了解Java泛型 <T extends Class> 和 <? extends Class>

Question

This may be a dumb question so please excuse my ignorance.

Lets say I have a class:

public class Foo<T extends Base> implements Bar<T> {
    private Bar<T> wrapped;
    public void setWrapped(Bar<T> input) {
        wrapped = input;
    }
}

If I call it with:

//Lets just assume methods getNewVal and getFoo exist
Bar<? extends Base> val = getNewVal();
Foo<? extends Base> foo = getFoo();
foo.setWrapped(val);

The compiler says foo.execute(val) is an error. With a message along the lines of The method setWrapped(Bar<capture#20-of ? extends Base>) in the type Foo<capture#20-of ? extends Base> is not applicable for the arguments (Bar<capture#22-of ? extends Base>) .

If I try to change Foo to be

public class Foo<T extends Base> implements Bar<T> {
    private Bar<T> wrapped;
    public void setWrapped(Bar<? extends Base> input) {
        wrapped = input;
    }
}

The call to foo.setWrapped(val) no longer errors. Instead wrapped = input is an error with a message along the lines of Type mismatch: cannot convert from Bar<capture#1-of ? extends Base> to Bar<T> .

What am I doing wrong? Is there not a way to get the compiler to be okay with a call like this without casting?

Answer 1

It looks like what you want is

public class Foo {
    private Bar<? extends Base> wrapped;
    public void setWrapped(Bar<? extends Base> input) {
        wrapped = input;
    }
}

It's not clear what the Bar interface Foo needs to implement is, though, and why Foo should have a Bar as well as be a Bar .