 # rootulp's solution

## to Accumulate in the Java Track

Published at Jul 13 2018 · 2 comments
Instructions
Test suite
Solution

Implement the `accumulate` operation, which, given a collection and an operation to perform on each element of the collection, returns a new collection containing the result of applying that operation to each element of the input collection.

Given the collection of numbers:

• 1, 2, 3, 4, 5

And the operation:

• square a number (`x => x * x`)

Your code should be able to produce the collection of squares:

• 1, 4, 9, 16, 25

Check out the test suite to see the expected function signature.

## Restrictions

Keep your hands off that collect/map/fmap/whatchamacallit functionality provided by your standard library! Solve this one yourself using other basic tools instead.

# Running the tests

You can run all the tests for an exercise by entering

``````\$ gradle test
``````

## Source

Conversation with James Edward Gray II https://twitter.com/jeg2

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### AccumulateTest.java

``````import org.junit.Test;
import org.junit.Ignore;

import java.util.Arrays;
import java.util.List;

import static org.junit.Assert.assertEquals;

public class AccumulateTest {

@Test
public void emptyAccumulateProducesEmptyAccumulation() {
assertEquals(expectedOutput, Accumulate.accumulate(input, x -> x * x));
}

@Ignore("Remove to run test")
@Test
public void accumulateSquares() {
List<Integer> input = Arrays.asList(1, 2, 3);
List<Integer> expectedOutput = Arrays.asList(1, 4, 9);
assertEquals(expectedOutput, Accumulate.accumulate(input, x -> x * x));
}

@Ignore("Remove to run test")
@Test
public void accumulateUpperCases() {
List<String> input = Arrays.asList("hello", "world");
List<String> expectedOutput = Arrays.asList("HELLO", "WORLD");
assertEquals(expectedOutput, Accumulate.accumulate(input, x -> x.toUpperCase()));
}

@Ignore("Remove to run test")
@Test
public void accumulateReversedStrings() {
List<String> input = Arrays.asList("the quick brown fox etc".split(" "));
List<String> expectedOutput = Arrays.asList("eht kciuq nworb xof cte".split(" "));
assertEquals(expectedOutput, Accumulate.accumulate(input, this::reverse));
}

private String reverse(String input) {
return new StringBuilder(input).reverse().toString();
}

@Ignore("Remove to run test")
@Test
public void accumulateWithinAccumulate() {
List<String> input1 = Arrays.asList("a", "b", "c");
List<String> input2 = Arrays.asList("1", "2", "3");
List<String> expectedOutput = Arrays.asList("a1 a2 a3", "b1 b2 b3", "c1 c2 c3");
assertEquals(expectedOutput, Accumulate.accumulate(
input1, c ->
String.join(" ", Accumulate.accumulate(input2, d -> c + d))
));
}
}``````
``````import java.util.List;
import java.util.ArrayList;
import java.util.function.Function;

public class Accumulate {
public static <T> List<T> accumulate(List<T> list, Function<T, T> f) {
List<T> results = new ArrayList<>();

for (T item : list) {
}

return results;
}
}`````` It is always best to initialize collections size, otherwise you could have performance issues. It is always best to initialize collections size, otherwise you could have performance issues.

### What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

• What compromises have been made?