Compare HashSet and TreeSet

Last time we talked about TreeSet in Java: Java TreeSet. This blog is just to provide a quick summary of the comparison between HashSet and TreeSet.

Characteristic of HashSet

• The order of the elements cannot be guaranteed. The order stored may be different from the order of addition and may change later
• HashSet is NOT synchronized. If multiple threads access a HashSet at the same time and threads modify the HashSet elements, the code must be used to ensure its synchronization
• The elements of HashSet can be null

Characteristic of TreeSet

• Elements in TreeSet is ordered. After the element is inserted, it will be arranged according to a specified order
• TreeSet is NOT synchronized.
• The elements of TreeSet can’t be null

Differences

• HashSet is implemented by hash table. The data in HashSet is unordered and can be put into null, but only one null is allowed.
• TreeSet is implemented by binary tree. The data in Treeset is sorted automatically, and null values are not allowed.
• The values in TreeSet and HashSet can not be repeated, just like the only constraint in the database.
• HashSet requires that the objects placed must implement the HashCode() method. The objects placed are identified by the hash code.
  • The String objects with the same content have the same hashcode, so the content cannot be repeated. But objects of the same class can be put into different instances.