Java Guild Meeting 5/2018

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Micro kata's

Here are a few micro kata's for you to solve, let's get to know some more possibilities of Streams!

If you want to know if your solution is optimal you can take look at the source & solutions.

The kata's are written in old-style Java code. Re-write them into beautiful one-liners using Streams.

alt text

Remember, you can find the documentation on Streams right here.

Flatten this multidimensional collection

// {
package streams;
import java.util.*;
import static java.util.stream.Collectors.*;
public class FlatMap {
// }
public static List<String> transform(List<List<String>> collection) {
    List<String> newCollection = new ArrayList<>();
    for (List<String> subCollection : collection) {
        for (String value : subCollection) {
            newCollection.add(value);
        }
    }
    return newCollection;
}
// {
}
// }

Get the oldest person from the collection

// {
package streams;
import java.util.*;
public class MaxAndComparator {
// }
    public static Person getOldestPerson(List<Person> people) {
        Person oldestPerson = new Person("", 0);
        for (Person person : people) {
            if (person.getAge() > oldestPerson.getAge()) {
                oldestPerson = person;
            }
        }
        return oldestPerson;
    }
// {
}
// }

Sum all elements of a collection, try to use the reduce operator with identity parameter instead of an IntStream

Get the names of all kids under the age of 18

// {
package streams;
import java.util.*;
public class UnderAge {
// }
    public static Set<String> getKidNames(List<Person> people) {
        Set<String> kids = new HashSet<>();
        for (Person person : people) {
            if (person.getAge() < 18) {
                kids.add(person.getName());
            }
        }
        return kids;
    }
// {
}
// }

Not very interesting as an exercise, but check out this example of how to easily generate numerical statistics from an IntStream.

Partition these people into adults and kids, you'll need a special collector for this one

// {
    public static Map<Boolean, List<Person>> partitionAdults(List<Person> people) {
        Map<Boolean, List<Person>> map = new HashMap<>();
        map.put(true, new ArrayList<>());
        map.put(false, new ArrayList<>());
        for (Person person : people) {
            map.get(person.getAge() >= 18).add(person);
        }
        return map;
    }
// {

Group these people by nationality, same kind as the previous exercise

// {
    public static Map<String, List<Person>> groupByNationality(List<Person> people) {
        Map<String, List<Person>> map = new HashMap<>();
        for (Person person : people) {
            if (!map.containsKey(person.getNationality())) {
                map.put(person.getNationality(), new ArrayList<>());
            }
            map.get(person.getNationality()).add(person);
        }
        return map;
    }
// {

Return a comma-separated string of all these people's names

// {
    public static String namesToString(List<Person> people) {
        String label = "Names: ";
        StringBuilder sb = new StringBuilder(label);
        for (Person person : people) {
            if (sb.length() > label.length()) {
                sb.append(", ");
            }
            sb.append(person.getName());
        }
        sb.append(".");
        return sb.toString();
    }
// {

Challenge

Write a method that returns a comma separated string based on a given list of integers. Each element should be preceded by the letter 'e' if the number is even, and preceded by the letter 'o' if the number is odd. For example, if the input list is (3,44), the output should be 'o3,e44'.

You can write this using only one operator and one collect statement. Go here and click 'Solution' if you want to see the optimal solution.

Return a comma-separated string of all these people's names

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4/11 Streams - Micro kata's

Micro kata's

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