Using the Stream API and the map method, we can transform elements in a stream to another object. Instead of using the map method, we can also write a custom Collector and transform the elements when we use the collect method as a terminal operation of the stream.

You may also like: A Guide to Streams: In-Depth Tutorial With Examples

First, we have an example where we transform String value using the map method:

package mrhaki;

import java.util.List;
import java.util.stream.Collectors;

public class CollectorString {
    public static void main(String[] args) {
        final var items = List.of("JFall", "JavaZone", "CodeOne");

        final List<String> upper =
                items.stream()
                     .map(String::toUpperCase)
                     .collect(Collectors.toUnmodifiableList());

        assert upper.equals(List.of("JFALL", "JAVAZONE", "CODEONE"));
    }
}

In our next example, we don't use the map method, but we write a custom Collector using the Collector.of method. As the first argument, we must provide the data structure we want to add elements too, the so-called supplier, which is an ArrayList.

The second argument is an accumulator where we add each element from the stream to the list and transform the value. The third argument is the combiner, and here, we combine multiple List instances to one List instance. The last argument is a finisher, and we make an immutable List to be returned.

package mrhaki;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collector;

public class CollectorString1 {
    public static void main(String[] args) {
        final var items = List.of("JFall", "JavaZone", "CodeOne");

        final List<String> upper =
                items.stream()
                     // Use collector to transform values
                     // in the items List.
                     .collect(upperCollect());

        assert upper.equals(List.of("JFALL", "JAVAZONE", "CODEONE"));
    }

    private static Collector<String, ?, List<String>> upperCollect() {
        return Collector.of(
                // First we specify that we want to add
                // each element from the stream to an ArrayList.
                () -> new ArrayList<String>(),

                // Next we add each String value to the list
                // and turn it into an uppercase value.
                (list, value) -> list.add(value.toUpperCase()),

                // Next we get two lists we need to combine,
                // so we add the values of the second list
                // to the first list.
                (first, second) -> { first.addAll(second); return first; },

                // Finally (and optionally) we turn the 
                // ArrayList into an unmodfiable List.
                list -> Collections.unmodifiableList(list));
    }
}

Written with Java 12.

Further Reading

A Guide to Streams: In-Depth Tutorial With Examples

Transforming Collections — Java 8 Collectors API 

The Ultimate Guide to the Java Stream API groupingBy() Collector