Question

下面的区别是什么?

new HashMap(); v. new HashMap<Integer, String>?

我从未使用过或看到过前者,但我是否知道有任何分歧?

Answer 1

First one doesnt know what type of data hashmap has. You can have any type of object in this map. Second one specifies what is key and value types for this map. You can only insert those types into map. This approach is called generics.

Answer 2

发言后将绘制一份地图,其关键价值类别为raw. (java.lang.Object)。

HashMap map=new HashMap();

In second statement, you are specifying type of Key and Value (Read Java Generics) .

HashMap<Integer,String> map=new HashMap<Integer,String>();

Answer 3

It should be new Hashmap<Integer, String>().
The first case will use Object type for keys as well as values and will return Object instances when you retrieve them. In the second case, you are specifying that the keys should be of Integer type and the values are of String type. So, you should add accordingly and when you retrieve, you ll get String object for values and Integer object for keys and you will not need a cast as in the previous case.

P.S. 我认为,你应当使用<代码>Integer <>/code>,而不是int<>t的理由是,如果某种方法需要返回null,则使用的类型应为“nullable”。

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