map method in java

Map method in Java: Basic methods: Get the value of the key, add key-value pairs, check whether the key or value exists, determine whether the map is empty, and get the number of key-value pairs. View method: Get key collection, value collection, key-value pair collection. Transformation methods: Add another map to the current map, clear the map, delete key-value pairs by key. Other useful methods: compare maps, get hash codes, perform operations on each key-value pair.

Map method in Java

The Map interface is an important data structure in the Java collection framework and is used to Store key-value pairs. It provides a set of methods to operate these key-value pairs, including:

1. Basic method

• get(key) : Gets the associated value based on the key (if it exists). Returns null if key does not exist.
• put(key, value): Add a key-value pair to the map. If the key already exists, replace the old value with the new value.
• containsKey(key): Checks whether the given key exists in the map.
• containsValue(value): Check whether the given value exists in the map.
• isEmpty(): Check whether the mapping is empty.
• size(): Returns the number of key-value pairs in the map.

2. View method

• keySet(): Returns the Set collection of all keys in the map.
• values(): Returns a Collection of all values ​​in the map.
• entrySet(): Returns a Set collection of all key-value pairs in the map, each key-value pair as an Entry object.

3. Conversion method

• putAll(Map<? extends K, ? extends V> m)： All key-value pairs from another map are added to the current map.
• clear(): Remove all key-value pairs from the map.
• remove(key): Remove a key-value pair from the map based on the key.

4. Other practical methods

• equals(Object o): Compare the current mapping with another mapping .
• hashCode(): Returns the mapped hash code.
• forEach(BiConsumer<? super K, ? super V> action): Perform the specified operation on each key-value pair in the map.

Through these methods, you can easily operate Map collections in Java, store, retrieve and manage key-value pairs.

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