1樓正解
& 在java中是and的意思，在泛型的應(yīng)用場景，含義基本不變。
public class EnumTypeHandler<E extends Enum<?> & BaseEnum> extends BaseTypeHandler<BaseEnum>
中的E extends Enum<?> & BaseEnum可以理解為 E extends (Enum<?> & BaseEnum)，結(jié)合extends的含義，及E 為 Enum<?> 和 BaseEnum 的子類

package TypeVarMembers;
class C {
    public void mCPublic() {}
    protected void mCProtected() {}
    void mCPackage() {}
    private void mCPrivate() {}
}
interface I {
    void mI();
}
class CT extends C implements I {
    public void mI() {}
}
class Test {
    <T extends C & I> void test(T t) {
        t.mI(); // OK
        t.mCPublic(); // OK
        t.mCProtected(); // OK
        t.mCPackage(); // OK
        t.mCPrivate(); // Compile-time error
    }
}

The type variable T has the same members as the intersection type C & I, which in turn
has the same members as the empty class CT, defined in the same scope with equivalent
supertypes. The members of an interface are always public, and therefore always inherited`
(unless overridden). Hence mI is a member of CT and of T. Among the members of C, all
but mCPrivate are inherited by CT, and are therefore members of both CT and T.
If C had been declared in a different package than T, then the call to mCPackage would
give rise to a compile-time error, as that member would not be accessible at the point where
T is declared.

來自 The Java? Language Specification, Java SE 8 Edition $ 4.4 Type Variables P58~59