Hello Java Streams

Streams in Java 8 are a way of creating operations to express sophisticated data processesing queries.

This example will just give a short on how to stream to very basic lists:

Sorting an integer list.
Filter and uppercase a string list and return it as a filtered list.

Setting up the JUnit tests

We will use two simple JUnit tests to ensure we get what we want.

// test/java/HelloStreamsTest.java
import org.junit.Test;
import java.util.Arrays;
import java.util.List;

import static org.junit.Assert.assertEquals;

public class HelloStreamsTest {
    @Test
    public void testBasicStream() {
        List<Integer> input = Arrays.asList(1, 2, 3);

        Integer output = 6;
        assertEquals(output, new HelloStreams().sum(input));
    }

    @Test
    public void testFilteredStream() {
        List<String> input = Arrays.asList("a2", "a3", "b1", "c2", "a1");
        List<String> output = Arrays.asList("A2", "A3", "A1");

        assertEquals(output, new HelloStreams().filterStream(input));
    }
}

In our first test, we are simply giving an input integer list that we want and asserting that the return value equates to the output value.

The same is true of the second test where we want to compare to string lists for our favoured result.

build.gradle

We'll use a build.gradle setup to be able to run our tests:

apply plugin: "java"
apply plugin: "eclipse"
apply plugin: "idea"

repositories {
    mavenCentral()
}

dependencies {
    testCompile "junit:junit:4.12"
}
test {
    testLogging {
        exceptionFormat = 'full'
        events = ["passed", "failed", "skipped"]
    }
}

HelloStreams class

We'll set up two methods of the class that themselves are short and functional.

The first function sum will take our integer list, sum it up and return the sum.

The second function filterStream will take a list of strings and also return a list of strings that will use a filter and map function chain to give us the result that we want.

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

class HelloStreams {
    Integer sum(List<Integer> input) {
        return input.stream().mapToInt(Integer::intValue).sum();
    }

    List<String> filterStream(List<String> input) {
        return input.stream().filter(t -> t.startsWith("a")).map(String::toUpperCase).collect(Collectors.toList());
    }
}

sum simply turns the input list into a stream, converts the values to ints and returns the sum.

filterStream converts out input list into a stream, will filter for values that only contain "a", then uppercase those results and collect them and convert them back into a list using the Collectors stream utility.