How to Store a HashMap Inside Another HashMap in Java?

What is the proper way to store a HashMap inside another HashMap in Java?

HashMap<String, HashMap<String, Integer>> outerMap = new HashMap<>();

HashMap<String, Integer> innerMap = new HashMap<>();
innerMap.put("key1", 1);
innerMap.put("key2", 2);

outerMap.put("outerKey", innerMap);

Storing a HashMap inside another HashMap in Java allows for more complex data structures, enabling you to group key-value pairs logically. This nested structure is useful in various situations like organizing hierarchical data or collecting related information under a single key.

// Declaring an outer HashMap
HashMap<String, HashMap<String, Integer>> outerMap = new HashMap<>();

// Creating an inner HashMap
HashMap<String, Integer> innerMap = new HashMap<>();
innerMap.put("A", 10);
innerMap.put("B", 20);

// Storing the inner HashMap in the outer HashMap
outerMap.put("FirstInnerMap", innerMap);

// Accessing elements
int value = outerMap.get("FirstInnerMap").get("A"); // Returns 10

Causes

• Creating nested data structures for complex data management.
• Grouping related key-value pairs under a single key for better organization.
• Facilitating multi-dimensional data access patterns.

Solutions

• Declare the outer HashMap with the value type as another HashMap.
• Instantiate the inner HashMap before storing it in the outer one.
• Use clear and consistent key naming conventions.

Mistake: Forgetting to initialize the inner HashMap before using it.

Solution: Always create an instance of the inner HashMap before adding it to the outer HashMap.

Mistake: Using incompatible types for keys and values.

Solution: Ensure that the keys and values match the declared types.