Use generics with legacy code

In my project, the external API we 开发者_运维问答used is not genericfied, so there's a class called ItemList which is an implementation of java.util.List, it holds a list of Item objects. however in our new code we express this as List<Item>, I want to write a method that can takes both ItemList and List<Item>, I tried this kind of signature:

public static void readList(List<?> list) {}

it works fine, but the problem is there is a cast from Object to Item inside this method, which is used when the argument is ItemList, and not necessary for List<Item>, is there a better way to do this?

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If ItemList is an implementation of List, and if you know it contains Item instances, just cast it to List<Item>. You'll get a type safety warning, but it's not less safe than casting each element of the ItemList to Item.

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Can you write your new API to handle List<Item> so it is clean moving forward, and then provide a static conversion method to act as an adapter for the legacy uses List<Item> asList(ItemList itemList)? This way you isolate where the ugly unchecked conversion happens.

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What about providing two methods - one for each parameter type. At least you only have only one type safety warning in a known place that has been discussed and accepted:

public static void readList(List<Item> list) {
    // do something
}

@SuppressWarnings("unchecked")
public static void readItemList(List<?> list) {
    readList((List<Item>)list); // Type safety warning tucked away in here 
}

Although you don't have type safety, the name of the method readItemList is at least a strong hint to coders of the list type they are expected to be passing in.

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If I am reading the API correctly, java.lang.Class:getTypeParameters() looks promising. Try:

if(list.getClass().getTypeParameters().length == 0) {
    // Code for non-generic
} else {
    // Genericized
}