CODEWITHSUNDEEP
java
weijia_yu
weijia_yu

Reputation: 1015

Why this String HashMap cannot get correct value by *char* key

I have this String to String map, and I am trying to pass char as a key

Map<String, String> phone = new HashMap<String, String>() {{
    put("2", "abc");
    put("3", "def");
    put("4", "ghi");
    put("5", "jkl");
    put("6", "mno");
    put("7", "pqrs");
    put("8", "tuv");
    put("9", "wxyz");
  }};

String letterList = phone.get('2');  //null
String letterList = phone.get(String.valueOf('2'));  //it works

Why does the first case not work? In my understanding, char can be implicitly converted to String "2", and HashMap use equals() to compare keys, so that it should retrieve the key in map?

Upvotes: 1

Views: 465

Answers (2)

Youans
Youans

Reputation: 5061

java.util.Map uses Objects only as a key so whenever you do map.get('c') since c is a char the compiler will do autoboxing operation parsing the char c primitive into Character Object (not String as you thought)

So at the end compiler will parse the following: map.get('2') into > map.get(Character.valueOf('2'))

And since Character.valueOf('2') key does not exist in your map null is returned

Upvotes: 2

Code-Apprentice
Code-Apprentice

Reputation: 83527

Why does the first case not work? In my understanding, char can be implicitly converted to String "2"

Your understanding is incorrect. A char will not be implicitly converted to String. If you look at the documentation, you will see this method get(Object key). I don't know why this isn't get(K key) instead. However, this explains why your first example compiles without any errors: the char constant is autoboxed into a Character object. Since the Character with the value '2' is not a key in your Map, get() returns null.

Upvotes: 5

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