Java Generic Type new instance

Question

public class PairT <T>{
    public T first;
    public T second;

    public T getFirst() {
        return first;
    }

    public void setFirst(T first) {
        this.first = first;
    }

    public T getSecond() {
        return second;
    }

    public void setSecond(T second) {
        this.second = second;
    }

    public PairT(T first, T second) {
        this.first = first;
        this.second = second;
    }
    public PairT() {
    }

What is the diffirence between these two makePair methods essentially? Why the second one is grammatically illegal?

First one:

public static <U> PairT<U> makePair(Class<U> cl) throws Exception {
        return new PairT<U>(cl.getConstructor().newInstance(),
                            cl.getConstructor().newInstance());
    }

Second:

public static <U> PairT<U> makePair(U cl) throws Exception {
        return new PairT<U(
                  cl.getClass().getConstructor().newInstance(), 
                  cl.getClass().getConstructor().newInstance());
    }

first essential difference: first is posted as text (easy to read, copy, ...); second is pasted as image (hard to read, impossible to copy, ...) || Why should I not upload images of code/data/errors when asking a question? (at least not only as image) — user16320675
– user16320675, Commented Jul 15, 2022 at 15:13
It's not grammatically illegal. Your grammar is fine. It's the type system that's complaining. U is erased to Object. cl.getClass() is Class<?>. The result of creating a class with that constructor is an instance of unknown type. — Michael
– Michael, Commented Jul 15, 2022 at 15:16
@user16320675 sorry, I just want to show the error message on the image — Jack Ma
– Jack Ma, Commented Jul 16, 2022 at 6:06
@Michael For the first case, what does param Class<U> look like after type erase? It becomes raw type Class ? — Jack Ma
– Jack Ma, Commented Jul 16, 2022 at 6:28

Pierre Carrier · Accepted Answer · 2022-07-16 08:47:05Z

Object.getClass() returns Class<?>, so you need to cast. makePair2 does so directly but at the end, makePair3 uses getClass which isolates where the casting is required.

    public static <U> PairT<U> makePair1(Class<U> cl) throws Exception {
        return new PairT<U>(cl.getConstructor().newInstance(),
                cl.getConstructor().newInstance());
    }

    public static <U> PairT<U> makePair2(U cl) throws Exception {
        return new PairT<U>(
                (U) cl.getClass().getConstructor().newInstance(),
                (U) cl.getClass().getConstructor().newInstance());
    }

    private static <U> Class<U> getClass(U u) {
        return (Class<U>) u.getClass();
    }

    public static <U> PairT<U> makePair3(U cl) throws Exception {
        return new PairT<U>(
                getClass(cl).getConstructor().newInstance(),
                getClass(cl).getConstructor().newInstance());
    }

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