CODEWITHSUNDEEP
javagenerics
Ralph
Ralph

Reputation: 4868

How to define a generic List of types in java?

I have implemented a java method with the following signature:

public <T> T getItemValue(String itemName, Class<T> itemType)  {
...
}

This allows a client to call the method in the following way to get for example a value of the type String or Integer:

String s = itemCol.getItemValue("_name", String.class);
int i = itemCol.getItemValue("_count", Integer.class);

This kind of method signature is also used by the Config interface of the new microprofile Config 1.3 API.

My question is how - or if - I can call the method with a Type of List of Types like List<String> to get for example a List of String objects. I was not able to formulate the client call. I tried something like this:

List<String> list = itemCol.getItemValue("_count", List<String.class>);

but this seems not to be the correct syntax.

EDITED:

As a result of the responses below I decided to add a separate method to get a List of a specific type.

public <T> List<T> getItemValueList(String itemName, Class<T> itemType) {
        ...
}

With this additional method signature a client can decide weather to get a single value or a list of a specific type.

Upvotes: 0

Views: 883

Answers (2)

Kayaman
Kayaman

Reputation: 73578

Not with List<String.class>, no. You need to use a TypeToken to get that sort of generic information. You also need to change your code to work with Type instead of Class.

Upvotes: 4

J&#246;rg W Mittag
J&#246;rg W Mittag

Reputation: 369623

Type Parameters are erased, they don't exist at runtime. There is no such thing as List<String> at runtime, there is only List.

So, the correct way to express a reflective proxy for List<String> is … you can't. You can only express a reflective proxy for List, the same way you do for any other class: List.class.

Upvotes: 5

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