Return from HashMap<String, String> when no key

Question

当我调用 map.get("key") 并且在 HashMap 中没有带有键“key”的条目时HashMap<String,String>返回什么？

Answer 1

It returns null . It's written in the documentation .

Returns: the value to which the specified key is mapped, or null if this map contains no mapping for the key

The first thing to do when you have such a specific question is to consult the documentation. Java APIs are documented reasonably well and tell you what is returned, what exceptions are thrown and what each argument means.

Answer 2

You can:

Check in your IDE

Map<String, String> map = new HashMap<String, String>();
map.put("foo", "fooValue");
System.out.println(map.get("bar")); // null

Check documentation - HashMap get() method description:

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

Answer 3

Careful - If you initialize it using

Map.of(key, val, key, val)

and then do a

get('key-that-isnt-there') 

then you'll get a null pointer exception.