Can I use object as a key in HashMap?

Can I use object as a key in HashMap?

Answer to your question is yes, objects of custom classes can be used as a key in a HashMap. If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

How are user defined objects stored in HashMap?

If you want to make a mutable object as key in hashmap, then you have to make sure that state change for key object does not change the hash code of object. This can be done by overriding the hashCode() method. But, you must make sure you are honoring the contract with equals() also.

What is a HashMap key?

HashMap is a Map based collection class that is used for storing Key & value pairs, it is denoted as HashMap or HashMap. This class makes no guarantees as to the order of the map. It is similar to the Hashtable class except that it is unsynchronized and permits nulls(null values and null key).

How get key from value in HashMap?

  1. import java. util. HashMap; import java. util. Map;
  2. // Program to get Map’s key from the value in Java.
  3. class Main. {
  4. public static K getKey(Map map, V value) {
  5. for (K key: map. keySet()) {
  6. if (value. equals(map. get(key))) { return key;
  7. } }
  8. return null; }

Which two methods HashMap key object should implement?

Any Object that we want to use as key for HashMap or in any other hash based collection data structure e.g. Hashtable, or ConcurrentHashMap must implement equals() and hashCode() method.

Which two methods are needed to implement for key object in HashMap?

In order to use any object as Key in HashMap, it must implements equals and hashcode method in Java.

Is HashMap immutable?

String, Integer and other wrapper classes are natural candidates of HashMap key, and String is most frequently used key as well because String is immutable and final,and overrides equals and hashcode() method. Other wrapper class also shares similar property.

Is HashMap thread safe?

HashMap is non-synchronized. It is not thread-safe and can’t be shared between many threads without proper synchronization code whereas Hashtable is synchronized.

Can a HashMap have duplicate keys?

HashMap doesn’t allow duplicate keys but allows duplicate values. HashMap allows null key also but only once and multiple null values.

How do I get the TreeMap key?

The process is divided into three steps:

  1. Use the entrySet() method of the TreeMap class to get a Set view of all the entries stored in the TreeMap object.
  2. Convert the entry set to an array using the toArray() method.
  3. And get TreeMap key or TreeMap value using index with the help of getKey() and getValue() method.