(1) List<?> myList = new ArrayList<?>();
(2) ArrayList<?> myList = new ArrayList<?>();
use (2) if code is the "owner" of the list. This is for example true for local-only variables. There is no reason to use the abstract type List instead of ArrayList. Another example to demonstrate ownership:
public class Test {
// This object is the owner of strings, so use the concrete type.
private final ArrayList<String> strings = new ArrayList<>();
// This object uses the argument but doesn't own it, so use abstract type.
public void addStrings(List<String> add) {
strings.addAll(add);
}
// Here we return the list but we do not give ownership away, so use abstract type. This also allows to create optionally an unmodifiable list.
public List<String> getStrings() {
return Collections.unmodifiableList(strings);
}
// Here we create a new list and give ownership to the caller. Use concrete type.
public ArrayList<String> getStringsCopy() {
return new ArrayList<>(strings);
}
}
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然后呢？
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我比较常用的是List<String> creditCardYears = new ArrayList<String>();