HashMap is aStuff list, the content it stores is a key-value map.
HashMap implements the Map interface, storing data according to the HashCode value of the key, with a very fast access speed, allowing up to one record to be null, and does not support it.Thread synchronization。
HashMap is unordered, that is, the order of insertion will not be recorded.
HashMap inherits from AbstractMap, implementing Map,Cloneable, interface.
Map<> map = new HashMap<>();This is the statement of HashMap, which can store key-value pairs or single-value values. There are many types of data stored.string, integer, character, etc.
Basic Operationfunction：
| clear() | Delete all key/value pairs in hashMap |
| clone() | Copy a hashMap |
| isEmpty() | Determine whether hashMap is empty |
| size() | Calculate the number of key/value pairs in hashMap |
| put() | Add key/value pairs to hashMap |
| putAll() | Add all key/value pairs to hashMap |
| putIfAbsent() | If the specified key does not exist in the hashMap, the specified key/value pair is inserted into the hashMap. |
| remove() | Delete the mapping relationship of the specified key in hashMap |
| containsKey() | Checks whether there is a mapping relationship corresponding to the specified key in hashMap. |
| containsValue() | Checks whether there is a mapping relationship corresponding to the specified value in hashMap. |
| replace() | Replace the value corresponding to the specified key in hashMap. |
| replaceAll() | Replace all mapping relationships in hashMap with the result executed by the given function. |
| get() | Get the specified key for the corresponding value |
| getOrDefault() | Get the specified key corresponding value. If key cannot be found, the default value is returned. |
| forEach() | Performs the specified action on each map in the hashMap. |
| entrySet() | Returns a collection view of all mapped items in hashMap. |
| keySet() | Returns a collection view composed of all keys in hashMap. |
| values() | Returns all value values present in hashMap. |
| merge() | Add key-value pairs to hashMap |
| compute() | Recalculate the value of the specified key in hashMap |
| computeIfAbsent() | Recalculate the value of the specified key in hashMap, and if this key does not exist, it is added to hasMap |
| computeIfPresent() | Recalculate the value of the specified key in hashMap, provided that the key exists in hashMap. |