CODEWITHSUNDEEP
javaclass
datafiddler
datafiddler

Reputation: 1835

How to access base class method?

Simplified demo code to show my problem.

class Base {
   public String toString() { return "Base"; }
};
class A extends Base {
   public String toString() { return "A"; }
};

class Test {
  public void test1() {
    A a = new A();
    Base b = (Base)a;    // cast is fine, but b is the same instance as a
    System.out.println(b.toString()); // want "Base", but get "A"
  } 

  private String testB(Base b) {
    return b.toString();   // this should return "Base"
  }   
  public void test2() {
    System.out.println( testB(new A()) );  // want "Base", but get "A"
  } 

};

I tried the cast approach (test1) , and the helper method (test2).

Up to now, I found to need a copy constructor for Base to create a real Base object.

Is there a method that does not need a duplicate object?

Some background info:

I get an instance of A, and I know its base class has a nice method, which I'd like to use instead of the overwritten version. I'd prefer to neither modify class A nor B (although a copy c'tor were a good enhancement anyway ;) )

Upvotes: 4

Views: 312

Answers (2)

Michael Berry
Michael Berry

Reputation: 72359

From class A directly, you can use super.toString(); to execute toString() on Base.

However, from outside class A, you can't call the superclass implementation in this way, doing so would break encapsulation. If you want to expose the superclass implementation then you still can, but you have to provide a separate method on A that exposes it directly.

Even using a trivial reflection based approach, you still won't be able to access it:

If the underlying method is an instance method, it is invoked using dynamic method lookup

System.out.println(Base.class.getMethod("toString", null).invoke(new A(), null)); //Prints "A"

...and using MethodHandles.lookup().findSpecial won't work either from outside the child class, as that has to be invoked where you have private access (otherwise you'll just get an IllegalAccessException.)

I concede that there may well be some weird and wonderful way of doing it directly in Java that I haven't thought of without bytecode manipulation, but suffice to say even if you can do it that way, you certainly shouldn't for anything but a quirky technical demonstration.

Upvotes: 7

cshapdev
cshapdev

Reputation: 152

You need to create the B instance(copy constructor), if you are using the A instance you will always get "A" no matter if you cast it or no.

Upvotes: 1

Related Questions