My Solution for https://dzone.com/articles/java-code-challenge-scrabble-sets

ScrabbleSets.java

package software.schwering.javacodechallenge;

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collectors;

public class ScrabbleSets {
	
	private static final Map<Character, Integer> COUNT_BY_LETTER = new HashMap<>();
	static{
		//see http://scrabblewizard.com/scrabble-tile-distribution/
		COUNT_BY_LETTER.put('A', 9);
		COUNT_BY_LETTER.put('B', 2);
		COUNT_BY_LETTER.put('C', 2);
		COUNT_BY_LETTER.put('D', 4);
		COUNT_BY_LETTER.put('E', 12);
		COUNT_BY_LETTER.put('F', 2);
		COUNT_BY_LETTER.put('G', 3);
		COUNT_BY_LETTER.put('H', 2);
		COUNT_BY_LETTER.put('I', 9);
		COUNT_BY_LETTER.put('J', 1);
		COUNT_BY_LETTER.put('K', 1);
		COUNT_BY_LETTER.put('L', 4);
		COUNT_BY_LETTER.put('M', 2);
		COUNT_BY_LETTER.put('N', 6);
		COUNT_BY_LETTER.put('O', 8);
		COUNT_BY_LETTER.put('P', 2);
		COUNT_BY_LETTER.put('Q', 1);
		COUNT_BY_LETTER.put('R', 6);
		COUNT_BY_LETTER.put('S', 4);
		COUNT_BY_LETTER.put('T', 6);
		COUNT_BY_LETTER.put('U', 4);
		COUNT_BY_LETTER.put('V', 2);
		COUNT_BY_LETTER.put('W', 2);
		COUNT_BY_LETTER.put('X', 1);
		COUNT_BY_LETTER.put('Y', 2);
		COUNT_BY_LETTER.put('Z', 1);
		COUNT_BY_LETTER.put('_', 2);
	}
	
	public static void printRemainingTiles(String tilesInPlay){
		Map<Character, Integer> remainingCount = new HashMap<>(COUNT_BY_LETTER);
		tilesInPlay.chars().forEach(i->remainingCount.put((char)i, remainingCount.get((char)i)-1));
		List<Character> errors = remainingCount.entrySet().stream().filter(e->e.getValue()<0).map(Entry::getKey).collect(Collectors.toList());
		if(errors.isEmpty()){
			Map<Integer, String> tilesByCount = remainingCount.entrySet().stream().collect(Collectors.groupingBy(Entry::getValue, Collectors.mapping(Entry::getKey, Collectors.mapping(String::valueOf,Collectors.joining(", ")))));
			tilesByCount.entrySet().stream().sorted((e1,e2)->e2.getKey().compareTo(e1.getKey())).forEachOrdered(e->System.out.printf("%s: %s\n",e.getKey(),e.getValue()));
		}else{
			errors.forEach(c->System.out.printf("Invalid input. More %s's have been taken from the bag than possible.\n",c));
		}
	}

}

ScrabbleSetsTest.java

package software.schwering.javacodechallenge;

import static org.hamcrest.CoreMatchers.equalTo;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;

import java.io.ByteArrayOutputStream;
import java.io.PrintStream;
import java.io.UnsupportedEncodingException;
import java.nio.charset.StandardCharsets;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

public class ScrabbleSetsTest {

	private ByteArrayOutputStream baos;
	private PrintStream out;

	@Before
	public void redirectOut() {
		baos = new ByteArrayOutputStream();
		out = new PrintStream(baos);
		System.setOut(out);
	}

	@After
	public void closeStreams() {
		out.close();
	}
	
	private String getOutputString() throws UnsupportedEncodingException{
		return baos.toString(StandardCharsets.UTF_8.name());
	}

	@Test
	public void testChallengeInput1() throws UnsupportedEncodingException {
		ScrabbleSets.printRemainingTiles("PQAREIOURSTHGWIOAE_");
		
		assertThat(getOutputString(), is(equalTo(
				"10: E\n" + 
				"7: A, I\n" + 
				"6: N, O\n" + 
				"5: T\n" + 
				"4: D, L, R\n" + 
				"3: S, U\n" + 
				"2: B, C, F, G, M, V, Y\n" + 
				"1: H, J, K, P, W, X, Z, _\n" + 
				"0: Q\n")));
	}

	@Test
	public void testChallengeInput2() throws UnsupportedEncodingException {
		ScrabbleSets.printRemainingTiles("LQTOONOEFFJZT");
		
		assertThat(getOutputString(), is(equalTo(
				"11: E\n" + 
				"9: A, I\n" + 
				"6: R\n" + 
				"5: N, O\n" + 
				"4: D, S, T, U\n" + 
				"3: G, L\n" + 
				"2: B, C, H, M, P, V, W, Y, _\n" + 
				"1: K, X\n" + 
				"0: F, J, Q, Z\n")));
	}

	@Test
	public void testChallengeInput3() throws UnsupportedEncodingException {
		ScrabbleSets.printRemainingTiles("AXHDRUIOR_XHJZUQEE");
		
		assertThat(getOutputString(), is(equalTo("Invalid input. More X's have been taken from the bag than possible.\n")));
	}

}