HashMap 的 get() 方法

Question

I have a HashMap:

Map stuff = new HashMap<String, ArrayList<Thing>();

I am trying to use the .get() method to get the ArrayList , but I have an error message saying,

Object cannot be converted to ArrayList;

I'm not really sure why I am getting this error message? Also, is there another way to acquire the ArrayList at the key?

Answer 1

Try parameterizing the Map declaration with <>.

Map<String, ArrayList<Thing>> stuff = new HashMap<>() ;

Without the type declaration it will be an untyped Map, so the compiler will only be able to assume that the results of get are the most generic possible result, an Object.

With the diamond operator the compiler will infer the type of the right hand side from the left hand declaration.

You can cast the result of get back into an ArrayList, but it is better to parameterize the Map declaration instead. A cast will require runtime checks which have some performance cost and can potentially introduce crashes once your code becomes more difficult to understand. If you use the parameterized form instead, the compiler can prove that it is the right type, preventing crashes and running faster.

If you're using an old java from before java 1.7 you'll need to write out the full type in the <>.

See this tutorial from oracle for more information.