Return from HashMap<String, String> when no key

What does a HashMap<String,String> return when I call map.get("key") and I don't have an entry with the key "ke开发者_如何学Cy" in the HashMap?

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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.

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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.

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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.