What does hashmap mean in java

HashMap is a data structure that stores key-value pairs, using hash tables to find and insert values ​​quickly and efficiently. Its features include: storing key-value pairs, the key is unique, and the value can be null. Based on hash tables, the average lookup complexity is O(1). The default is not thread-safe and requires synchronization mechanism.

What is HashMap?

HashMap is a hash table, which is a data structure that can efficiently store key-value pairs. In Java, HashMap is a class provided by the java.util package that maps keys to values ​​using a hash function.

Characteristics of HashMap in Java:

• Storage key-value pairs: HashMap stores key-value pairs, where the key is a unique identifier , the value is the data associated with the key.
• Based on hash table: HashMap is implemented using a hash table, which hashes the key to an integer index that is used to quickly look up the value.
• Fast search: Due to the use of a hash table, HashMap can quickly find values ​​by key, with an average time complexity of O(1).
• Allow null values: HashMap can store null keys and values.
• Not thread-safe: By default, HashMap is not thread-safe, which means that multiple threads cannot modify the same HashMap at the same time unless appropriate synchronization mechanisms are used.

Using HashMap:

To use HashMap, you can perform the following steps:

1. Import java.util. HashMap class.
2. Create a HashMap object.
3. Use the put() method to add key-value pairs to the HashMap.
4. Use the get() method to get the value based on the key.
5. Use the containsKey() method to check whether a key exists in the HashMap.
6. Use the remove() method to remove key-value pairs from a HashMap.

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