Java generics restrictions with interfaces

Question

abstract class

public abstract class Animal {

private int id;
private String name;

public Animal(int id, String name) {
    this.id = id;
    this.name = name;
}}

_child of animal 1

public class Tiger extends Animal implements Dangerous {

public Tiger(int id, String name) {
    super(id, name);
} }

_child of animal 2

public class Panda extends Animal implements Harmless{

public Panda(int id, String name){
    super(id, name);
}}

_ Two attribute interfaces

public interface Dangerous {}
public interface Harmless {}

public class Zoo {

public static <T extends Animal & Harmless> void tagHarmless(Animal animal) {
    System.out.println("this animal is harmless");
}

public static <T extends Animal & Dangerous> void tagDangerous(Animal animal) {
    System.out.println("this animal is dangerous");
}}

public class App {
public static void main(String[] args) {

    Animal panda = new Panda(8, "Barney");
    Animal tiger = new Tiger(12, "Roger");

    Zoo.tagHarmless(panda);
    Zoo.tagHarmless(tiger);

}}

-result

this animal is harmless
this animal is harmless

Process finished with exit code 0

i try to restrict the methods of the class "zoo" with the interfaces "Dangerous" and "Harmless".

with the code

public static <T extends Animal & Harmless> void tagHarmless(Animal animal).

The Tiger doesnt have this Interface, so it actually should not work, does it? But the tiger can also be added into this method tagHarmless.

I don't see the mistake.

Thanks for help.

Eran · Accepted Answer · 2021-07-27 09:42:01Z

8

You are declaring a generic type parameter T, but your method is never using it. Your method accepts an Animal argument, which means any Animal is acceptable.

It should be:

public static <T extends Animal & Harmless> void tagHarmless(T animal) {
    System.out.println("this animal is harmless");
}

As for your main method, you are assigning the Panda and Tiger instances to Animal variables. Therefore, changing tagHarmless as I suggested means that neither the panda nor the tiger variables can be passed to tagHarmless (since an Animal doesn't implement Harmless).

If you change your main to:

Panda panda = new Panda(8, "Barney");
Tiger  tiger = new Tiger(12, "Roger");

Zoo.tagHarmless(panda);
Zoo.tagHarmless(tiger);

The call to Zoo.tagHarmless(panda); will pass compilation, and the call to Zoo.tagHarmless(tiger); will not.

edited Jul 27, 2021 at 9:42

answered Jul 27, 2021 at 9:37

Eran

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1 Comment

Rido Over a year ago

then i get a problem with the class App "Zoo.tagHarmless(panda);". Required type T , provided Animal

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