Java泛型-意外范围

Question

No Problems here

public class MyList<E extends Number> extends ArrayList<E> {
}

.

Unexcepted bound. What does this mean? And why is it wrong? Thanks for help.

public class MyList<E extends Number> extends ArrayList<E extends Number> {

}

Answer 1

class MyList<E extends Number> is OK because you declare a type parameter, so you have to give it a name ( E ) and you can optionally declare it as bounded ( extends Number ).

In extends ArrayList<E> instead you just have to "use" a type parameter: with " <E> " you refer to the parameter declared in your class, for which a bound is already given in its declaration. " <? extends Number> " (with ? in place of E ) would also be accepted by the compiler (although it would be not what you want). Instead, " <E extends Number> " is taken as a type parameter declaration, so it is a compile error.

Answer 2

public class MyList<E extends Number> extends ArrayList<E> {
    private static final long serialVersionUID = -1025575227555594680L;

}

This should work with no compilation error and no warnings even. Let me know if you still have the same error.