Map Interface

Basics#

  • maps Key to Value -- <K, V>
  • no duplicated Key allowed, or it will overwrite previous value with the same Key, aka. a key can only map to at most one value
  • deplicated Values are allowed

Put#

  • Returns previous value in the Key associated:
HashMap hm = new HashMap();
System.out.println(hm.put(1, 1)); // returns Null
System.out.println(hm.put(1, 2)); // 1
System.out.println(hm.put(1, 3)); // 2

Remove#

  • HashMap remove can uses key as a parameter:
map.remove(c); // c is key

Pitfall - ConcurrentModificationException#

for(int num : nums){
ctr.put(num, ctr.getOrDefault(num, 0) + 1);
if(ctr.size() > k){
for(int key : ctr.keySet()){
if(ctr.get(key) == 1){
ctr.remove(key);
} else ctr.put(key, ctr.get(key) -1);
}
}
}

Iterate Through Values#

  • use for loop — use values() to iterate through values in a HashMap:
HashMap<Integer, Integer> count = new HashMap<>();
for (int i : count.values())
res = Math.max(res, i);
return res;
  • use Map.Entry<K, V> entry: map.entrySet() to iterate through both values and keys:
for(Map.Entry<Character, Integer> entry: hm.entrySet()){
entry.getKey();
entry.getValue();
}