List、List<?>、List<T>、List<E>和List<Object>之间的区别 [英] Difference between List, List<?>, List<T>, List<E>, and List<Object>
问题描述
List、List、List、List 和 List?
What are the differences between List, List<?>, List<T>, List<E>, and List<Object>?
List:是原始类型，因此不是 typesafe.它只会在铸造错误时产生运行时错误.当演员表不好时，我们想要一个编译时错误.不推荐使用.
List: is a raw type, therefore not typesafe. It will only generate a runtime error when the casting is bad. We want a compile time error when the cast is bad. Not recommended to use.
List 是无界通配符.但我不确定它是做什么用的?我可以毫无问题地打印 List:
List<?> is an unbounded wildcard. But I'm not sure what it's for? I can print a List<?> without issue:
public static void test(List<?> list){
System.out.println(list); // Works
}
为什么我不能将项目添加到List?
public static void test(List<?> list){
list.add(new Long(2)); // Error
list.add("2"); // Error
System.out.println(list);
}
3.列表
public static void test(List<T> list){ // T cannot be resolved
System.out.println(list);
}
我不明白这个语法.我看到了这样的东西，它有效:
I don't understand this syntax. I saw something like this, and it works:
public <T> T[] toArray(T[] a){
return a;
}
有时，我看到，
Sometimes, I see <T>, or <E>, or <U>, <T,E>. Are they all the same or do they represent something different?
这给出了错误方法test(List不适用于参数List":
This gives the error "The method test(List<Object>) is not applicable for the argument List<String>":
public static void test(List<Object> list){
System.out.println(list);
}
如果我尝试这个，那么我得到无法从 List 转换为 List":
If I try this then I got "Cannot cast from List<String> to List<Object>":
test((List<Object>) names);
我很困惑.String 是 Object 的子类，那么为什么 List 不是 List?
I am confused. String is a subclass of Object, so why isn't List<String> a subclass of List<Object>?
推荐答案
1) 正确
2) 您可以将其视为只读"列表，您可以在其中不关心项目的类型.可以例如由返回列表长度的方法使用.
2) You can think of that one as "read only" list, where you don't care about the type of the items.Could e.g. be used by a method that is returning the length of the list.
3) T、E 和 U 是相同的，但人们倾向于使用例如T 代表类型，E 代表元素，V 代表值，K 代表键.编译的方法说它接受了某个类型的数组，并返回了一个相同类型的数组.
3) T, E and U are the same, but people tend to use e.g. T for type, E for Element, V for value and K for key. The method that compiles says that it took an array of a certain type, and returns an array of the same type.
4) 你不能把橙子和苹果混在一起.如果您可以将字符串列表传递给需要对象列表的方法，您就可以将一个对象添加到您的字符串列表中.(并不是所有的对象都是字符串)
4) You can't mix oranges and apples. You would be able to add an Object to your String list if you could pass a string list to a method that expects object lists. (And not all objects are strings)
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