RouletteWheel.java

import java.util.Random;

public class RouletteWheel {

    private boolean bouncing = false;
    private int ballNumber = 0;
    private long elapsedSpinTime = 0;
    private long spinStartTime = 0;

    public boolean isBouncing() {
        return bouncing;
    }

    public int getBallNumber() {
        return ballNumber;
    }

    public void spin(long spinTime) {
        this.spinStartTime = System.currentTimeMillis();

        do {
            bounce();
        } while (elapsedSpinTime < spinTime);

        this.bouncing = false;
    }

    private void bounce() {
        this.bouncing = true;
        generateRandomBallNumber();
        this.elapsedSpinTime = System.currentTimeMillis() - spinStartTime;
    }

    private void generateRandomBallNumber() {
        this.ballNumber = new Random().nextInt(37);
    }

    public long getElapsedSpinTime() {
        return elapsedSpinTime;
    }

}

RouletteWheelTest.java

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;
import static org.junit.Assert.fail;

import java.util.HashSet;
import java.util.Set;

import org.junit.Test;

public class RouletteWheelTest {

    @Test
    public void rouletteWheelBallInitiallyNotBouncing() {
        RouletteWheel rouletteWheel = new RouletteWheel();
        assertFalse(rouletteWheel.isBouncing());
    }

    @Test
    public void rouletteBallStartsAtZero() {
        RouletteWheel rouletteWheel = new RouletteWheel();
        assertEquals(0, rouletteWheel.getBallNumber());
    }

    @Test
    public void rouletteWheelSpinsForTwentySeconds() {
        RouletteWheel rouletteWheel = new RouletteWheel();
        rouletteWheel.spin(20000);
        assertEquals(20000, rouletteWheel.getElapsedSpinTime());
    }

    @Test
    public void rouletteWheelBallLandsBetweenZeroAndThirtySix() {
        RouletteWheel rouletteWheel = new RouletteWheel();
        Set<Integer> ballNumberResults = new HashSet<Integer>();
        for (int i = 0; i < 1000; i++) {
            rouletteWheel.spin(5);
            int ballNumber = rouletteWheel.getBallNumber();
            if (ballNumber < 0 || ballNumber > 36) {
                fail("Ball number out of range");
            }
            ballNumberResults.add(Integer.valueOf(ballNumber));
        }
        assertTrue(ballNumberResults.size() == 37);
    }

    @Test
    public void rouletteWheelBallStopsBouncing() {
        RouletteWheel rouletteWheel = new RouletteWheel();
        rouletteWheel.spin(5);
        assertFalse(rouletteWheel.isBouncing());
    }

    @Test
    public void rouletteWheelStopsWhenElapsedTimeIsGreaterThanSpinTime() {
        RouletteWheel rouletteWheel = new RouletteWheel();
        rouletteWheel.spin(-5);
        assertFalse(rouletteWheel.isBouncing());
    }

}

Kata suggested by @jjeffries1 https://docs.google.com/document/d/18mBn4R_DsuHPSuZHCqTgXrhXfyPJW-Gd46EJS_mWy2k/edit