How do I add elements from one map to another if it doesn't exist already or the value is blank?

In my Spring Boot application I have two Maps:

Map<String, String> defaultValuesMap = Map.of(
    "BMW", "Something",
    "Audi", "Something else",
    "Nissan", "Something else"
);

Map<String, String> myMap = Map.of(
    "Mercedes", "Something",
    "Suzuki", "Whatever",
    "Audi", "Dummy data",
    "Nissan", ""
);

I'm trying to make some logic that takes alle the Entries in defaultValuesMap and say if that entry does not exist in myMap OR it exists but the value is blank, then replace that key/value pair with the one from defaultValuesMap.

The result would look like this:

Map<String, String> myMap = Map.of(
    "Mercedes", "Something",
    "Suzuki", "Whatever",
    "Audi", "Dummy data",
    "Nissan", "Something else",
    "BMW", "Something"
);

I've tried something like this:

defaultValuesMap.entrySet()
    .stream()
    .map(entry -> {
        if (!myMap.containsKey(entry) || myMap.get(entry).isBlank)
                        myMap.put(
                                entry.getKey(),
                                entry.getValue());
                    return entry;
                });

This didn't work, then I tried to add collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)); to the end of it but then it just replaced the elements even if they were not missing or blank...

Unmodifiable map

Map.of returns an unmodifiable map, which as the name implies cannot be modified.

Create a third result map or if you want to do it in place like in your example, make the second map modifiable. Then you can use map.compute to get the desired result:

Map<String, String> defaultValuesMap = Map.of(
        "BMW", "Something",
        "Audi", "Something else",
        "Nissan", "Something else");

Map<String, String> myMap = new HashMap<>(Map.of(
        "Mercedes", "Something",
        "Suzuki", "Whatever",
        "Audi", "Dummy data",
        "Nissan", ""));

defaultValuesMap.forEach((k,v) ->{
    myMap.compute(k, (k1, prev) -> prev == null || prev.isBlank() ? v : prev);
});

System.out.println(myMap);

there is no way to achieve this with iteration on one map. the reason is that the result must include mapping that exists in only one map (either myMap or the default) so iteration on one map will always exclude keys that are present only in the other.

so first you need to construct a set that includes union of keys from both maps

	Set<String> unionKeys = new HashSet<>(myMap.keySet());
	unionKeys.addAll(defaultValuesMap.keySet());

this set is the basis of the iteration, the value can be taken using the ternary operator from myMap or the default

	Map<String, String> resultMap = unionKeys.stream()
		.collect(Collectors.toMap(
			Function.identity(),
			key -> myMap.containsKey(key) && !myMap.get(key).isBlank() ? 
				myMap.get(key) : defaultValuesMap.get(key)
			)
		);