Generic Type Array - Cannot Select from a Type Variable

Question

I'm attempting to convert the following method:

public List<AliasIp> getStoreAsList(String storeName) {
    return new java.util.ArrayList<>(Arrays.asList(mGson.fromJson(getStoreRaw(storeName), AliasIp[].class)));
}

To a generic version similar to:

public <T> List<T> getStoreAsList(String storeName) {
    return new java.util.ArrayList<T>(Arrays.asList(mGson.fromJson(getStoreRaw(storeName), T[].class)));
}

But I'm running into troubles with the T[].class which doesn't compile with the error:

"Cannot select from a type variable"

What should the syntax be to avoid that error?

You face the "type erasure" of Java. Take a look at stackoverflow.com/questions/529085/…, it's not 100% equal to your uestion, but in the answer to this question you can read why compiler can't understand what T[].class is. — AGV
– AGV, Commented Feb 7, 2016 at 10:43
When Java process you code, it is always remove T expression with definite class (type erasure). Java never leaves T as is. So, when it comes across T[], it has no idea, what class should be here. I suggest you follow solution of @Kolesnikovich Dmitry, and pass class as method parameter. — Ken Bekov
– Ken Bekov, Commented Feb 7, 2016 at 10:57
By the way Arrays.asList already returns a list. There is no need to copy it into an ArrayList, unless the method guarantees to return the specific type ArrayList, which is not recognizable from its current signature. — Holger
– Holger, Commented Feb 7, 2016 at 10:58
@Holger the reason for the redundant ArrayList is Arrays.asList is a wrapper around an array and does not allow list.Add() type functionality. See stackoverflow.com/questions/1624144/… — Baker
– Baker, Commented Feb 7, 2016 at 15:05
@KenBekov After reading angelikalanger.com/GenericsFAQ/FAQSections/… I imagine that T is removed during Type erasure as T is used as a Type argument. So that T[].class becomes [].class, which is clearly not a statement. Am I understanding the process correctly? — Baker
– Baker, Commented Feb 7, 2016 at 16:14

Dmitry Kolesnikovich · Accepted Answer · 2016-02-07 10:52:05Z

7

You can include Class<T[]> klass into method parameters like this:

public <T> List<T> getStoreAsList(String storeName, Class<T[]> klass) {
    return new java.util.ArrayList<T>(Arrays.asList(mGson.fromJson(getStoreRaw(storeName), klass)));
}

Then just use it like this:

List<MyStore1> myStores1 = getStoreAsList("myStoreName1", MyStore1[].class);
List<MyStore2> myStores2 = getStoreAsList("myStoreName2", MyStore2[].class);

edited Feb 7, 2016 at 10:52

answered Feb 7, 2016 at 10:40

Dmitry Kolesnikovich

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