How to achieve a multi valued hashmap like HashMap<String,Hashmap<String,String>> in Java

Question

I want an outer map to have duplicate keys, therefore cannot use a traditional hashmap.

I want to achieve something like below:

Key Value
Col--->apple-->Apple
Col--->ball--->Ball
Col1--->tree--->Tree

so map would look like,
Key. Value
Col-->[apple-->Apple],[ball-->Ball]
Col1-->[tree--->Tree]

Please help!

Answer 1

A nice, compact way to do this in modern Java is:

Map<String, Map<String, String>> map = new HashMap<>();
map.computeIfAbsent("Col", k -> new HashMap<String,String>()).put("apple", "Apple");
map.computeIfAbsent("Col", k -> new HashMap<String,String>()).put("ball", "Ball");
map.computeIfAbsent("Col", k -> new HashMap<String,String>()).put("tree", "Tree");

A little verbose but I'd assume you'd actually do this in a like in this example:

String[][] values = {
  {"Col", "apple", "Apple"},
  {"Col", "ball", "Ball"},
  {"Col1", "tree", "Tree"},      
};

Map<String, Map<String, String>> map = new LinkedHashMap<>(); 
for (String[] row : values) {
  map.computeIfAbsent(row[0], k -> new LinkedHashMap<String, String>()).put(row[1], row[2]);
}

Note: I used LinkedHashMap to preserve order.