(3);
    private int code;
    Color(int code){
        this.code=code;
    }
    public int getCode(){
        return code;
    }
  }