Java generics super usage

Question

I am trying to understand why the following is not compiling.

public class AnimalHolder<T super Animal> {
        T animal;
        public static void main(String[] args) {
                AnimalHolder<Object> objectHolder = new AnimalHolder<Object>();

        }
}

As I understand I can use anything that IS A Animal or super type of Animal(in this case Object). Can any one explain?

Compiler Message:

AnimalHolder.java:15: error: > expected
public class AnimalHolder<T super Animal> {
                           ^
AnimalHolder.java:15: error: <identifier> expected
public class AnimalHolder<T super Animal> {
                                        ^
AnimalHolder.java:17: error: illegal start of expression
        public static void main(String[] args) {
        ^
AnimalHolder.java:17: error: illegal start of expression
        public static void main(String[] args) {
               ^
AnimalHolder.java:17: error: ';' expected
        public static void main(String[] args) {
                     ^
AnimalHolder.java:17: error: '.class' expected
        public static void main(String[] args) {
                                         ^
AnimalHolder.java:17: error: ';' expected
        public static void main(String[] args) {
                                             ^
AnimalHolder.java:21: error: reached end of file while parsing
}
 ^
8 errors

If you have a question about compilation failure, **always** post the compilation error message! — Oliver Charlesworth, Apr 20 '14 at 11:46
Possibly useful thread http://stackoverflow.com/questions/4902723/why-cant-a-java-type-parameter-have-a-lower-bound — tzima, Apr 20 '14 at 11:59

score 2 · Accepted Answer · answered Apr 20 '14 at 12:05

2

The problem is you can't specify a lower bound for a generic parameter:

public class AnimalHolder<T super Animal> { // Can't do this

You can only specify an upper bound:

public class AnimalHolder<T extends Animal> { // Must use "extends"

answered Apr 20 '14 at 12:05

Bohemian

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To be honest, I didn't understand the difference between the two. That is why I posted this question. – Apr 20 '14 at 12:07
`T super X` isn't very useful if you think about it. All you can say for sure is T is `Object`, which tells you nothing. Read the duplicate question for more info. – Bohemian Apr 20 '14 at 12:10

Java generics super usage

1 Answers1