Java Map

  • 3/12/2022
  • 2-minute read

In this post, I will introduce HashMap, LinkedHashMap and Hashtable in Java as well as their corresponding methods.

Introduction to Map

  • An element in a map contains two values, a key and a value.

  • Keys in the map are not allowed to be repeated.

  • HashMap is implemented by hash table; therefore, its access efficiency is super fast.

  • Before JDK 1.8, hash table is implemented by array plus linked list.

  • After JDK 1.8, hash table is implemented by array plus linked list / red black tree (if the length is greater than 8).

  • LinkedHashMap is an ordered collection.


Common Methods

// put(k, v): If k is not duplicated, it returns null. Else it returns the value that is replaced.
private static void show01() {
    Map<String, String> map = new HashMap<>();

    String v1 = map.put("Jason", "Kevin");
    System.out.println(v1); // null

    String v2 = map.put("Jason", "Joey");
    System.out.println(v2); // Kevin

    System.out.println(map); // {Jason=Joey}
}

// remove(k): If k is existed, it returns v. Else it returns null.
private static void show02() {
    Map<String, Integer> map = new HashMap<>();
    map.put("Jason", 1);

    Integer v1 = map.remove("Jason");
    System.out.println(v1); // 1

    Integer v2 = map.remove("Kevin");
    System.out.println(v2); // null
}

// get(k): If k is existed, it returns v. Else it returns null.
private static void show03() {
    Map<String, Integer> map = new HashMap<>();
    map.put("Jason", 1);
    System.out.println(map.get("Jason")); // 1
    System.out.println(map.get("Kevin")); // null
}

// containsKey(k): If k is existed, it returns true. Else it returns false.
private static void show04() {
    Map<String, Integer> map = new HashMap<>();
    map.put("Jason", 1);
    System.out.println(map.containsKey("Jason")); // true
    System.out.println(map.containsKey("Kevin")); // false
}

Iterate Map Approach 1: (Key -> Value)

// Use keySet() method to collect all keys of the map
private static void demo01() {
    Map<String, Integer> map = new HashMap<>();
    map.put("Jason", 177);
    map.put("Chan", 172);
    map.put("Hung", 178);

    Set<String> set = map.keySet();
    Iterator<String> it = set.iterator();
    while (it.hasNext()) {
        String key = it.next();
        System.out.println(key + ": " + map.get(key));
    }
}

Iterate Map Approach 2: (Map.Entry<K, V>)

// entrySet() stores all entries out of map in a set
private static void demo01() {
    Map<String, Integer> map = new HashMap<>();
    map.put("Jason", 177);
    map.put("Chan", 172);
    map.put("Hung", 178);

    Set<Map.Entry<String, Integer>> set = map.entrySet();
    Iterator<Map.Entry<String, Integer>> it = set.iterator();
    while (it.hasNext()) {
        Map.Entry<String, Integer> entry = it.next();
        String key = entry.getKey();
        Integer value = entry.getValue();
        System.out.println(key + ": " + value);
    }
}

Hashtable

  • ’null’ cannot be used as key or value in Hashtable.

  • Hashtable and Vector are all replaced by more advanced collections (HashMap, ArrayList) after JDK 1.2.

  • However, ‘Properties’ (A derived class of Hashtable) is still popular.

JavaData Structure