I have a nested HashMap with String keys that contains either List, Map, or String values. I would like to flatten them like the below.
Here is the data:
import java.util.*;
import java.util.stream.*;
public class MyClass {
public static void main(String args[]) {
Map<String, Object> dates = new HashMap<String, Object>() {{
put("1999", new HashMap<String, Object>() {{
put("3", Arrays.asList("23", "24", "25"));
put("4", Arrays.asList("1", "2", "3"));
}});
put("2001", new HashMap<String, Object>() {{
put("11", new HashMap<String, Object>() {{
put("7", Arrays.asList("23", "24", "25"));
put("9", Arrays.asList("1", "2", "3"));
}});
put("12", "45");
}});
}};
System.out.println(dates);
}
}
Map looks like:
{2001={11={7=[23, 24, 25], 9=[1, 2, 3]}, 12=45},
1999={3=[23, 24, 25], 4=[1, 2, 3]}}
The flattening of map should look like this:
{2001.11.7.1=23, 2001.11.7.2=24, 2001.11.7.3=25, 2001.11.9.1=1, 2001.11.9.2=2,
2001.11.9.3=3, 2001.12=45, 1999.3.1=23, 1999.3.2=24, 1999.3.3=25,
1999.4.1=1, 1999.4.2=2, 1999.4.3=3}
Note: the level of nested arrays or maps is unknown, it may go more than 2 levels.
You can use recursion to flatten the Map. Each time you encounter a Map, recurse by flattening that Map; when you encounter a List, iterate over it and add the index to the current key. A single value can be trivially set otherwise. See the below code in action here.
public static Map<String, Object> flatten(final Map<String, Object> map) {
return flatten("", map, new HashMap<>());
//use new TreeMap<>() to order map based on key
}
@SuppressWarnings("unchecked")//recursive helper method
private static Map<String, Object> flatten(final String key, final Map<String, Object> map,
final Map<String, Object> result) {
final Set<Map.Entry<String, Object>> entries = map.entrySet();
if (!entries.isEmpty()) {
for (final Map.Entry<String, Object> entry : entries) {
//iterate over entries
final String currKey = key + (key.isEmpty() ? "" : '.') + entry.getKey();
//append current key to previous key, adding a dot if the previous key was not an empty String
final Object value = entry.getValue();
if (value instanceof Map) {//current value is a Map
flatten(currKey, (Map<String, Object>) value, result);//flatten Map
} else if (value instanceof List) {//current value is a List
final List<Object> list = (List<Object>) value;
for (int i = 0, size = list.size(); i < size; i++) {
result.put(currKey + '.' + (i + 1), list.get(i));
}
//iterate over the List and append the index to the current key when setting value
} else {
result.put(currKey, value);//set normal value
}
}
}
return result;
}
public static void main(final String[] args){
final Map<String, Object> flattened = flatten(dates);
System.out.println(flattened);
}
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