The Deaf Rats of Hamelin Challenge in Java

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public class DeafRatsOfHamelin {
    public static int countDeafRats(final String town) {
        String t = town.replaceAll(" ","");
        int count = 0;
        for (int i = 0 ; i < t.length() ; i+=2) if (t.charAt(i) == 'O') count++;
        return count;
    }
}

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import java.util.stream.IntStream;

public class DeafRatsOfHamelin {
  public static int countDeafRats(final String town) {
    final String s = town.replaceAll("\\s+", "");
    return (int) IntStream.range(0, s.length()).filter(i -> s.charAt(i) == '~' && i%2 > 0).count();
  }
}

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public class DeafRatsOfHamelin {
  public static int countDeafRats(final String town) {
    int DeafRats=0;   
    String CompactTown;   
    CompactTown = town.replace(" ","");
    for(int i=0;i<CompactTown.length();i+=2){   
        if(CompactTown.charAt(i) == 'O') DeafRats++;     
    }
    return DeafRats;
  }
}

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import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class DeafRatsOfHamelinTest {
    @Test
    public void ex1() {
        assertEquals(0, DeafRatsOfHamelin.countDeafRats("~O~O~O~O P"));
    }
    @Test
    public void ex2() {
        assertEquals(1, DeafRatsOfHamelin.countDeafRats("P O~ O~ ~O O~"));
    }
    @Test
    public void ex3() {
        assertEquals(2, DeafRatsOfHamelin.countDeafRats("~O~O~O~OP~O~OO~"));
    }
}

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import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class SolutionTests {
  @Test
  public void ex1() {
    assertEquals(0, DeafRatsOfHamelin.countDeafRats("~O~O~O~O P"));
  }
  
  @Test
  public void ex2() {
    assertEquals(1, DeafRatsOfHamelin.countDeafRats("P O~ O~ ~O O~"));
  }
  
  @Test
  public void ex3() {
    assertEquals(2, DeafRatsOfHamelin.countDeafRats("~O~O~O~OP~O~OO~"));
  }

  @Test
  public void rats() {
    assertEquals(8, DeafRatsOfHamelin.countDeafRats("O~~OO~~OO~~OO~P~OO~~OO~~OO~~O"));
    assertEquals(8, DeafRatsOfHamelin.countDeafRats("O~~OO~~OO~~OO~ P~OO~~OO~~OO~~O"));
    assertEquals(8, DeafRatsOfHamelin.countDeafRats("O~~OO~~OO~~OO~P ~OO~~OO~~OO~~O"));
  }
  
  @Test
  public void highlander() {
    assertEquals(0, DeafRatsOfHamelin.countDeafRats("~OP"));
    assertEquals(0, DeafRatsOfHamelin.countDeafRats("PO~"));
    assertEquals(1, DeafRatsOfHamelin.countDeafRats("O~P"));
    assertEquals(1, DeafRatsOfHamelin.countDeafRats("P~O"));
  }

  @Test
  public void empty() {
    assertEquals(0, DeafRatsOfHamelin.countDeafRats("         P"));
    assertEquals(0, DeafRatsOfHamelin.countDeafRats("P         "));
    assertEquals(0, DeafRatsOfHamelin.countDeafRats("         P      "));
    assertEquals(0, DeafRatsOfHamelin.countDeafRats("P"));
  }

  // ====================================
  
  // Reference implementation for the Random test cases
  private static class DeafRatsOfHamelin {
  
    static int countDeafRats(final String town) {
      String s = "";
      for (int i = 0; i < town.length(); i++) {
        final char c = town.charAt(i);
        if (c == '~') { s += "R"; i ++; }
        else if (c == 'O') { s += "L"; i ++; }
        else if (c == 'P') { s += c; }
      }
      final String s1 = s.substring(0, s.indexOf("P")), s2 = s.substring(s.indexOf("P")+1);
      int deaf = 0;
      // Rats to left of the Piper should be going right
      deaf += s1.replace("R","").length();
      // Rats to right of the Piper should be going left
      deaf += s2.replace("L","").length();
      return deaf;
    }
  
  }
  
  private static String makeRatFacing(final char mostly) {
    final double d = Math.random() * 3;
    if (mostly == 'L') {
      // Mostly facing left
      if (d < 2.5) return "O~";
      if (d < 2.8) return "~O";
    } else {
      // Mostly facing right
      if (d < 2.5) return "~O";
      if (d < 2.8) return "O~";
    }
    return "  ";
  }
  
  private static String makeTown(char piperPos) {
    final int ratsLeft = (int)(Math.random() * 20) + 5;
    final int ratsRight = (int)(Math.random() * 20) + 5;
    String town = "";
    boolean piper = false;
    if (piperPos == 'L') { town += "P"; piper = true; }
    for (int r = 0; r < ratsLeft; r++) {
      town += makeRatFacing(piper ? 'L' : 'R');
    }
    if (piperPos == 'M') { town += "P"; piper = true; }
    for (int r = 0; r < ratsRight; r++) {
      town += makeRatFacing(piper ? 'L' : 'R');
    }
    if (piperPos == 'R') town += "P";
    return town;
  }
  
  @Test
  public void random() {
    for (int r = 1; r <= 200; r++) {
      final int p = (int)(Math.random() * 3);
      final char piper = p == 0 ? 'L' : p == 1 ? 'M' : 'R';
      final String town = makeTown(piper);
      final int expected = DeafRatsOfHamelin.countDeafRats(town);
      System.out.println(String.format("Random test %d : <span style='color:green'>%s</span> has %d deaf rats", r, town, expected));      
      assertEquals(expected, DeafRatsOfHamelin.countDeafRats(town));
    }
  }
}