javalangIllegalStateException Duplicate key

The exception java.lang.IllegalStateException: Duplicate key is thrown when you try to insert a duplicate key into a data structure that does not allow duplicates, such as a Set or a Map with unique keys.

Here are a few possible reasons for this exception:

1. Inserting a duplicate key into a Set: If you are using a Set implementation like HashSet or TreeSet, they do not allow duplicate elements. So, if you try to add an element that already exists in the set, it will throw this exception.

2. Inserting a duplicate key into a Map: If you are using a Map implementation like HashMap or TreeMap, they do not allow duplicate keys. So, if you try to put a key-value pair with a key that already exists in the map, it will throw this exception.

3. Using a custom class as a key without overriding hashCode and equals methods: If you are using a custom class as a key in a Set or Map, you need to make sure that you have properly implemented the hashCode and equals methods. If these methods are not implemented correctly, the data structure may not be able to detect duplicate keys, leading to this exception.

To fix this exception, you should review your code and make sure that you are not inserting duplicate keys into a data structure that does not allow duplicates. If you are using a custom class as a key, make sure to override the hashCode and equals methods properly