The paperback edition of Mastering CodingBat (Java), Vol. 2: Intermediate A is now available for pre-order. It will be released on 29 April. A Kindle ebook will be released soon after that date. Check out the product page of this book for further information, including a sample PDF.

I just added the solutions to the three new problems in the Map-1 section of CodingBat and fixed the problem of the code samples on that page being incorrectly rendered.

Mastering CodingBat (Java), Vol. 1: Basics is now also available as an ebook for Amazon Kindle. Amazon also offers a free reader app for computers and mobile phones. Check out the product page of this book for further information, including a sample PDF.

For further help with Coding Bat (Java), please check out my books. I am also available for tutoring.

Here are my solutions to the Map-2 section on CodingBat.

word0:

public Map<String, Integer> word0(String[] strings) {
  
  Map<String, Integer> map = new HashMap<String, Integer>();
  
  for (int i = 0; i < strings.length; i++) {
    map.put(strings[i], 0);
  }
  
  return map;
}

wordLen:

public Map<String, Integer> wordLen(String[] strings) {
    
  Map<String, Integer> map = new HashMap<String, Integer>();
  
  for (int i = 0; i < strings.length; i++) {
    String tmp = strings[i];
    map.put(tmp, tmp.length());
  }
  
  return map;
}

pairs:

public Map<String, String> pairs(String[] strings) {
  
  Map<String, String> map = new HashMap<String, String>();
  
  for (int i = 0; i < strings.length; i++) {
    String tmp   = strings[i];
    String first = String.valueOf(tmp.charAt(0));
    String last  = String.valueOf(tmp.charAt(tmp.length() - 1));
    map.put(first, last);
  }
  
  return map;
}

wordCount:

public Map<String, Integer> wordCount(String[] strings) {
  
  Map<String, Integer> map = new HashMap<String, Integer>();
  
  for (int i = 0; i < strings.length; i++) {
    
    String tmp = strings[i];
    
    if (map.containsKey(tmp)) {
      int count = map.get(tmp);
      map.put(tmp, count + 1);
    } else {
      map.put(tmp, 1);
    }
    
  }
  return map;
}

firstChar:

public Map<String, String> firstChar(String[] strings) {
  
  Map<String, String> map = new HashMap<String, String>();
  
  for (int i = 0; i < strings.length; i++) {
    
    String key = String.valueOf(strings[i].charAt(0));
    
    if (map.containsKey(key)) {
      String val = map.get(key) + strings[i];
      map.put(key, val);
    } else {
      map.put(key, strings[i]);
    }
    
  }
  return map;
}

wordAppend:

public String wordAppend(String[] strings) {
 
 Map<String, Integer> map    = new HashMap<String, Integer>();
 String               result = "";
 
 for (int i = 0; i < strings.length; i++) {
   
   String key = strings[i];
   
   if (map.containsKey(key)) {
     int val = map.get(key);
     val++;
     if (val % 2 == 0) {
      result += key;
     }
     map.put(key, val);
   } else {
     map.put(key, 1);
   }
   
 }
 
 return result;
}

wordMultiple:

public Map<String, Boolean> wordMultiple(String[] strings) {
  
  Map<String, Integer> counts = new HashMap<String, Integer>();
  Map<String, Boolean> result = new HashMap<String, Boolean>();
  
  for (int i = 0; i < strings.length; i++) {
    String key = strings[i];
    
    if (counts.containsKey(key)) {
      int val = counts.get(key);
      val++;
      counts.put(key, val);
    } else {
      counts.put(key, 1);
    }
    
    result.put(key, counts.get(key) >= 2);
  }
  
  return result;
}

allSwap:

public String[] allSwap(String[] strings) {
  
  Map<String, Integer> map = new HashMap<String, Integer>();
    
  for (int i = 0; i < strings.length; i++) {
    
    String key = String.valueOf(strings[i].charAt(0));
    
    if (map.containsKey(key)) {
      
      // swap
      int    pos   = map.get(key); 
      String tmp   = strings[pos];
      strings[pos] = strings[i];
      strings[i]   = tmp ;
      
      // delete
      map.remove(key);
      
    } else {
      map.put(key, i);
    }
    
  }
  
  return strings;
}

firstSwap:

public String[] firstSwap(String[] strings) {
  
  Map<String, Integer> map = new HashMap<String, Integer>();
    
  for (int i = 0; i < strings.length; i++) {
    
    String key = String.valueOf(strings[i].charAt(0));
    
    if (map.containsKey(key)) {
      
      int val = map.get(key);
      if (val == -1) {
        continue;
      }
      
      // swap
      int    pos   = map.get(key); 
      String tmp   = strings[pos];
      strings[pos] = strings[i];
      strings[i]   = tmp ;
      
      // set a flag
      map.put(key, -1);
      
    } else {
      map.put(key, i);
    }
    
  }
  
  return strings;
}

For further help with Coding Bat (Java), please check out my books. I am also available for tutoring.

For further help with Coding Bat (Java), please check out my books. I am also available for tutoring.

Nick Parlante updated CodingBat with two new sets of Java exercises, which both focus on maps. My solutions for Map-1 are below.

mapBully:

public Map<String, String> mapBully(Map<String, String> map) {
  if (map.containsKey("a")) {
    String tmp = map.get("a");
    map.put("a", "");
    map.put("b", tmp);
  }
  return map;
}

mapShare:

public Map<String, String> mapShare(Map<String, String> map) {
  if (map.containsKey("a")) {
    String tmp = map.get("a");
    map.put("b", tmp);
  }
  map.remove("c");
  return map;
}

mapAB:

public Map<String, String> mapAB(Map<String, String> map) {
  if (map.containsKey("a") && map.containsKey("b")) {
    String tmp = map.get("a") + map.get("b");
    map.put("ab", tmp);
  }
  return map;
}

topping1:

public Map<String, String> topping1(Map<String, String> map) {
  if (map.containsKey("ice cream")) {
    map.put("ice cream", "cherry");
  }
  map.put("bread", "butter");
  return map;
}

topping2:

public Map<String, String> topping2(Map<String, String> map) {
  if (map.containsKey("ice cream")) {
    map.put("yogurt", map.get("ice cream"));
  }
  if (map.containsKey("spinach")) {
    map.put("spinach", "nuts");
  }
  return map;
}

topping3:

public Map<String, String> topping3(Map<String, String> map) {
  if (map.containsKey("potato")) {
    map.put("fries", map.get("potato"));
  }
  if (map.containsKey("salad")) {
    map.put("spinach", map.get("salad"));
  }
  return map;
}

mapAB2:

public Map<String, String> mapAB2(Map<String, String> map) {
  if (map.containsKey("a") && map.containsKey("b")) {
    if (map.get("a").equals(map.get("b"))) {
      map.remove("a");
      map.remove("b");
    }
  }  
  return map;
}

mapAB3:

public Map<String, String> mapAB3(Map<String, String> map) {
  if (map.containsKey("a") && !map.containsKey("b")) {
      map.put("b", map.get("a"));
  } else if (!map.containsKey("a") && map.containsKey("b")) {
      map.put("a", map.get("b"));
  }
  return map;
}

mapAB4:

public Map<String, String> mapAB4(Map<String, String> map) {
  if (map.containsKey("a") && map.containsKey("b")) {
    String a = map.get("a");
    String b = map.get("b");
    if      (a.length() > b.length()) map.put("c", a);
    else if (b.length() > a.length()) map.put("c", b);
    else { 
      map.put("a", "");
      map.put("b", "");
    }
  }
  return map;
}

For further help with Coding Bat (Java), please check out my books. I am also available for tutoring.