Master the essence of Java Map, a necessary skill for advanced learners

php editor Zimo recommends learning Java Map, which is a key skill for advanced learners. Mastering the essence of Java Map can not only improve programming skills, but also help learners understand the application of data structures and algorithms more deeply, laying a solid foundation for future programming.

Map has many uses in Java. For example, you can use a Map to store user IDs and passwords, product IDs and prices, or file names and file contents. Maps are also great for storing configuration settings within an application.

There are three built-in Map implementations in Java: HashMap, TreeMap and LinkedHashMap. HashMap is a Map implementation based on hash tables, and it is the most widely used Map implementation. TreeMap is a Map implementation based on red-black trees. It sorts key-value pairs in key order. LinkedHashMap is a Map implementation based on Linked List, which retains the insertion order of key-value pairs.

HashMap

HashMap is the most commonly used Map implementation. It uses a hash table to store key-value pairs. A hash table is an

array

where each element is a linked list. When you add a key-value pair to a HashMap, the HashMap calculates the hash code of the key and stores the key-value pair in the same linked list as the hash code. When you retrieve a value from a HashMap, the HashMap calculates the hash code of the key and then searches for the key in the same linked list as the hash code. The complexity of HashMap is O(1), which means that on average, the time to find, add or delete a key-value pair is constant. However, in the worst case, the complexity of a HashMap may degenerate to O(n), where n is the number of key-value pairs in the Map. This is because the hash table can become unbalanced, causing some linked lists to become very long.

TreeMap

TreeMap is a Map implementation that sorts key-value pairs in key order. It uses red-black trees to store key-value pairs. A red-black tree is a self-balancing binary search tree, which means that its height is always O(log n), where n is the number of nodes in the tree.

The complexity of TreeMap is O(log n), which means that on average, the time to find, add or delete a key-value pair is O(log n). However, in the worst case, the complexity of TreeMap may degenerate to O(n), where n is the number of key-value pairs in the Map. This is because the red-black tree can become unbalanced, causing the height of the tree to become O(n).

LinkedHashMap

LinkedHashMap is a Map implementation that preserves the insertion order of key-value pairs. It uses linked lists to store key-value pairs. When you add a key-value pair to a LinkedHashMap, the key-value pair will be added to the end of the linked list. When you retrieve a value from a LinkedHashMap, the LinkedHashMap will traverse the linked list until it finds a key-value pair that matches the given key.

The complexity of LinkedHashMap is O(n), where n is the number of key-value pairs in the Map. This is because LinkedHashMap must traverse the linked list to find the key-value pair that matches the given key.

Choose the appropriate Map implementation

When choosing a suitable Map implementation, you need to consider the following factors:

Performance:
• You need to consider the performance requirements of Map. If you need a high-performance Map, then you should use HashMap. If you need a Map that sorts key-value pairs in key order, then you should use a TreeMap. If you need a Map that preserves the insertion order of key-value pairs, then you should use LinkedHashMap.
Thread safety:
• You need to consider whether the Map needs to be threadsafe. If your Map will be accessed by multiple threads simultaneously, then you should use ConcurrentHashMap. ConcurrentHashMap is a thread-safe Map implementation that allows you to use Map safely in a concurrent environment.
Memory usage:
• You need to consider the memory usage requirements of Map. HashMap generally uses less memory than TreeMap and LinkedHashMap.

Demo code

The following code demonstrates how to use HashMap to store user IDs and passwords:

Map<String, String> users = new HashMap<>();
users.put("alice", "passWord1");
users.put("bob", "password2");
users.put("charlie", "password3");

String password = users.get("alice");

The following code demonstrates how to use TreeMap to store product IDs and prices:

Map<Integer, Double> products = new TreeMap<>();
products.put(1, 10.0);
products.put(2, 20.0);
products.put(3, 30.0);

Double price = products.get(2);

The following code demonstrates how to use LinkedHashMap to store file names and file contents:

Map<String, String> files = new LinkedHashMap<>();
files.put("file1.txt", "This is the content of file1.txt.");
files.put("file2.txt", "This is the content of file2.txt.");
files.put("file3.txt", "This is the content of file3.txt.");

String content = files.get("file2.txt");

in conclusion

Map is a very useful data structure in Java. It allows you to store and retrieve values ​​using keys. There are many different implementations of Map, each with its own pros and cons. When choosing a suitable Map implementation, you need to consider performance and thread safety

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