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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.