Java stream toArray() convert to a specific type of array

Question

Maybe this is very simple but I'm actually a noob on Java 8 features and don't know how to accomplish this. I have this simple line that contains the following text:

"Key, Name"

and I want to convert that line into a String array, separating each value by the comma (,), however, I also want to trim every field before returning the final array, so I did the following:

Arrays.stream(line.split(",")).map(String::trim).toArray();

However, this returns an Object[] array rather than a String[] array. Upon further inspection, I can confirm that the contents are actually String instances, but the array itself is of Object elements. Let me illustrate this, this is what the debugger says of the returned object:

Object[]:
    0 = (String) "Key"
    1 = (String) "Name"

As far as I can tell, the problem is in the return type of the map call, but how can I make it return a String[] array?

Solution 1

Use toArray(size -> new String[size]) or toArray(String[]::new).

String[] strings = Arrays.stream(line.split(",")).map(String::trim).toArray(String[]::new);

This is actually a lambda expression for

.toArray(new IntFunction<String[]>() {
        @Override
        public String[] apply(int size) {
            return new String[size];
        }
    });

Where you are telling convert the array to a String array of same size.

From the docs

The generator function takes an integer, which is the size of the desired array, and produces an array of the desired size. This can be concisely expressed with an array constructor reference:

 Person[] men = people.stream()
                      .filter(p -> p.getGender() == MALE)
                      .toArray(Person[]::new);

Type Parameters:

A - the element type of the resulting array

Parameters:

generator - a function which produces a new array of the desired type and the provided length

Solution 2

String[]::new is a function that invokes the new "pseudo-method" for the String[] type just like String::trim is a function that invokes the real trim method of the String type. The value passed to the String::new function by toArray is the size of the collection on the right-hand side of the .toArray() method invocation.

If you replaced String[]::new with n->new String[n] you might be more comfortable with the syntax just like you could replace String::trim with the less cool s->s.trim()