// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        // 1. initialize a hash map
        Map<Integer, Integer> hashmap = new HashMap<>();
        // 2. insert a new (key, value) pair
        hashmap.putIfAbsent(0, 0);
        hashmap.putIfAbsent(2, 3);
        // 3. insert a new (key, value) pair or update the value of existed key
        hashmap.put(1, 1);
        hashmap.put(1, 2);
        // 4. get the value of specific key
        System.out.println("The value of key 1 is: " + hashmap.get(1));
        // 5. delete a key
        hashmap.remove(2);
        // 6. check if a key is in the hash map
        if (!hashmap.containsKey(2)) {
            System.out.println("Key 2 is not in the hash map.");
        }
        // 7. get the size of the hash map
        System.out.println("The size of hash map is: " + hashmap.size()); 
        // 8. iterate the hash map
        for (Map.Entry<Integer, Integer> entry : hashmap.entrySet()) {
            System.out.print("(" + entry.getKey() + "," + entry.getValue() + ") ");
        }
        System.out.println("are in the hash map.");
        // 9. clear the hash map
        hashmap.clear();
        // 10. check if the hash map is empty
        if (hashmap.isEmpty()) {
            System.out.println("hash map is empty now!");
        }
    }
}

Scenario I - Provide More Information

/*
 * Template for using hash map to find duplicates.
 * Replace ReturnType with the actual type of your return value.
 */
ReturnType aggregateByKey_hashmap(List<Type>& keys) {
    // Replace Type and InfoType with actual type of your key and value
    Map<Type, InfoType> hashmap = new HashMap<>();
    for (Type key : keys) {
        if (hashmap.containsKey(key)) {
            if (hashmap.get(key) satisfies the requirement) {
                return needed_information;
            }
        }
        // Value can be any information you needed (e.g. index)
        hashmap.put(key, value);    
    }
    return needed_information;
}

​# 1. initialize a hash map
hashmap = {0 : 0, 2 : 3}
# 2. insert a new (key, value) pair or update the value of existed key
hashmap[1] = 1
hashmap[1] = 2
# 3. get the value of a key
print("The value of key 1 is: " + str(hashmap[1]))
# 4. delete a key
del hashmap[2]
# 5. check if a key is in the hash map
if 2 not in hashmap:
    print("Key 2 is not in the hash map.")
# 6. both key and value can have different type in a hash map
hashmap["pi"] = 3.1415
# 7. get the size of the hash map
print("The size of hash map is: " + str(len(hashmap)))
# 8. iterate the hash map
for key in hashmap:
    print("(" + str(key) + "," + str(hashmap[key]) + ")", end=" ")
print("are in the hash map.")
# 9. get all keys in hash map
print(hashmap.keys())
# 10. clear the hash map
hashmap.clear();
print("The size of hash map is: " + str(len(hashmap)))