Java - calling a method by passing child instance to a function with parameter of parent class type

Question

Can anyone please help how I can call a method (eat) of an child class (Tiger) externally by passing the instance of the child class (tiger) but the argument type is of parent class (Animal) and the parent calls has no method defined.

I have a requirement where I cannot make changes to the ParentClass as its a third-party class

Meaning the following code breaks (where parent class has no defined method eat):

package learnings;
import static java.lang.System.out;


abstract class BaseAnimal {
}

class Tiger extends BaseAnimal {
    public String eat(String x) {
        out.printf("Tiger eats %s\n", x);
        return x;
    }
}

class Lion extends BaseAnimal {
    public String eat(String x) {
        out.printf("Lion eats %s\n", x);
        return x;
    }
}

public class LearnJavaGenerics2 {

    // simple example of generic with bounded type
    // Same error with public static void callEat(BaseAnimal animal, String y) {
    public static <T extends BaseAnimal> void callEat(T animal, String y) {
        out.printf("animal Type: %s\n", animal);
        animal.eat(y);
    }

    public static void main(String[] args) {
        Lion lion = new Lion();
        Tiger tiger = new Tiger();
        callEat(lion, "foo");
        callEat(tiger, "bar");
    }

}

With Exception:

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    The method eat(String) is undefined for the type T

    at learnings.LearnJavaGenerics2.callEat(LearnJavaGenerics2.java:31)
    at learnings.LearnJavaGenerics2.main(LearnJavaGenerics2.java:37)

But the following code passes (where parent class has a defined method):

package learnings;
import static java.lang.System.out;


abstract class BaseAnimal {
    public String eat(String x) {
        out.printf("BaseAnimal eats String %s\n", x);
        return x;
    }
}

class Tiger extends BaseAnimal {
    public String eat(String x) {
        out.printf("Tiger eats %s\n", x);
        return x;
    }
}

class Lion extends BaseAnimal {
    public String eat(String x) {
        out.printf("Lion eats %s\n", x);
        return x;
    }
}

public class LearnJavaGenerics2 {

    // simple example of generic with bounded type
    public static <T extends BaseAnimal> void callEat(T animal, String y) {
        out.printf("animal Type: %s\n", animal);
        animal.eat(y);
    }

    public static void main(String[] args) {
        Lion lion = new Lion();
        Tiger tiger = new Tiger();
        callEat(lion, "foo");
        callEat(tiger, "bar");
    }

}

With Output:

animal Type: learnings.Lion@1b6d3586
Lion eats foo
animal Type: learnings.Tiger@4554617c
Tiger eats bar

Give BaseAnimal an abstract method: `abstract public String eat(String x);` or cast to child class (with inherent risks) — Hovercraft Full Of Eels, Jun 15 '18 at 00:47
I can give the BaseAnimal an abstract method. I don't have control over that class — amulllb, Jun 15 '18 at 00:49
Why does BaseAnimal have the method, but it's commented out? If you can't change BaseAnimal, then you're going to have to cast to the child class, but this is not without risk of class cast exception. — Hovercraft Full Of Eels, Jun 15 '18 at 00:51
Your other problem is lack of decent `public String toString()` method overrides. — Hovercraft Full Of Eels, Jun 15 '18 at 00:53
Also you state, `"I have a requirement where I cannot have the Parent class defining the method"`, but you don't state that there is no restriction on the parent class ***declaring*** the abstract method without defining it, and *this* is what you should do! — Hovercraft Full Of Eels, Jun 15 '18 at 00:56
Thank u, i have updated the question stating i can't even declare the abstract method as i don't have access to modify the class — amulllb, Jun 15 '18 at 00:59
You want `public abstract String eat(String x);` inside `BaseAnimal`. This can be overridden in the various subclasses. — Dawood ibn Kareem, Jun 15 '18 at 01:59
@DawoodibnKareem: see above in comments as we've already been through all this — Hovercraft Full Of Eels, Jun 15 '18 at 02:13
Oh yeah, I see. I hate questions like this. I have to do XYZ but I'm not allowed to do XYZ, so how can I do XYZ? By the way, I can't do XYZ. I bought a Ferrari, but I push it around town, because I'm not allowed to drive. — Dawood ibn Kareem, Jun 15 '18 at 03:05

Java - calling a method by passing child instance to a function with parameter of parent class type

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