How does a Java HashMap handle different objects with the same hash code?

user_802778 0
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As per my understanding I think:

  1. It is perfectly legal for two objects to have the same hashcode.
  2. If two objects are equal (using the equals() method) then they have the same hashcode.
  3. If two objects are not equal then they cannot have the same hashcode

Am I correct?

Now if am correct, I have the following question: The HashMap internally uses the hashcode of the object. So if two objects can have the same hashcode, then how can the HashMap track which key it uses?

Can someone explain how the HashMap internally uses the hashcode of the object?

hash-function hashcode hashmap java
15 Answers

Bearing in mind the explanations here for the structure of a hashmap, perhaps someone could explain the following paragraph on Baeldung :-

Java has several implementations of the interface Map, each one with its own particularities.

However, none of the existing Java core Map implementations allow a Map to handle multiple values for a single key.

As we can see, if we try to insert two values for the same key, the second value will be stored, while the first one will be dropped.

It will also be returned (by every proper implementation of the put(K key, V value) method):

Map<String, String> map = new HashMap<>();
assertThat(map.put("key1", "value1")).isEqualTo(null);
assertThat(map.put("key1", "value2")).isEqualTo("value1");
assertThat(map.get("key1")).isEqualTo("value2");
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