为什么在父类中必须强制有一个子方法以引用父类的引用来调用它，该父类引用子类的对象？

Question

public class A
{  }

public class B extends A
{
  public void add()
  {
   System.out.println("add in B");
  }
}

Now here if we call add in following way hen it gives an error: A a1 = new B; a1.add();

But when we add the add() method in class A and then call in the similar fashion then add() method of child class is called.

ie

public class A
{
  public void add()
  {
   System.out.println("add in A");
  }
}

public class B extends A
{
  public void add()
  {
   System.out.println("add in B");
  }
}

call:

A a1 = new B;
a1.add();

output:

add in B

Why is it so?

Answer 1

at the method invocation of a1.add() the compiler checks if the method is present. But it only knows that a1 is a reference to an object of class A, which does not have that method. So the compilation fails.

In this trivial example it would probably be easy for the compiler to deduct the correct type. But in more general cases it wouldn't. And therefore this kind of logic is not part of the specs.

Answer 2

Because java does not know at compile time that a1 will refer to an instance of B at runtime. It only knows the declared type, so it only allows calls that work with the declared type.

Answer 3

When the Java compiler looks at the reference a1 , it knows what methods are available. In your first example, class A does not have add() in its API. It is legal in this case to perform a cast of a1 to B like so:

((B)a1).add();

and the compiler will not complain.

Answer 4

您希望在声明的类型A的对象上调用方法，但仅在A的子类B中实现它。在这种情况下，通常将A作为抽象类，并在A中将add（）声明为抽象方法。

Answer 5

Good answers...I had a doubt too regarding this but now I am clear :) And one more thing ..you don't have to go into the trouble of declaring an abstract method,just make an empty method with the same name and signature in your parent class and " voila " all compilation errors are gone ;)

In your case you can add a void add(){} method like this and you wont have any problems