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# jlacar's solution

## to Darts in the Java Track

Published at May 01 2019 · 3 comments
Instructions
Test suite
Solution

#### Note:

This exercise has changed since this solution was written.

Write a function that returns the earned points in a single toss of a Darts game.

Darts is a game where players throw darts to a target.

In our particular instance of the game, the target rewards with 4 different amounts of points, depending on where the dart lands:

• If the dart lands outside the target, player earns no points (0 points).
• If the dart lands in the outer circle of the target, player earns 1 point.
• If the dart lands in the middle circle of the target, player earns 5 points.
• If the dart lands in the inner circle of the target, player earns 10 points.

The outer circle has a radius of 10 units (This is equivalent to the total radius for the entire target), the middle circle a radius of 5 units, and the inner circle a radius of 1. Of course, they are all centered to the same point (That is, the circles are concentric) defined by the coordinates (0, 0).

Write a function that given a point in the target (defined by its `real` cartesian coordinates `x` and `y`), returns the correct amount earned by a dart landing in that point.

## Setup

Go through the setup instructions for Java to install the necessary dependencies:

https://exercism.io/tracks/java/installation

# Running the tests

You can run all the tests for an exercise by entering the following in your terminal:

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

Use `gradlew.bat` if you're on Windows

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by removing the `@Ignore("Remove to run test")` annotation.

## Source

Inspired by an exercise created by a professor Della Paolera in Argentina

## Submitting Incomplete Solutions

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

### DartsTest.java

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

import static org.junit.Assert.assertEquals;

public class DartsTest {

@Test
public void testDartOutsideTarget() {
Darts darts = new Darts(-9, 9);
assertEquals(0, darts.score());
}

@Ignore("Remove to run test")
@Test
public void testDartLandsOnBorderOfTarget() {
Darts darts = new Darts(0, 10);
assertEquals(1, darts.score());
}

@Ignore("Remove to run test")
@Test
public void testDartLandsInOuterCircle() {
Darts darts = new Darts(4, 4);
assertEquals(1, darts.score());
}

@Ignore("Remove to run test")
@Test
public void testDartLandsInBorderBetweenOuterAndMiddleCircles() {
Darts darts = new Darts(5, 0);
assertEquals(5, darts.score());
}

@Ignore("Remove to run test")
@Test
public void testDartLandsOnMiddleOfCircle() {
Darts darts = new Darts(0.8, -0.8);
assertEquals(5, darts.score());
}

@Ignore("Remove to run test")
@Test
public void testDartLandsOnBorderBetweenMiddleAndInnerCircles() {
Darts darts = new Darts(0, -1);
assertEquals(10, darts.score());
}

@Ignore("Remove to run test")
@Test
public void testDartLandsInTheInnerCircle() {
Darts darts = new Darts(-0.1, -0.1);
assertEquals(10, darts.score());
}

@Ignore("Remove to run test")
@Test
Darts darts = new Darts(0.4, 0.8);
assertEquals(10, darts.score());
}

@Ignore("Remove to run test")
@Test
Darts darts = new Darts(2, 4);
assertEquals(5, darts.score());
}

@Ignore("Remove to run test")
@Test
Darts darts = new Darts(4, 8);
assertEquals(1, darts.score());
}

}``````
``````import java.util.List;
import java.util.function.Function;

import static java.lang.Math.*;
import static java.util.Comparator.comparing;

/* Stream version */

class Darts {

private final int score;

Darts(double x, double y) {
double hit = sqrt(x * x + y * y);
score = List.of(ring(1.0, 10),
ring(5.0, 5),
ring(10.0, 1))
.stream()
.map(fn -> fn.apply(hit))
.max(comparing(Integer::valueOf))
.get();
}

int score() {
return this.score;
}

/* Creates a closure for a ring with given radius and equivalent score */
private static Function<Double, Integer> ring(double radius, int score) {
return hit -> (hit <= radius) ? score : 0;
}
}``````

Solution Author
commented over 1 year ago

Note: It looks more complicated than a straightforward imperative solution, which I did for my first iteration. I wanted to see and compare a stream-based solution so this is what I came up with. I kind of like it, even though it may take a few seconds to figure out. Note also that there is no explicit if-statement although one could argue that there is a conditional branch in the `max()` method. <shrug></shrug>

(edited over 1 year ago)
Solution Author
commented over 1 year ago

Note: Score is calculated in the constructor itself and the x and y cartesian coordinates are not stored as fields. This is because the API suggests that `Darts` is immutable. If x and y never change, then the score will never change. If the score never changes, why should we calculate it every time it is queried? Better to calculate it once in the constructor. I did the same thing with my imperative solution.

Solution Author
commented over 1 year ago

Note: Someone elsewhere pointed out that the `Math.hypot()` method can be used instead of `sqrt()` - I think that was an excellent suggestion. Thanks, Campbell R.!

(edited over 1 year ago)

### What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™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?