键值映射

/*
 * MapSum class implements a map that supports two operations using Java Streams: 
 * 1. insert(String key, int val): Insert a key-value pair into the map. If the key already exists, update the value.
 * 2. sum(String prefix): Return the sum of all values of keys that start with the given prefix.
 */
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;
 
public class MapSum {
    private Map<String, Integer> map;
 
    // Constructor to initialize the map
    public MapSum() {
        map = new HashMap<>();
    }
 
    // Insert a key-value pair into the map
    public void insert(String key, int val) {
        map.put(key, val);
    }
 
    // Return the sum of values for all keys starting with the given prefix using Java Streams
    public int sum(String prefix) {
        return map.entrySet().stream()
                   .filter(entry -> entry.getKey().startsWith(prefix))
                   .collect(Collectors.summingInt(Map.Entry::getValue));
    }
}