Silly example about converting an array of strings to a single comma-separated, uppercased string. Note the difference in the effect of orElseGet() vs the effect of orElse() (which calls makeEmpty() regardless of if it's needed)

TryStream.java

import org.junit.Test;

import java.util.Arrays;
import java.util.Optional;

import static org.hamcrest.core.Is.is;
import static org.junit.Assert.assertThat;

/**
 * Created by ton on 09/12/16.
 */
public class TryStream {

    private String[] arr1 = { null, "two", "three"};
    private String[] arr2 = { "one", null, "three"};
    private String[] arr3 = { "one", "two", null};
    private String[] single = { "one" };
    private String[] empty = {  };

    @Test
    public void testConvert0() {
        assertThat(convert0(arr1), is("TWO,THREE"));
        assertThat(convert0(arr2), is("ONE,THREE"));
        assertThat(convert0(arr3), is("ONE,TWO"));
        assertThat(convert0(single), is("ONE"));
        assertThat(convert0(empty), is(""));
    }

    @Test
    public void testConvert1() {
        assertThat(convert1(arr1), is("TWO,THREE"));
        assertThat(convert1(arr2), is("ONE,THREE"));
        assertThat(convert1(arr3), is("ONE,TWO"));
        assertThat(convert1(single), is("ONE"));
        assertThat(convert1(empty), is(""));
    }

    @Test
    public void testConvert2() {
        assertThat(convert2(arr1), is("TWO,THREE"));
        assertThat(convert2(arr2), is("ONE,THREE"));
        assertThat(convert2(arr3), is("ONE,TWO"));
        assertThat(convert2(single), is("ONE"));
        assertThat(convert2(empty), is(""));
    }

    private static String convert0(String[] strings) {
        return Arrays.stream(strings)
                .filter (s -> s != null)
                .map(String::toUpperCase)
                .reduce((s1, s2) -> s1 + "," +s2)
                .orElseGet(TryStream::makeEmpty);
    }

    private static String makeEmpty() {
        System.out.println("makeEmpty()");
        return "";
    }

    private static String convert1(String[] strings) {
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i< strings.length; i++) {
            if (strings[i] != null) {
                if (builder.length() > 0) {
                    builder.append(",");
                }
                builder.append(strings[i].toUpperCase());
            }
        }

        return builder.toString();
    }

    private static String convert2(String[] strings) {
        return Arrays.stream(strings)
                .map(Optional::ofNullable)
                .filter(Optional::isPresent)
                .map(Optional::get)
                .map(String::toUpperCase)
                .reduce((s1, s2) -> s1 + "," +s2)
                .orElse(makeEmpty());
    }
}