how to declare multiple classes in a file and how can we use them in java - beginner level

//-----class A----//
class A{
   public static void main(String[] arg){

        B objB=new B();
       objB.show();

       C objC=new C();
       objC.show();


   }
}//end class A

//----class B----//
class B{
  public void show(){
   System.out.println("Class B");
  } 
}//end class B

//----class C----//
class C{
 public void show(){
   System.out.println("Class C");
 }
}//end class C