Output seems correct, but validation fails. Why?

right, that makes sense. thanks a lot for the quick answer! I saw your comments also in the other questions - are you still studying yourself or mostly helping others out by now? ;)

Programmers are never done studying 😄. I answer a lot of questions here because: 1) it helps me to continue to learn. Plenty of times I need to actually do research before answering questions. This has led me to learn more from the help section than what I did with the course; and 2) I also do it just because I enjoy interacting with people all over the world.

I'm just trying to understand how to use the merge() method. Looking at the API, I have a partial understanding. But what does this code mean: String::concat or Integer::sum ?

I honestly have to still look into that myself. I first wrote the code below instead of the map.merge line, then Intellij suggested to replace it with the one you see above.

if (map.get(thisByte) != null)
                map.put(thisByte, map.get(thisByte) + 1);
            else
                map.put(thisByte, 1);

Yes, the latter code is very logical and easy to understand:)

and the merge method does the same thing. I suppose in case of Strings, it concatenates them, and accounts for null too.

merge takes three arguments. The first two are values you want to work with and the third is a BiFunction that you can use to tell those two values how to interact. String::concat and Integer::sum are method references that are definition identical to the BiFunctions apply method. That's why you can use these (but better ignore that for now as it is hard to visualize that if you haven't used method references for some time). Better implement it with a dedicated class or an anonymous class. When you had a look at the merge method than you probably looked at the BiFunction, too. It just is a functional interface with one method. That method takes two arguments (of any type) and returns any type. Here you have two Integers and it returns an int. Perfect. So here the long version

map.merge(thisByte, 1, new BiFunction<Integer, Integer, Integer>() {
    @Override
    public Integer apply(Integer integer, Integer integer2) {
        return Integer.sum(integer, integer2);
    }
});

You see that you have the two values, thisByte and 1 and the BiFunction. The apply method is used on those two values (they are passed to the method). Inside the method I just could have written integer + integer2 but I used Integer.sum (hahah, I bet you know why - that's already the possible short form). Integer.sum is definition compatible to the BiFunctions method (it takes 2 arguments and returns one). You could use any method that is compatible. You also could write one of your own like

public static int add(int a, int b) {
    return a + b;
}

You could use that, too if you replace Integer::sum with Solution::add But imho merge is overkill. There are alternative one liner that'll do that without anon class, method references etc.

like

map.put(thisByte, map.getOrDefault(thisByte, 0) + 1);

this pretty much does what aijo wrote above. It puts data for the key thisByte. The value it takes from the map, if there's no such key it uses a default value (0) the existing value or the 0 we increment and, as said, put it into the map again.

thank you, Lisa! I much prefer your one line solution 👌 simple and easy to understand.

Lisa, thanks for both comments! Very helpful. Now I understand. (But I apologize in advance if I get a bit confused again next time a method reference occurs, but again in a new context...)

anytime... and it is not so easy to understand the concept of passing behavior instead of state. Don't invest to much brain power into that kind of coding atm.