java 为什么 List<Object> list = new ArrayList<String>() 这会导致 TypeMismatch 错误
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Why List<Object> list = new ArrayList<String>() this give TypeMismatch error
提问by Raghvendra Garg
List<Object> list = new ArrayList<String>()
When I use the above line compiler gives me type mismatch error. But as I understand Object is super class of String and if I create list of object then it should also accept String. So why above statement is wrong. I am looking for an explanation.
当我使用上述行时，编译器给了我类型不匹配错误。但据我所知，Object 是 String 的超类，如果我创建对象列表，那么它也应该接受 String。那么为什么上面的说法是错误的。我正在寻找一个解释。
回答by PermGenError
One sentence, because
一句话，因为
Generic types are not polymorphic
泛型类型不是多态的
i.e., even though java.lang.Stringis a subtype of java.lang.Objectpolymorphism doesn't apply to generic types. It only applies to collection types. thus
即，即使java.lang.String是java.lang.Object多态的子类型也不适用于泛型类型。它仅适用于集合类型。因此
List<Object> list = new ArrayList<String>(); //this isn't valid
? List<Object> list = new ArrayList<Object>(); //valid
List<? extends Object> list = new ArrayList<String>();//valid
回答by keiki
Because you define what possible list can be associated with.
因为您定义了可以关联的可能列表。
The correct way would be
正确的方法是
List<?> list = new ArrayList<String>();
List<?> list = new ArrayList<String>();
which is the same as
这与
List<? extends Object> list = new ArrayList<String>();
List<? extends Object> list = new ArrayList<String>();
回答by Jiangzhou
Ais a super type of Bdoes not imply List<A>is a super type of List<B>. If such proposition holds, consistency of type system will be violated. Consider the following case:
A是超类型的B并不意味着List<A>是超类型的List<B>。如果这样的命题成立，就会违反类型系统的一致性。考虑以下情况：
// ! This code cannot pass compilation.
List<String> list1 = new ArrayList<String>();
List<Object> list2 = list1; // Error
// Previous conversion causes type contradiction in following statements.
list2.add(new SomeOtherClass());
String v = list1.get(0); // list1.get(0) is not String!
That's why List<Object>should not be super type of List<String>.
这就是为什么List<Object>不应该是超类型的List<String>.
回答by Festus Tamakloe
Just replace:
只需更换：
List<Object> list = new ArrayList<String>()
with
和
List<String> list = new ArrayList<String>()
or
或者
List<Object> list = new ArrayList<Object>()
It is important here that you operate with the same data type
在这里使用相同的数据类型很重要
Or you can also use
或者你也可以使用
List<?> list = new ArrayList<Object>();
回答by amicngh
The type of the variable declaration mustmatch the type you pass to the actual object type. If you declare
List<Foo>foo then whatever you assign to the foo reference MUSTbe of the
generic type . Not a subtype of <Foo>. Not a supertypeof <Foo>.
变量声明的类型必须与您传递给实际对象类型的类型相匹配。如果你声明
List<Foo>foo ，那么你分配给 foo 引用的任何东西都必须是泛型类型。不是<Foo>. 不是supertype的<Foo>。
Simple Generic types are not polymorphic
List<Parent> myList = new ArrayList<Child>() \\ Invalid
List<Parent> myList = new ArrayList<Child>() \\ Invalid
