Question

Lets suppose we were given the following two arrays

```
String[] keys = new String[] {"a", "b", "c", "aa", "d", "b"}
int[] values = new int[] { 1 , 2 , 3 , 4 , 5 , 6 }
```

And by merging these 2 arrays into HashTable we get the following

```
// pseudo-code
Map<String, Integer> dictionary = new HashTable<>(
("a" => 1)
("b" => 8) // because "b" appeared in index 1 and 5
("c" => 3)
("aa" => 4)
("d" => 5)
);
```

How can we do this using java Lambda style?

So far I have the following:

```
// this loops through the range given (used for index start and end)
// and sums the values of duplicated keys
tree.listMap = IntStream.range(range[0], range[1]).boxed().collect(
Collectors.toMap(i - > dictionary[i], i - > i 1, Integer::sum, TreeMap::new)
);
```

However, I'd like to take 2 arrays, merge them by key and value, where value is the sum of all values for duplicated keys. How can we do this?

Solution

There you go:

```
Map<String,Integer> themap =
IntStream.range (0, keys.length).boxed()
.collect (Collectors.toMap(i->keys[i],
i->values[i],
Integer::sum,
TreeMap::new));
```

Output:

```
{a=1, aa=4, b=8, c=3, d=5}
```

This is quite similar to the snippet you posted, though, for some reason, the snippet you posted contains no reference to the `keys`

and `values`

arrays.