Java Data Structure Cheat Sheet - Hash Table

HashMap is the hash table implementation in Java. Check this post for time complexity.

https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html

The methods include:

• V put(K key, V value)
• boolean containsKey(Object key)
• V get(Object key)
• V remove(Object key)
• Set<K> keySet()
• etc

Example

import java.util.HashMap;
import java.util.Map;

public class HashMapExample {

  public static void main(String[] args) {
    Map<String, Integer> map = new HashMap<String, Integer>();

    map.put("a", 1);
    map.put("b", 2);
    map.put("c", 3);

    System.out.println(map.get("a")); // 1
    System.out.println(map.get("z")); // null
    if (map.containsKey("b")) {
      System.out.println(map.get("b")); // 2
    }

    for (String key : map.keySet()) {
      int val = map.get(key);
      System.out.println(val); // 1, 2, 3 (in undefined order)
    }
  }

}

Sort

• To sort by key, you can convert the key set into List and Collections.sort(), or you may consider using TreeMap instread.
• To sort by value (and still keep the key-value relation), you may convert the pairs into a list of objects then sort it by the value.