Java：声明为父类，但实例化为Child，然后在Child中使用唯一方法

Question

I ran into this problem because I need to instantiate based on user input. Considering the following.

   public class Parent{
       public void foo(){
         System.out.println("foo");
       }
   }

   public class Child extends Parent{
       public void UniqChildMethod(){
          System.out.println("I am unique")
       }
   }

I need to create parent or child based on user input.So lets say if the input is 0. mother object is created, if the input is 1, child is created. Since a bunch of share methods will be call on this object, i dont want to write the same code twice under if/else condition. So my workaround is that, I create a parent object set to null, and instantiate base on user input.

Here is when problem came:

public static void main(String argv[]) {
        Parent obj = new Child();

        obj.UniqueChildMethod();   <--- The method is underfined for the type parent
   }

is there a workaround for this issue?

Answer 1

If you know your Parent is indeed a Child object reference, yo can cast it and call child methods:

((Child) obj).UniqueChildMethod();

In typical app, one should check for object type before class to avoid ClassCastException :

if (obj instanceof Child) {
   Child childObj = (Child) obj;
   childObj.UniqueChildMethod(); 
}

Answer 2

You can use switch to solve this problem.

 public static void main(String argv[]) {
    Parent obj = null;
    Scanner scanner = new Scanner(System.in);
    int x = scanner.nextInt();

    switch (x){
        case 0 :    obj = new Parent();
                    obj.foo();
                    break;
        case 1 :    obj = new Child();
                    ((Child) obj).UniqChildMethod();
                    break;
                    default: obj = null;
    }
}