Will the compiler throw an error or warning if an immutable object is used as a key in a hash table?

The Java compiler will not produce an error or warning if an immutable object is used as a key in a hash table (for example, in HashMap). It is important that the object is correctly implemented in terms of the hashCode() and equals() methods. Using an immutable object as a key is even good practice, as changing the state of the key after placing it in a hash table can lead to search problems.

Example:

Map<String, Integer> map = new HashMap<>();
String key = "immutableKey";
map.put(key, 1);
// String is an immutable object, safe to use as a key

If the object is mutable and its fields involved in hashCode() and equals() change after being added to the map, this can lead to errors, but the compiler will not detect this.