dictionary - java hashmap : enexpected behaviour for containsKey - Stack Overflow

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Here is a code snippet :

public static void main(String[] args) {
    final byte key = 0;
    Map<Integer, Integer> test = new HashMap<>();
    test.put(0, 10);
    System.out.println(test.containsKey(key));
}

and I get false printed on console. Only if I cast key to int I get true, like this

System.out.println(test.containsKey((int)key));

Can someone explain what is going on here?

Here is a code snippet :

public static void main(String[] args) {
    final byte key = 0;
    Map<Integer, Integer> test = new HashMap<>();
    test.put(0, 10);
    System.out.println(test.containsKey(key));
}

and I get false printed on console. Only if I cast key to int I get true, like this

System.out.println(test.containsKey((int)key));

Can someone explain what is going on here?

• java
• dictionary
• boxing
• autoboxing

Share Improve this question Follow edited Nov 20, 2024 at 1:10 Basil Bourque 341k122122 gold badges934934 silver badges1.3k1.3k bronze badges asked Nov 19, 2024 at 21:11 DmitryDmitry 2,16833 gold badges2323 silver badges3434 bronze badges 1

• This question is similar to: Will HashMap's containsKey method in java, check for an integer if I put a character in it?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. – Joe Commented Nov 20, 2024 at 1:30

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1 Answer 1

Sorted by: Reset to default 7

Boxing

put takes an Integer as the key. So the primitive numeric literal 0 in put(0, 10) gets boxed to an Integer object.

But containsKey takes an Object (see also). So the byte gets boxed to a Byte object. This Byte object does not equals the Integer object that was put into the map since they are completely different classes. Therefore containsKey returns false.

Primitive type	Object type, after auto-boxing
int	java.lang.Integer
byte	java.lang.Byte

---

See Auto-boxing, at Wikipedia. And Autoboxing tech note at Oracle.

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